Download Mineral Grain–Complete

The Mineral Grain
and Mineral-grain
Surfaces:
a low-technology
approach, description,
and use
By Wallace D. Kleck
Notice—I claim copyright protections for this work and its contents. However, I freely
grant anyone permission to make one paper or electronic copy for their own personal use. If any
of the contents are quoted, there must be an appropriate technical reference. The total or any part
of this work, may not be copied for sale or sold without the direct, personal permission of
Wallace D. Kleck or his estate. I may be contacted by standard-mail at 23940 Basin Harbor
Court, Tehachapi, CA, 93561 or by e-mail at [email protected].
So stated, May, 10, 2012, Tehachapi, California.
ii
TABLE OF CONTENTS
Note, page numbers are shown for 1st and 2nd
level headings
------section heading---------------------------------------------------------------------page
ABSTRACT
1
INTRODUCTION
1
DEFINITIONS
2
FORMATION OF GRAINS
4
GRAIN SURFACES—PROCESSES OF FORMATION
5
Grain Growth
5
Growth-free
Growth-partially-free
Growth-restricted
Dissolution (s.l.--sensu lato)
6
Mechanical Modification
6
Other (Grains at or Near Equilibrium)
6
MINERAL GRAIN AND CRYSTAL
7
MINERALOID
8
GRAIN SURFACES—DETAILS
8
Growth Surfaces (redefined; new)
9
Type-1 surfaces
Type-2 surfaces
Type-3 surfaces
Type-4 surfaces
Type 4-1 surfaces; mutual growth and relative bond strength
Type 4-1 surfaces; metamorphic environments
Type 4-1 surfaces; igneous environments
Type-5 surfaces
Paramorphic-surfaces (redefined)
22
Dissolution (s.l.) (modified)
23
Features of melt surfaces
Features of a chemical solution surface
Features of dissolution surfaces on quartz phenocrysts in rhyolite
Recognizing dissolution surfaces
Mechanically modified (new)
28
Phantom Surface (modified)
30
Artificial
30
Mineraloid (modified)
30
Gel-mineral (modified)
Grain-replaced (new)
Crystallized gel (new)
Grain-metamict.
Grain-domain Containing (new)
THREE EXAMPLES
34
Pegmatite-Example 1
34
Quartz in Rhyolite (Crystal Tuff)-Example 2
35
iii
Spodumene Crystal-Example 3
DISCUSSION
Other Surfaces
Crystals
Mixed and Complex Surfaces
Surfaces on Grains from Pegmatite Rocks
Grains in Cavities36
Some Comments on the Nature of Selected Minerals
Feldspar (in crystal tuffs)
Β-quartz (in crystal tuffs)
Mica (in a variety of environments)
Graphite (in tactite)
CREATION OF SUITABLE GRAINS FOR STUDY
Weathering
Cavities
Artificially Crushed, Broken, or Naturally Fragmented Rocks
Separation by Chemical or Physical Properties
ACKNOWLEDGEMENTS
REFERENCES CITED
iv
36
36
36
37
37
37
42
43
43
44
44
44
44
45
The Mineral Grain and Mineral-grain
Surfaces: a low-technology approach and
uses
Wallace D. Kleck
[email protected]
23940 Basin Harbor Court, Tehachapi, California 93561, USA; retired--no present college
affiliation
ABSTRACT
A ‘grain’ (mineral grain) is redefined as the fundamental unit of mineralogy. Grain surfaces (in
3-dimensions) provide considerably more information about the geologic history of grains in a
rock than do a small number of thin sections through grains. Grain surfaces are formed by one or
a combination of growth, dissolution, and mechanical modification. Herein, grain surfaces are
divided into six classes and ten subclasses (in parentheses)—growth (type-1, type-2, type-3,
type-4, type 4-1, type-5), paramorphic, dissolution, mechanically modified (cleavage, parting,
fracture, smoothed), phantom, and artificial. In addition, mineraloid is redefined and divided into
five subclasses (gel-mineral, grain-replaced, crystallized gel, grain-metamict, and grain-domain
containing).
Type 4-1 growth surfaces are a newly defined type of surface, resulting from competitive growth
(primarily) in igneous and metamorphic environments. It is proposed that the determining factor
in the ‘goodness of development’ of crystal surfaces on such grains is the relative difference in
bond strength. An imprecise measure of the difference in bond strength is the relative difference
in mineral hardness (ΔH). Where the difference of hardness between two competing grains is
approximately >2, recognizable crystal surfaces develop.
INTRODUCTION
For approximately 100 years, petrologists and mineralogists have dealt with the interpretation of
the features of mineral grains primarily in section. This is a useful and well developed technique,
but it ignores features that can provide a great deal of information about the environment of
formation of mineral grains—i.e. the nature of and the features on the surfaces of the mineral
grains. These surface features are the last to form and then are the latest of the mineral-grain
surface features. For a compete interpretation of the grains, this information needs to be added to
information about the internal (and former grain surfaces) features gained by section studies.
This paper has three purposes:
 To gather and develop information on the origin and nature of grain surfaces;
1

To develop a classification system that places this information in a coordinated
and logical format;
 To demonstrate how this information can be used.
In doing this, it is necessary to develop a certain amount of new terminology and to modify some
existing terminology; if this approach is valid, the new and modified terminology will need to
become a part of mineralogy and petrology. It is suggested that information on grain surfaces
become one of the standard types of information collected and interpreted for mineral grains in
rocks.
Further, this paper is primarily concerned with natural, solid grains that have formed at or near
the surface of Earth. It is not that other substances or environments are uncommon or
uninteresting, but for the practical reasons of length and coherence of topic. The surface and near
surface processes of the planetary body called Earth are complex enough. But as we scientists
begin to consider the interior of Earth, outer-space, other planetary bodies, etc., the job of
understanding natural solids grows to unimagined proportions. Truly, we are cursed (or blessed)
and live in interesting times.
Note that Geosphere (the GSA online journal) is planning an issue on high- and new-technology
imaging of 3-D grain surfaces. However, before we can use the high technology information, we
need to know the ‘introductory’ information. This article is concerned with the low- and currenttechnology study of 3-D grain surfaces. This is available using 1-20X magnification and a
stereomicroscope.
Three types of mineral-grain surfaces (crystal, cleavage, fracture) were characterized by the late
1700’s. Observations about these three types of surfaces were keys to the eventual development
of atomic theory and the nature of crystalline material. For example, in 1669, Steno observed that
the angles between the ‘same’ crystal faces on different quartz crystals were equal. In 1723,
Cappeller found that this was the case for different minerals and that each mineral crystal had its
own characteristic angles. In 1773, Bergmann observed that finely crushed calcite produced
uniform rhombohedra which he suggested may be the building blocks of crystals. By 1900 basic
crystallography, optical properties, and the relationships between these three surfaces of grains
and the internal structure of the mineral grain itself was generally understood and widely
utilized.
From a practical standpoint, from the early 1800’s, it appears that the presence, absence, and
nature of the three surfaces (cleavage, crystal faces, fracture) were used to provide key
identification features for minerals. These were used long before it was recognized that they are
unique to each mineral species and that they were controlled by the nature of bonding and
structure in individual mineral species. Gliding-planes were recognized and discussed by the
1880’s; this probably led to understanding parting as a type of surface. Most of the information
in this and the preceding paragraph was developed from the references listed in Dana and Foord
(1932).
With the passage of time and the development of technology, additional types of surfaces have
been recognized, but seldom related to known processes. It may now be appropriate to describe,
relate, and catalog all of these surfaces.
2
DEFINITIONS
In most cases where an adjective is used with a noun, the noun will be placed first with a dash
followed by the adjective. The following new and/or modified terms are proposed herein:
 anhedral—lacking (for one reason or another) crystal growth surfaces;
 chemical replacement—the original material of a mineral grain has been replaced by (or
changed to) a different chemical material and, typically, changed in its physical form
(from that a crystalline solid to that of an aggregate of grains) with or without change to
the shape of the surface of the original grain;
 cleavage—modified only to the extent that it is now a part of mechanically modified
surfaces;
 crystal—a grain consisting of at least 75% growth-type-1 surfaces;
 delta-H (ΔH)—the difference in hardness (~bond strength) between two growing grains
which are competing for space;
 dissolution—general term for the process of dissolving; applies to all process whereby
grains have been dissolved and made smaller;
 dissolution-faceted—dissolution which produces surfaces resembling crystal faces;
 dissolve—the verb ‘to dissolve’ associated the process of dissolution;
 euhedral—a growth-type-1 surface with well-developed crystal surfaces;
 fracture—modified only to the extent that this is now a part of mechanically modified
surfaces;
 fragment—a term useable for a piece of a mineral or a natural substance of uncertain or
unknown characteristics;
 gel-like—an imprecise, catchall term for gels, former gels, and extremely fine-grained,
silica-rich materials (excluding glasses); used particularly when detailed study has not
been conducted and the exact nature of the material is uncertain;
 gel mineral—opal and other gel-like substances that lack long-term order, but may have
short-term order;
 grain-aggregate—a grain with inclusions of smaller sub-grains;
 grain-crystal—a mineral grain bounded by greater than 75% growth-type-1, -2 or 4-1
surfaces;
 grain-domain—an ill defined portion of a grain with a (slightly) different structure and
composition (currently, only applicable to the plagioclase feldspars);
 grain-lithic-fragment—a grain composed of sub-grains; typically a rock fragment;
 grain-paramorphic—a grain with a relic surface formed under a different set of
equilibrium conditions, but the surface survived the inversion to current conditions; i.e.
the internal atomic structure has changed to that stable under current conditions, but the
external shape has not;
 grain-replaced—a chemically replaced or physically modified grain; a general term
which includes pseudomorph as a specific type of replacement;
 grain-metamict—a former mineral grain where the original atomic structure has been
destroyed by radioactivity or shock;
 growth-exsolved—growth resulting in a perthite (currently, only applicable to the
feldspars);
3




















growth-free—growth in an environment where no physical boundaries interfere with the
grain growth and a (nearly) complete growth-type-1 grain results;
growth-partially-free—growth in an environment where there is some interference with
growth;
growth-restricted—growth in an environment where boundaries exist in all three
dimensions;
growth-type-1—growth resulting in crystal faces on all (or most) surfaces;
growth-type-2—growth where surface formation is modified by abundant inclusions;
growth-type-3—growth where surface formation is modified by preexisting surfaces;
growth-type-4—growth where surface formation is modified by another, simultaneous
growing grain of similar bond strength (ΔH < 2);
growth-type 4-1—growth were relative bond strength (ΔH > 2) determines the
development of the type of growth surface between two or more growing, competing
grains;
growth-type-5—growth producing a surface on a gel-mass (a non-crystal growth);
mechanically modified surface—a grain surface produced by mechanical process (such as
abrasion, faulting, glaciation, fracturing, cleaving, etc.) resulting in a new surface; the
surface may or may not be irregular (i.e., cleavage and parting result in a structurallycontrolled, regular surface);
mineraloid—a generalized term applied to certain mineral-like grains;
mineral grain (also grain)—fundamental and general unit in mineralogy and petrology;
parting—modified only to the extent that this is now a part of mechanically modified
surface;
pseudomorph—a chemically replaced or physically modified grain which retains the
crystal shape of the original mineral;
sub-grain—term applied to grains within a larger grain such as a rock fragment when it is
a part of a rock;
subhedral—a growth surface with poorly developed crystal surfaces;
surface-etched—a dissolution surface covered by small pits and rough on a very small
scale (pits are observable only 10X or greater);
surface-melted—a dissolution surface created by melting of the grain.
surface-phantom —a surface on within a grain covered by additional growth after a pause
in growth;
surface-pitted—a dissolution surface covered by pits large enough to be observed at less
than 10X;
FORMATION OF GRAINS
As geologists explore the margins of classic rock classifications, we find that these (in selected
environments) grade into one another. Granted, some of the gradational volumes are small and
specialized, but, never-the-less, we delight in exploring them and arguing about the compartment
in which particular rocks best fit. These compartments, then, are indistinct and have fuzzy
boundaries. But these compartments hold the most common of the range of natural materials.
These classifications are useful for building a structure of ideas and for instructing the beginning
student in geology.
4
Further, it appears that the generalized processes of growth and dissolution of grains are nearly
the same no matter the:
 source of the growth units (atoms and groups of atoms);
 medium in which this takes place (solid, liquid, gas, fluid, silicate melt, water-based
fluid, etc.);
 temperature;
 pressure;
 composition of the material.
The physical nature of the grains is apparently independent of the material from which they form
and the conditions of formation. Yes, different P, T, X conditions results in different composition
of grains, but the processes of nucleation, growth, dissolution, and modification are basically the
same--bonds form, rupture, and change.
The following examples, then, are chosen without regard to their classical source (although each
example will be described in relation to its classical origin). They are selected to illustrate the
physical nature of and the processes of formation of the grain surfaces. Note that by determining
the nature of the grain surface, we can sometimes infer the last of the processes that affected the
grain. Note also, as we build an understanding of the last processes, it increases our
understanding of the former surfaces that we can observe (in sections) in the interior of the grain.
GRAIN SURFACES—PROCESSES OF FORMATION
The three primary processes involved in the formation of grain surfaces are:
 growth;
 dissolution;
 mechanical modification.
Grain Growth
This term is herein restricted (with the exception of growth-type 5) to process(es) of enlargement
of a crystalline, solid, natural grain in a geologic environment. Such growth may be free-growth,
partially-free-growth, or restricted-growth and the nature of the growth may grade from one to
another of these. Physical boundaries to such growth may consist of pre-existing grains, other
growing grains, pre-existing rocks, interfaces between different material (solid-liquid, liquidgaseous, immiscible liquids, etc.). As well, early formed grains may become unstable and
disappear as the physical or chemical conditions surrounding the grains change.
Growth-free
Such growth occurs where there are no physical boundaries to interfere with the attachment of
atoms or groups of atoms to the surface of an existing grain. The general result of such freegrowth is a single grain bounded by at least 75 percent crystal surfaces (this allows for
attachment of the growing grain to a pre-existing surface).
5
This process is common but not ubiquitous in geologic environments. In addition, such a process
is many times not preserved in the final rock because it grades into restricted growth or into
dissolution. For example, the initial process of growth in an intrusive magma body may be
homogeneous nucleation of isolated grains which undergo free-growth, but grades into partiallyfree-growth and then into restricted growth as the proportions of melt and solid change; as well,
the initial grains may become unstable and undergo dissolution. Such growth can exist in other
than igneous environments—for example, a snow flake may grow freely until it accumulates as a
mass of grains on the ground surface or until it passes into a volume of air of different
temperature or different water content.
Such free-growth surfaces may be preserved:
 as growth zones in the interior of irregular grains;
 as phenocrysts; including early crystallizing grains preserved in pumice or tuffs;
 in the solidification of complex melts, liquids, or fluids as different minerals become
stable and form or accumulate (and are protected by enclosure);
 in recrystallization of certain metamorphic or sedimentary materials;
 in the accumulation of certain sedimentary materials.
Growth-partially-free
In this situation, growth is restricted in less than three directions, but not in all directions. For
example, if growth results in a crust of grains on a surface in a hydrothermal environment or
growth occurs in an inclusion-rich environment (see Figure 3 following).
Growth-restricted
In this situation, growth is restricted by physical barriers in all directions. An example would be
the final growth in the solidification of magma.
Dissolution (s.l.--sensu lato)
This term (in the strict sense) is commonly used by current geochemists for the process(es) of the
reaction(s) of grains with a melt. The term, herein, is expanded and adapted to mean the
processes of 1) transport of reactants to and from an interface; 2) reaction(s) at the interface; 3)
transfer of heat at the interface. The ‘interface’ is the contact of solid, liquid, gas, melt, or fluid
with a given grain. This set of processes is adapted and adopted from Kuo and Kirkpatric, (1985,
p. 52), and they may serve both for dissolution and growth. The difference would appear be the
nature of the heat term (exothermic for growth, endothermic for dissolution, and zero for
equilibrium). This may serve as a simplified and generalized model for growth, equilibrium, and
dissolution.
In its (herein) expanded and modified form, the term applies to all geologic environments where
material is transported to or away from a grain surface and the grain is decreasing in size. It is
considered that melting, solution (in water and other fluids) and reaction between grains and
solids, liquids, melts or fluids are all variations on the same three-step process. It may not be
known, after the event(s), which of the various specific processes have produced a naturally
6
rounded grain. If a person wishes to use the term strictly for melt-reaction processes use
‘dissolution (s.s.)’ where ‘(s.s.)’ stands for ‘sensu stricto’. In the most common cases, such
dissolution results in the rounding of the grain in question (see further discussion under ‘Other
(Grains at or Near Equilibrium)’ following).
If ‘dissolution’ becomes the term for the process, then the verb should be ‘to dissolve’. So ‘to
dissolve’ now is also a general verb covering a single process in a variety of environments.
Mechanical Modification
Any number of natural or artificial processes can result in the physical disruption of a grain (i.e.
breaking of the grain into two or more fragments) or the modification of grains by abrasion. This
process results in the formation of several types of surfaces. The distinction between natural and
artificial will be indicted by ‘mechanical modification (artificial)’. Fracture, parting, and
cleavage (herein) are a part of this category.
Other (Grains at or Near Equilibrium)
It is suggested that (under certain circumstances) both growth and loss of mass for a grain may
happen simultaneously. For example, a formerly free-growing grain reaches and exists in a state
of equilibrium with the surrounding material wherein growth and dissolution (as dynamic
processes) both occur but are balanced in rate; as a result, the grain has neither a net gain nor loss
of atoms or groups-of-atoms.
Donaldson (1985b, p. 130) notes that in several reported experiments that ‘dissolution facets’
appear to occur in cases where “melt or solution is only slightly under saturated” and attributes
the development of facets to interface kinetics. It is proposed that near the boundaries of overunder-saturation and over-under-heat that both addition and removal of atoms or groups-ofatoms is occurring. In a state of equilibrium, at a phase boundary, the two rates are balanced and
the two processes are both occurring. As the system is shifted away from equilibrium, the
process is shifted so that one rate is greater than the other, but the two dynamic processes are still
occurring. It is only when the system is ‘significantly far’ from the boundary that one process
dominates to the near exclusion of the other. In cases where both processes are occurring at near
equal rates, the removal of material will preferentially occur where atoms or groups of atoms are
slightly less strongly attached. This will lead to the selective survival of surfaces of more
strongly attached atoms (hence dissolution facets).
If this model is valid, some (? many) phase boundaries are not a sharp change from one process
to another but a place where growth and dissolution rates are in balance. As well, the boundary is
not a narrow sharp line, but is a broad, fuzzy line across which the relative rates change. This,
then, may be the process by which faceted grains develop during dissolution. In general, grain
growth tends to produce crystal surfaces and grain dissolution generally tends to produce
rounded grains although close to boundary a grain may both loose mass and maintain a general
crystal form (faceted dissolution). As well, solution by a very weak dissolving agent may be
controlled by slight differences in bonding. In a natural system and long after the formation of
the grains, the fact that a faceted grain had a net loss of mass cannot necessarily be differentiated
7
from a grain that had a net gain of mass; this can only be discerned in a closely monitored
artificial system. Also note that the slowest of artificial systems is still fast when considered over
geologic time.
MINERAL GRAIN AND CRYSTAL
Under the system developed herein, ‘mineral grain or grain’ becomes the fundamental and
general unit in mineralogy just as ‘rock’ is the fundamental and general unit for petrology. A
mineral grain is a fragment of a naturally-occurring solid with a regular, long-range, threedimensional, characteristic, and homogeneous, internal structure composed of bonded atoms.
This is, basically the definition of ‘crystalline’, and most single mineral grains are crystalline
(exceptions to and terminology for this will be explored later, i.e. glasses, etc.). The mineral
grain may vary in chemical composition within restricted limits as long as the internal structure
is homogeneous and without physical, internal boundaries. The naturally allowed variation of
composition is determined by the atom size and bond requirements of grain structural sites.
Different atoms (sizes and bond types) may be accommodated in very small amounts (a few
parts per thousand) by small distortions or flaws in the mineral structure; omissions may also be
accommodated. Omissions (vacant structural sites) may well turn out to be more common than
we now realize; this is due, in part, to past and present short-comings of our analytical
techniques.
Chemical zoning is a common phenomenon. This most commonly consists of concentric bands
made up slightly different amounts of similarly sized and charged atoms substituting for one
another. Zoning may also be gradational and may involve different charges so long as the
charges are balanced locally in some manner. Such chemical zoning is not considered to
represent a boundary between grains. For the purposes of this definition, internal twinboundaries are also not considered to be such physical boundaries because the same structure
(but at a specific, different angle) is present on both sides of the twin boundary.
The term ‘fragment’ (mineral fragment, rock fragment, glass fragment) can be used for any
substance of uncertain character or not meeting these criteria. This term also deals with
troublesome things like glass shards, glass fragments, and pumice fragments in tuffs.
From a physical chemistry point of view, glass is a substance distinct from the liquid which
preceded it (see Mysen, 2005 for a extensive treatment); however, from the point of view of
petrology, glass may be treated as a cold, highly-viscous liquid with out much of a problem--so
long as it is realized that this is a simplification..
The terms ‘grain-aggregate’, ‘rock-fragment’, or ‘glass fragment’ can be used for a complex
fragment composed of smaller grains, and the smaller grains referred to as ‘sub-grains’. This
will then deal with the problem of lithic fragments in certain complex materials. Note that
defining a rock as an aggregate of mineral grains makes several mono-mineralic rocks much
easier to define and describe. Various textures now become a matter of defining relationships
between grains.
8
Individual grains are externally bounded by one or more physical-surface boundaries (grain
surfaces). A crystal is just a grain with growth surfaces; hence ‘grain’ is the broader, more
encompassing term. A crystal can then be defined in these terms—a crystal is mineral grain
mostly to completely bound by growth type-1, type-2, and/or type 4-1 surfaces.
MINERALOID
The classification of certain types of natural substances is herein changed and their terminology
modified.
 mineraloid (modified)
o gel-minerals (modified)
o grain-replaced (new)
o crystallized gel (new)
o grain-metamict
o grain-domain containing (new)
GRAIN SURFACES--DETAILS
The surfaces on a grain consist of one or some combination of the following (where the meaning
of the term is new or modified, that will be so indicated):
 growth (redefined, new);
o type-1—crystal surfaces (poor to excellent) (redefined)
o type-2—inclusion-modified growth (new);
o type-3—growth against a preexisting surface (new)
o type-4---growth against another growing surface (new)
o type 4-1 (new)
o type-5---surfaces on non-crystalline substances (new)
 paramorphic (redefined);
 dissolution (modified);
 mechanically modified (new);
o cleavage (redefined)
o parting (redefined)
o fracture (redefined)
o smoothed (new)
 phantom surface (modified)
 artificial (direct human activity only).
The geometry of cleavage, parting, growth type-1, growth type-2, growth type 4-1 and
paramorphic surfaces is controlled by the internal crystal structure, and the surfaces generally are
repeatable in form, location, and angles. Cleavage, parting, and fracture are redefined only to the
extent that they become part of the ‘mechanically modified’ section.
Growth Surfaces (redefined; new)
Growth surfaces are divided into six types—type-1, 2, 3, 4, 4-1, and 5.
Type-1 surfaces
9
Type-1 surfaces are crystal faces and form during growth. They may range between barely
recognizable (poor) and excellent. Several figures (herein) show various crystal faces and
surfaces. In the best cases, these are flat (on an atomic scale), lustrous, planer surfaces which
have specific, relationships to the underling crystal structure. Classical mineralogy, three terms
were devised to describe the variation in perfectness (see discussion)—euhedral (well-developed
crystal faces), subhedral (moderate- to poorly-developed crystal faces), anhedral (lacking crystal
faces). Anhedral grains are generally due to growth-interference, dissolution, and mechanical
modification. The very best crystals (generally) form from gasses (or fluids).
The formation of type-1 surfaces is well recognized in many environments. However in the two
cases of competitive solid-state and melt growth, grains may also develop anhedral to euhedral
grains (see discussion under ‘Type 4-1 Surfaces; Mutual Growth and Relative Bond Strength’).
Even under the best growth conditions, grains develop crystal surfaces of considerably different
‘goodness’. One of the better examples of this phenomenon is spodumene grains from pegmatite
miarolitic cavities; these grains can range from mostly free-growth (with small attachment
surfaces) to restricted growth and can range from excellent crystal surfaces to those of uncertain
origin (Fig. 1).
Figure 1. Spodumene grains (illustrating several types of surfaces) from pegmatite cavities,
purchased, Nuristan, Afghanistan; ‘upper’ and ‘lower’ = surfaces // to page, ‘top’ and ‘bottom’ =
surfaces toward the top and bottom of page; A--single, euhedral crystal (with phantom surfaces)
(complex growth; see Fig. 27); view is ~parallel to the a axis and the c axis is vertical; B--grain
is lying on {110} cleavage (upper, lower, right, left sides are cleavages) and c is vertical; the
large cleavage surface (facing up) under >10X shows growth hillocks which do not significantly
modify the surface; C--the upper surface is striated (alternating {110} surfaces) creating a
surface which is // to {010}, c is vertical; top and bottom of grain are were originally fractures
and are now covered by multiple crystal faces (phantom overgrowth with no visible boundary;
growth type 1) as an overgrowth; color and lack of imperfections make this grain suitable for
gemstone cutting purposes; D--bottom of grain is a fracture surface, remainder are crystal faces;
10
view is ~parallel to a and c is vertical; E--a flat, euhedral crystal resting on a combination of very
small{110} and {010} faces; both top and bottom contain visible phantom overgrowths.
Type-2 surfaces
Type-2 surfaces are growth-surfaces modified by inclusions and are due to growth in an
environment where there is an abundance of inclusions. In some cases, the inclusions are
sufficient to interfere with the free development of the surface. The result is a grain-aggregate,
but the underling control of the sample is the processes of crystal-surface formation. Agreed, this
grades into all three other surfaces, but since it grades into all, it was placed in a separate
category. Crystals that grow in a silicate melt may grow cleanly (producing well developed
crystal surfaces); they may also grow in an environment that is so full of other grains that they
incorporate a considerable number of ‘inclusions’ (>50%). This, of course, results in a complete
gradation; the resulting crystal surfaces may then grade from excellent to not recognizable (Figs.
2, 3). If the growth surfaces have recognizable, roughly planer surfaces that are related to the
Figure 2. K-feldspar, Carlsbad twin, from a rhyolite dike near Jawbone Canyon, Kern Co.,
California. The feldspar grain is partially altered (by surface weathering) and has been artificially
freed from the enclosing rock. Note the euhedral grain surfaces (growth type-1—free growth as a
phenocryst); identity—K-feldspar/growth type-1 (euhedral)/artificial mechanical removal of
rock.
11
Figure 3. Two K-feldspar crystals (phenocrysts) from a granite porphyry in the Sheephole
Mountains, San Bernardino Co., California. The grains have been freed from rock; A-left-Carlsbad Twin; B-right--untwined grain; identity—K-feldspar/growth type-2 (subhedral).
crystal structure of the mineral, it should be classified as growth-type-1 or type-2 (with a
modifier ‘subhedral’, ‘poor’, etc.).
A similar process (but in a different environment) results in ‘sand crystals’. These ‘minerals’
(most commonly calcite, gypsum, halite, barite) typically grow from a water solution in a porous
material (mud, soil, sand). They will form crystal surfaces that range from excellent to those
which are not recognizable (Figs. 4, 5). How and why these growing grains sometimes exclude
12
Figure 4. Gypsum grain from Willow Creek (near Nanton), Alberta, Canada; purchased. The
grain was enclosed in fine-grained, clay-rich mudstone and was freed by soaking in water. Note
the well-developed crystal faces, internal clarity, and lack of inclusions; identity—
gypsum/growth type-1 (euhedral).
Figure 5. The image is of a cluster of gypsum ‘sand crystals’ from Ho-puff (phonetic, actual
name is unknown), Saudi Arabia; gift. Three intergrown grains with abundant sand grains as
inclusions (sub-grains); identity—gypsum/growth type-2.
all or most of the surrounding material during growth or can even grow in such an inclusion-rich
environment poses a problem. Rarely, in inclusion-rich environments, high quality crystal
surfaces (those with no to few inclusions such as in Figure 3) will form; such grains seem to
occur more often during growth in water-saturated, dispersed, clay mud. Perhaps the small
(colloidal?) clay particles have a surface charge which results in repulsion by the unsatisfied
bonds on the growing crystal surface, and hence, the tiny clay particles are kept at a sufficient
distance (a few µm) from the surface so as to not be engulfed.
Type-3 surfaces
13
Type-3 surfaces form where a pre-existing surface controls the surface shape of the growing
grain (grain-aggregate or mineraloid). If growth is against an existing crystal (or a mold of the
crystal surfaces), then flat surfaces may result. This type of surface is one which may have planer
elements or geometric shapes (Fig. 6); as such, it may be confused with planer or geometric
Figure 6. Mold of garnet crystal faces in a hydrothermal quartz aggregate of grains; sample is
from the Tungsten King mine dump, near Havilah, Kern Co., California; identity—quartz
(aggregate)/growth type-3.
surfaces controlled by the mineral structure and is specifically described to avoid confusion.
Type-4 surfaces
Type-4 surfaces are surfaces developed as two or more growing grains (having nearly the same
hardness; see following section ‘Type 4-1 surfaces; mutual growth and relative bond strength’)
compete for the available atoms (or groups of atoms) and space. Such grains commonly
preferentially separate at such boundaries when fractured. This type of surface most commonly
occurs during the final solidification of igneous rocks (Fig. 7) and during the formation of some
metamorphic rocks.
14
Figure 7. Tourmaline grain from the Southern Pacific Silica Quarry pegmatite body, Riverside
Co., California. The grain illustrates two types of growth surfaces—the mid-grain, parallel
grooves result from growing against preexisting tourmaline grains (growth type-3); the
somewhat rounded surface extending from the bottom to top left developed by simultaneous
growth against another growing grain (growth type-4), probably tourmaline; identity—
tourmaline/growth type-3 and type-4/fractures (unknown origin).
Type-4 surfaces, then, are the favored boundaries for separation of grains in mineral aggregates
making up certain igneous, metamorphic, and hydrothermal rocks. This would also be true for
some non-detrital sedimentary rocks, except that the common minerals in such rock (calcite,
halite, gypsum) have such an easy and well-developed cleavage. However, under careful
examination, growth-interference surfaces may be found on grains in coarsely crushed samples
of these rocks.
Growth during exsolution (such as a perthite) is included under growth-type-4. Note that
perthitic growth is, at least, partially controlled by bonding. Recognizing this type of surface
feature can be difficult; note however, that the environment and nature of both the host and the
exsolved mineral will be indicative (see figure 18, following).
The growing grains on both sides of most type-4 surfaces or the growing grain on one side of a
type-3 contact surface, will have a random crystallographic orientation (with reference to the
surface). As a result, atomic level attractive forces will be weak to absent (perhaps even weakly
repulsive) and the main thing holding the grains together is the mechanical interlocking of
irregular surfaces. Such interlocking surfaces are, in many instances, weaker than the bonding in
any of the adjacent grains, and will in all instances be as weak as the weakest mineral involved.
These surfaces will typically be randomly oriented and non-repeatable; they will, only by
chance, have some relationship to the structure of the minerals involved. It may turn out,
however, that the relative difference in bond strength will influence the quality of such a grain
surface, and they (see ‘type 4-1 surfaces’ following) will approach or grade into a growth-type-1
surface.
15
Type 4-1 surfaces; mutual growth and relative bond strength
An approximate measure of bond strength for mineral grains is available in any mineralogy book
as the Mohs hardness. Note that this is only an approximation because bond strengths on
different crystal surfaces or in different crystallographic directions are slightly to significantly
different (for example, kyanite H = 5 one direction, H = 7 a different direction; see the following
Figure 8 and discussion). It is herein proposed that crystal growth surfaces develop on grains
during mutual growth in metamorphic and igneous environments when there is a difference in
hardness of approximately >2 (refer to this approximate difference as ‘ΔH’).
Note, that in this section, there is a problem in terminology—i.e., there are crystal growth
surfaces developing on grains with mutual, competing growth surfaces and these are referred to
as ‘growth-type-4’; these grains may rival growth-type-1 surfaces in ‘goodness’. Herein, this
problem will be dealt with in the following way:
 in known, understood environments for anhedral grains use ‘growth-type-4’; it will be
understood that ΔH < 2 unless otherwise stated;
 in known, understood igneous and metamorphic environments for euhedral grains use
‘growth-type-4-1, euhedral (good, poor, etc.)’; it will be understood that ΔH > 2;
 in unknown situations use ‘growth-type-?, (euhedral, subhedral, anhedral, etc.);
 a particular type of type 4-1 surfaces are somewhat of a problem to classify—mineral
grains with a particular ‘growth habit’ such as tourmaline and α-quartz (elongate growth
habit). These types of grains will be classified as type 4-1 (anhedral); see Figures 11, 12,
13.
In igneous rocks, then, there is a difference between phenocrysts and growth type 4-1 grains.
Both may have good to poor crystal surfaces, but they have formed for different reasons, at
different times, and by different processes.
Type 4-1 surfaces; metamorphic environments
Metamorphic crystallization occurs in the solid state (although fluids are an important factor).
However, grains in such rocks do grade from those with surfaces that are highly irregular (and
obviously are not crystal faces) to those that rival the best of type-1 surfaces (see figures 8, 9,
10). The best (of these types of) grains occur in tactites (calcite with enclosed silicate grains;
note that some petrologists use the term ‘skarn’ for such rocks); these are also easiest to free
from the enclosing rock (by solution with acid). Cataclastic rocks seldom produce other than
fragmented grains, but as cataclastic rocks grade into migmatites, the growth of grains becomes
an important process. There exists an extensive literature on porphyroblastic textures and
‘crystallization force’, but this usually deals with non-carbonate rocks (Misch, 1971 and
discussion with replies; also see Yardley, 1974). It is suggested that type 4-1 processes are
responsible.
Two additional factors have some influence in the metamorphic environment—time and fluids.
These affect the diffusion of atoms or molecules through the rock. The longer the time the rock is
held at the temperature and pressure of growth, the more diffusion can occur. The presence of
fluids tends to increase both transportation and diffusion. It appears that the coarser-grained
tactites (approximately cm-grained) tend (?) to have better type-1 surfaces.
16
In certain metamorphic rocks (tactites), grains with ΔH <2 typically develop growth-type-4
surfaces. However, when ΔH >2 the harder grain develops a crystal surface of lesser or greater
‘goodness’ at the expense of the softer grain. This is best illustrated by the tactites where calcite
(H = 3) competes with pyroxene (group), humite (group) or periclase (H = 5 – 7; ΔH = 2 to 4) ,
and the minerals can be easily separated by solution in acid.
In a mm-grained tactite, where minerals of the humite-group (hardness 6) are growing in (and
competing with) calcite (hardness 3) and periclase (hardness 6), certain directions of the grains
of the humite-group minerals tend to have subhedral forms (Fig. 8). Note that both calcite and
17
Figure 8. Humite-group grains (red) with periclase (white, partly dissolved) grains; sample is
acid etched from tactite from the Jensen Quarry, Riverside, Co., California. Note that the humite
grains have some crystal faces (growth vs. calcite; growth type 4-1) and some rounded surfaces
(? growth vs. periclase; growth type-4); sample is magnified and the box containing the sample is
2 cm square; identity—humite/growth type 4-1 (euhedral and subhedral) and periclase/growth
type-4 and acid etching (artificial; rounding) and calcite/acid solution/removed (artificial).
Samples like these are viewed at 15X with a stereomicroscope.
periclase are soluble in acid, but periclase is not as quickly dissolved and tends to remain as
somewhat rounded grains. Humite-group grains commonly have some surfaces slightly rounded
(‘melted’ in appearance). Note that in these samples, it could not be determined if the rounding
was due to contact with periclase (ΔH>2, now dissolved). However, the humite-group minerals
were always in intimate contact with periclase at this locality (Jensen Quarry) in all the samples
prepared and observed (approximately 10).
If (as suggested) the degree of development of the faces is controlled by the relative bond
strength, then some minerals will have a different degree of development of certain crystal faces.
This phenomenon is particularly illustrated by the graphite grains found in some of the tactite
(generally mm-grained) in and near Riverside, California. The calcite-rich parts of the tactites
have been extensively quarried for cement production and are well exposed in numerous
abandoned quarries. The surfaces of the graphite grains enclosed in calcite have a remarkable
difference in development of the hexagonal-prism faces (m)-{10ī0} (anhedral; the grains are
typically oval) as opposed to basal pinacoid faces (c)-{0001} bright, shinny, euhedral) (Fig. 9).
18
Figure 9. Graphite and ? periclase from the Jensen Quarry, Riverside, Co., California. The
scattered, smaller chalk-white grains are probably remnants of periclase. The graphite grains are
flakes with euhedral {0001} surfaces and anhedral surfaces where {10ī0} would occur. The
grains are deformed and consist of single grains, and clusters which may contain 10+ grains;
identity—graphite (growth type 4-1 and 4)/calcite and periclase/artificial acid solution. These
samples are best viewed at 15X with a stereomicroscope.
The bonds along the prism are weak Van der Waals bonds where as the bonds in the basalpinacoid faces are covalent and approach the strength of the bonds in diamond (see graphite in
most any modern mineralogy book).
In the case of tactites with large silicate grains (better/longer-time diffusion; factor 2) the grains
tend (?) to have better developed growth surfaces (Fig. 10).
19
Figure 10. Diopside grain (with an attached mica grain, on left side,) from Dog Lake, Ontario,
Canada; purchased. Sample is probably from the Grenville Marble which is well known for
diopside-hedenbergite grains (Sinkankas, 1966, p. 492-493); identity—diopside (euhedral),
growth type 4-1 and (probable) calcite/chemical solution (acid removed; artificial).
Type 4-1 surfaces; igneous environments
In most igneous rocks (those composed predominately of silicates), the hardness of the typical
minerals fall between 6 and 8 (hence ΔH<2). Note, however, than some minerals have ‘growth
habits’ which are the result of differing bond strengths in different directions (i.e. the elongate
habits of some minerals). As a result, the grains in most igneous rocks have type-4 surfaces with
some minerals having elongate growth habits (such as, quartz and tourmaline—see Figures 11,
12).
In some granitic, igneous rocks (particularly in pegmatite bodies), graphic-granite occurs and
makes an excellent example of type-4 growth (plus elongate habit). This is a poly-mineralic
aggregate that is composed of quartz and K-feldspar grains (ΔH<2) growing at the same time and
in the same direction. In the miarolitic cavities of pegmatite bodies, because temperatures
probably are less than 573o C (see Chap. 15 in London, 2008), the quartz is α-quartz (the lowquartz paramorph). This paramorph is trigonal and elongate in form. In the mixture of minerals,
the quartz (which is slightly harder and has an elongate habit) forms tapering grains with steplike growth-type-4 surfaces on the sides (Fig. 11A) and a crude trigonal outline (Fig. 11B). This
20
Figure 11A.
Figure 11B. Two views of a single sample of graphic granite (simultaneous growth of quartz and
K-feldspar) from the Southern Pacific Silica Quarry, Riverside Co., California. (A) The view is
perpendicular to the growth of the quartz (grey) and feldspar (white) (growth type-4); (B) The
view is parallel to the growth and of the right side of ‘A’. This surface has been cut and polished
and shows the trigonal habit of the quartz (grey); identity—quartz and K-feldspar (rock; graphic
granite)/growth type-4 (4-1?)/artificial cut and polished sample.
21
paragraph prompts two question concerning graphic-granite: 1) does this pattern also occur in
graphic-granite formed at >573o C; 2) does (classic) graphic-granite form only at less than 573o
C? Note that the hardness of β-quartz does not appear to have been measured (above 573o C) but
it is suspected that (because of lower density) it is slightly less than α-quartz in hardness.
Tourmaline does have a similar elongate habit (Fig. 12) in some types of pegmatite rocks.
Figure 12.Medium grained pegmatite rock showing tapering growth of tourmaline (larger in
direction of growth), Lakeview Mountains, Riverside Co., California; identity—tourmaline in a
quartz + feldspar pegmatite rock/growth type-4 (4-1?)/mechanical disruption (artificial).
Carbonatites are both rare and small-volume igneous rocks. But, they serve as examples; i.e. they
commonly are composed of minerals of differing bond strengths with ΔH = 1 to 3 (commonly
calcite, dolomite and/or ankerite vs. parisite, apatite, pyrochlore, perovskite, or magnetite). Note
that other minerals are also present, but tend to be subject to alteration (see for example, HornigKjarsgaard, 1998). Many of the articles on carbonatites do not discuss the nature of the grains,
but two articles (Hornig-Kjarsgaard, 1998; Gaspar and Wyllie, 1983) do. Hornig-Kjarsgaard
(1998) describes the calcite grain relationships as “equant, xenomorphic” and the apatite,
perovskite, and pyrochlore as euhedral. Gaspar and Wyllie (1983) describe magnetite as
nucleating throughout the solidification of the carbonatites (p. 205 and 208) and as being
anhedral to perfectly octahedral (p. 197-198) and, in some cases, being rounded by resorption.
In a carbonatite and in the case where the two minerals have different growth habits and nearly
the same hardness such as parisite (elongate habit; probably slightly stronger bonds in the
elongate direction) growing with dolomite-ankerite (ΔH 0 to 1), the parisite will develop
elongate grains, larger in the growth direction, and bounded by type-4 surfaces (Fig. 13); note
22
Figure 13. Parisite (tapering grain chipped free of enclosing carbonatite); grain is larger in
direction of growth. The parisite was enclosed in dolomite-ankerite; Snow Bird claim, Lolo Pass,
Montana; identity—parisite in carbonatite/growth type 4 (4-1?) /artificially partially separated
grain.
the resemblance to the growth-habit for tourmaline in pegmatite rock (see Figure 12).
Type-5 surfaces
Chert, chalcedony, and opal can originate in a variety of complex ways (see for example the
sections on chert in Blatt and Tracy, 1996 and Twenhofel, 1939); this may involve precipitation
as one or a mixture of colloids, gels, minuet fibrous grains, inorganic and organic opaline
masses; the later nucleation and/or growth of sub-grains makes these materials even more messy.
Irrespective of the exact process of formation, these materials, if not originally colloids or gels,
tend to be extremely fine grained, and many contain opal or chalcedony. The resulting surfaces,
then, tend to be smooth, slightly- to highly-irregular surfaces. In the absence of detailed study of
these materials and for the purpose of surface studies, it is suggested that they be considered as
originally gel-like. So the term ‘gel-like’ herein is an ill-defined, catchall term; in one sense, a
place-holder while further and detailed study is pursued.
Gel-like and former gel surfaces tend to be smoothly irregular to regular (but not planar). Once
a gel crystallizes, it most commonly is composed of tiny, fibrous grains. The fractures of former
gel-like substances tend to be controlled by the grain-size of the material, and, where the
23
substance is micro-grained, fractures are curved and smooth (for example, as in a chert). In such
materials, the fracture is controlled by the grain size—the smaller the grains the smoother and
more conchoidal the fracture; the larger the grains, the more the nature of the mineral controls
the surfaces resulting from mechanical modification.
Paramorphic-surfaces (redefined)
Some minerals crystallize and form crystals with characteristic structures, shapes, and surfaces
under one set of physical conditions only to undergo a (minor) change in the structure as a result
of change to different physical conditions. The new structure is not reflected by a corresponding
change in the shape and surfaces of the crystal. Many members of polymorphic sets have a
sufficiently different internal structure so that there is a resistance to change, and the structure
may continue to exist under conditions where it is not stable (i.e., it is metastable).
The two polymorphs of quartz are a common and a good example of a paramorphic set of
surfaces which do not change in spite of a change in the internal structure. This involves the βquartz (high-quartz) to α-quartz (low-quartz) structural change. The difference in the structures is
minor and involves only a small shift in bond angles (a degree or so), and it apparently does not
require the breaking and reforming of bonds (for more detail see Heaney and Veblem, 1991).
Much of the mineral quartz (in silica-rich igneous rocks) forms at temperatures > 573oC (one
atmosphere of pressure; note that there is a small composition and pressure effect). When βquartz crystals (typically hexagonal dipyramids in form; Fig. 14) cool through ~573oC, the
Figure 14. Quartz phenocrysts from the Bishop Tuff, east side of Sherwin Grade near scenic
pullout. These grains show a range of rounding from grains with all corners and edges rounded
24
to grains with nearly complete euhedral form; as well many grains are etched and a few have
almost pristine surfaces; fractures are common; identity—phenocrysts from rhyolite
tuff/paramorph, α-quartz after β-quartz/some fracturing/artificial separation by panning. Grains
like these are best viewed at 15X with a stereomicroscope.
structure readily changes from hexagonal to trigonal without any corresponding and obvious
change in the crystal form; some twinning in α-quartz may be due to the change, but this is not
observed on the external form of the grain. Note that for samples commonly labeled β-quartz in
collections, the crystal form and faces are those of β-quartz, but the internal structure is that of αquartz; the structure compared to the crystal form and crystal surfaces do not match. Note also
that the hexagonal-dipyramid crystal form is suggestive (not definitive) of β-quartz (see Frondel,
1944, p. 119). Such samples are sometimes labeled as ‘pseudomorphs’. However, this particular
process is distinct and should be labeled (for example) ‘α-quartz paramorphic after β-quartz’.
The feldspars pose similar concerns (concerning paramorphs and paramorphic surfaces). Deer
and others (1992, p. 407 and 411) note that twinning occurs on thermal transformation to lower
symmetry (they do not use the term ‘paramorph’); it is suggested, then, that most (?) twinned
plagioclase and K-feldspar crystals are paramorphic grains. The fine-scale, multi-twinning
characteristic of these two minerals has resulted as an accommodation of the structures to the
temperature-caused change from monoclinic (high-temperature form, initial crystal) to triclinic
(low-temperature form, derived crystal). Deer and others (1992, p. 405) note that triclinic
feldspars do not deviate greatly from monoclinic symmetry (generally <3o on any axis; see also
relevant sections in Smith and Brown, 1988).
Dissolution (s.l.) (modified)
Dissolution must, of necessity, happen to preexisting surfaces. This then becomes a two-part
problem; 1) recognizing a dissolution surface as such, and 2) (in some cases) determining the
nature of the preexisting surface.
Various factors control the rate of dissolution (concentration of selected materials in a melt or
solution, amount of excess heat, viscosity of the fluids, density of the fluids, etc.), and the rate of
dissolution appears to then control the nature of dissolution. Experiments with artificial systems
show that rapid dissolution tends to produce rounded grains, and slow dissolution tends to
produce dissolution-facets (Donaldson, 1985a, 1985b). Two problems, however: 1) dissolution in
some natural systems can extend over geologic time which is orders of magnitude longer than
experimental times; 2) only in closely monitored artificial systems can it be told for certain that a
faceted grain lost mass. Note that because of the great time difference between artificial and
natural systems, that dissolution facets may be rare in natural systems.
Features of melt surfaces
Definite melt surfaces may be difficult to differentiate from chemical reaction dissolution
surfaces in natural systems which are only observed after the event (most times long after the
event). However, melt surfaces observed for this study appear to be lustrously smooth and
rounded—edges and corners are modified first. This, like many surfaces described herein, may
25
require modification to suggestions in this paper as different melt surfaces are examined over the
next several years. An inference can sometimes be made based on the chemistry of the grain and
the surrounding melt. If the grains would normally be in equilibrium with the likely melt
composition and the temperature is likely to be above the liquidus, it is probably melting. In the
case of the grain in Figure 15, the grain occurs as a xenocryst in basaltic tephra at Dish Hill
Figure 15. Basaltic lava with a single grain of mantle derived kaersutite that has been subject to
melting. Sample is from Dish Hill (a basaltic cinder cone), San Bernardino Co., California (see
text for more detailed location). Two cleavage surfaces survive on the grain: 1) the top side has a
fresh, clean amphibole cleavage surface; 2) on the left top side is a companion cleavage which
has suffered minor melting (or solution); the top cleavage surface happened during or after
extrusion; the side cleavage surface happened before extrusion. The image has been significantly
adjusted to particularly show the melt surfaces; identity—kaersutite (as a foreign, included grain
in basaltic lava)/series of unknown events including melting/cleavage/some solution or
melting/eruption and cleavage surface/artificial partial separation of grain by mechanical means.
crater, San Bernardino Co., CA (34.613o N Lat., 115.945o W Long.; also see Wilshire and
Trask, 1971). It is unlikely that the kaersutite is in equilibrium with the alkali-basalt magma and
it is likely that the temperature of the magma is above the melting point of the gain; perhaps both
melting and chemical reaction have occurred. These grains and their surfaces probably are
preserved only because of rapid cooling of the magma due to rapid extrusion.
Features of a chemical solution surface
26
The current usage of the term ‘solution’ implies water solution, but that is only a special case of
dissolution in general. In the usage herein, ‘dissolution’ will include water and chemical solution
but only as particular types of dissolution.
The dissolution surfaces (Fig. 16) shown here are artificial and may be faceted dissolution, but it
Figure 16. This grain was a rhombohedral cleavage grain of calcite (39.9g) of an unknown
commercial source. The image is the same grain after being partially dissolved (now 10.0g; 26%
of the original weight) while suspended in weak, dilute HCl (~1N); note that the grain has
retained an approximate shape of the original grain although the surfaces are slightly irregular;
identity—calcite/cleavage surfaces/artificial solution.
serves to illustrate the nature of the surfaces may result from any number of geologic (or
artificial) processes.
Dissolution in mixtures of minerals may be very specific—affecting only certain of the minerals
in intimate contact. This is illustrated in two samples from miarolitic cavities in two different
pegmatites: sample one--quartz has dissolved and the adjoining cleavelandite-plagioclase is
totally unaffected—not even etched (Fig. 17); the identity of the dissolved mineral is not certain,
27
Figure 17. Cleavelandite (multiple, bladed-grains of albite plagioclase in radiating aggregates)
with solution pits (formerly occupied by quartz(?)). One is at the top, one at the right, and one at
the bottom, cut surface. The sample is from a miarolitic cavity opened and excavated in 1991;
White Queen Mine and pegmatite body, Pala Pegmatite District, San Diego Co., California;
identity—cleavelandite and quartz(?) growth type-4 and -1/dissolution quartz(?)/series of
unknown events.
but the shape of the cavities, the expected minerals at this stage of pegmatite development, and
the presence of quartz crystals in samples from nearby miarolitic cavities all indicate that is was
quartz; sample two—K-feldspar has dissolved leaving ribs of albite which were perthitic
exsolution grains in the K-feldspar as well as a slightly different K-feldspar (Fig. 18). Note,
‘perthite’ now becomes a grain-aggregate with sub-grains (formed by growth-type-4). The term
describes a certain (exsolution) texture and is not a mineral name.
Figure 18. Perthitic feldspar that has been subject to solution; sample was collected from the
mine dump of the Ocean View Mine and pegmatite body, Pala Pegmatite District, San Diego
28
Co., California. Most likely, this sample came from a miarolitic cavity excavated in ~1991.
Preliminary powder-x-ray studies indicate two generations of K-feldspar; the first-generation Kfeldspar is an intermediate microcline and makes up the interior of the grain; the secondgeneration (cleavage is continuous with the first-generation) consists of domains (?) of
maximum-microcline + orthoclase. The ribs (<%5 of the sample) are thin, exsolution lamina of
albite; the ribs continue without interruption from the overgrowth surface into the interior of the
grain. Note that solution appears to have occurred after both generations crystallized and has
specifically affected the first-generation but not the second-generation K-feldspar.
Features of dissolution surfaces on quartz phenocrysts in rhyolite
In the course of conducting this study, several samples of crystal tuffs were examined because
they are common in western U.S. and because they can be excellent sources of mineral grains.
Crystal tuffs examined:
1. Bishop Tuff; several road-cuts on Shirwin Grade just north of Bishop, CA; collected
1981, 1989, 2010;
2. a tuff; road-cut near Bandelero, NM; collected 1988;
3. Island Park Caldera tuff; Yellowstone Nat. Park; collected 1968 under permit (NSF
fieldtrip);
4. a tuff; road-cut just north of Jordan Valley, OR; collected 2010
5. a tuff; road-cut near Ashton, ID, collected 1968 and 1976
6. Mazama Tuff; road-cut just northeast of Crater Lake Nat. Park, collected 2010; not a
rhyolite;
7. Sugar Loaf Dacite; near Hover Dam; collected 1991; not a rhyolite.
All five of the five rhyolitic crystal tuffs, have a common feature (see Figure 14)—the quartz
had:
 a range of sizes, typically 1 mm-1 cm;
 a range of degree of rounding; in some cases from completely to only slightly
rounded;
 etched and/or pitted surfaces (in two cases, melted).
The feldspar grains had similar surfaces, but the quartz was easier to work with, and hence the
quartz grains are the ones described herein.
Similar grains were also observed as a part of a rhyolitic dike in eastern Kern Co. CA (Fig. 19).
29
Figure 19. This image is of quartz paramorphic grains (α-quartz paramorphic after β-quartz)
from a rhyolitic dike intersecting Water Canyon, Kern Co., California and collected in 2003.
Weathering has freed the quartz grains; most of the grains are fractured by post-crystallization
events, and the grains shown here represent ~1% of the grains on the ground surface. All grains
are etched, and rounding of the grains ranges from slight to almost complete; identity—α-quartz
paramorphic after β-quartz, growth type-1/solution to various degrees by crystal settling
(?)/emplacement of melt+grains as a dike/solidification of rock/paramorphic change to αquartz/fracturing/weathering to free grains.
There are minor differences between the different rhyolite crystal tuffs, but the features on the
quartz grains are basically the same. This will be discussed later in the second of ‘Two
Examples’.
Recognizing dissolution surfaces
The general recognition of dissolution surfaces is somewhat uncertain; different minerals in
different environments may produce different dissolution surfaces. The nature of dissolution
surfaces tends to be mineral specific; recognition of such surfaces, then, requires the examination
of several different grains of this particular mineral from different localities. For example, the
presence of a well-developed and intersecting cleavage appears (on dissolution and chemical
alteration) to produce a splinter-like surface; this was observed for some crystals of feldspar (in
the crystal tuffs) and for some large spodumene crystals from the Etta Mine (see Figure 25). It is
not presently known if splinter-like surfaces result primarily from dissolution of surfaces with
cleavage intersection or not, but it appears to be the case.
However, all of the following features have been observed (singly and in combination) on grains
in environments consistent with solution; note that it is much preferred that multiple grains are
observed to have the feature and/or that more than one type of feature is present. The following
are probabilities and suggestions:
30

Irregular solution pits (size, shape, location)—pits which have a range of sizes, a
range of shape, and which have various locations on a crystal face are due to
dissolution. Both the cause and location these features are probably minor flaws in the
atomic structure on that growth surface. A single pit which is centered and has a
negative crystal shape is most likely due to growth.
 Etched surface (frosted surface)—the entire or the exposed surface is covered by
small scale irregularities (not resolvable at <10X magnification); this resembles the
appearance of a surface that has been sand-blasted or coarsely ground. A
mechanically frosted surface can generally be distinguished by the environment in
which the grain is located—such as a rounded and frosted grain in a sedimentary
environment.
 Splintery surface—a rough, splintery surface is consistent with and believed to be due
to the intersection of cleavage(s) with the surface.
 Melted surface—(only three such grains have been observed) a smooth, glossy,
enveloping surface believed to be due to and consistent with a temperature above the
melting point of the mineral in that given environment.
 Rounding of edges, corners, and points—this rounding may occur in association with
flat, apparently unmodified growth surfaces; note that at edges, corners, and points,
there are (?usually) more unsatisfied bonds available than on a flat surface, therefore
edges, corners, and points are preferentially subject to dissolution.
Alteration of the grain (accompanying the above features) can be an additional feature.
Alteration may only represent dissolution where the reactants are either non-soluble or are not
removed from the grain surface.
Some experimental work needs to be done on the nature of the 3D surfaces of mineral grains at
slow dissolution rates (reference--faceted dissolution). Variation of surfaces on quartz grains
from single samples of tuffs when ~50 grains are examined is common. If the nature of the
grains surfaces is to be inferred from standard thin-section examination, multiple thin sections
(showing at least 30+ grains needs to be utilized.
Mechanically modified (new)
Note that ‘cleaved’, ‘fractured’, ‘ground’, ‘scratched’, ‘grooved’, ‘frosted’, and ‘polished’ only
indicate the details. This not only applies to the individual grains, but also to the grains in an
aggregate) for example; modified surfaces may occur as:
 surfaces created by glacial action (glacial polish, glacial striations, etc.);
 smoothed surfaces along faults (slickenside) (Fig. 20);
31
Figure 20AB. Samples of slickenside--light reflects from the polished surfaces of the grains. (Aleft) A quartz aggregate from a mine dump, Skidoo Mining District, Inyo Co., California with
fault polished grain surfaces. (B-right) An aggregate (rock) of serpentine from a road-cut
exposure, just west and near San Luis Obispo, California (a cold intrusion).

tumble smoothed (to polished, rarely) grains or rock aggregates in sedimentary
environments (Fig. 21);
Figure 21. Four grains of garnet from Emerald Creek, Benewah Co., Idaho. These are
sedimentary grains derived from a nearby outcrop of garnet-bearing schist; they grade from non32
smoothed on the left to broken and artificially polished on the right. These are selected from
approximately 100 grains and less than 1% of the grains retained unmodified crystal shapes or
surfaces.
 (rarely) tumble smoothed to polished mineral grains from gas-pressure ruptured
miarolitic cavities in pegmatites (such material occurred in ruptured, miarolitic cavities in
the Little Three pegmatite body, Ramona, CA (Gene Foord and Louis Spaulding,
personal communication; L. Spaulding provided samples for examination; also see
London, 2008, p. 300, Fig. 17-21);
 cleavage, parting, or fractures; such surfaces represent a common result on both natural
and artificial mechanically modified grains and are well described and understood.
Some surfaces termed fractures (in the past) may really have been due to growth (type-3 or -4)
surfaces. These are excluded from ‘fracture’ and put under the respective type of growth. In the
case of uncertainty as to the origin, use ‘fracture (?)’ or ‘type-3(?)’. Cleavage, parting, and
fracture (the terms) are modified to the extent that they are now a detail under ‘Mechanically
modified’.
Phantom Surface (modified)
This type of surface is somewhat complex and may be created by one or more overgrowths on
any of the types of surfaces. The phantom surface is always overgrown and hence is ‘buried’.
Such surfaces are many times not readily visible except in ‘clear’ grains.
The most commonly occurring and readily visible of such surfaces are phantom crystals (see
Figure 1, sample E; also see for example Sinkankas, 1966, p. 75-76). Less common would be the
surface beneath an epitaxial overgrowth.
Artificial
Artificial surfaces are those resulting from any direct human activity (for example, in Figure 22
(the polished grain). Striking a sample to create a surface to aid identification is a common and
useful activity. There may be a practical difficulty of determining if the surface was created by
human activity or by natural events—such as blasting while mining in a fault zone and such as
collecting samples from a bulldozer created rubble pile. In cases of uncertainty, a ‘?’ can be
used—‘? artificial’.
Mineraloid (modified)
The term ‘mineraloid’ is modified, made a general term, and expanded to include several
difficult to classify substances. In current mineralogy usage, this term is applied to gel-minerals
(such as opal). In standard English usage, the suffix ‘oid’ is used to modify a word or make a
new word. The meaning of this suffix is ‘having the form of’, ‘like’ (Onions, 1964), and
‘resembling’ (Thatcher, 1952). So, the term ‘mineraloid’ (herein) will consist of substances
which have the form of, resemble, or are like a mineral grain. The term will be subdivided into
the five following categories.
33
Gel-mineral (modified)
This term currently is an alternative to ‘mineraloid’ but will be restricted and used for opal and
other mineral-like substances which are gels or gel-like. Such substances lack long-term internal
structure or order but may have short-term order.
Grain-replaced (new)
A part (or the total) of a grain may be chemically altered without great change in the general
shape of the original surface. The replacing mineral is typically in the form of a fine aggregate.
This most commonly happens between closely-related mineral species such as azurite-malachite
(see Figure 22 following), borax-tincalconite, laumontite-leonhardite. Detailed examination of
the replacing material will commonly reveal growth-interference surfaces between the verysmall, individual grains.
The term ‘pseudomorph’ has been used when the replacing grain(s) mimics the original
crystal form, but this term did not apply to irregular grains. Chemical-replacement (herein) will
be applied to aggregates of all altered and replaced grains and ‘pseudomorph’ will be reserved
for those samples that mimic the original crystal (if one existed). A compound phrase will be
used for pseudomorphs like these (see Figures 22, 23, 24), i.e. ‘ulexite after borax’-- meaning the
ulexite mimics the shape of the original borax crystals).
There are cases where the process or processes of formation can probably be determined (such
as):
 atom for atom replacement or substitution (with formation of multiple grains);
 replacement (partial to complete), addition, or loss of a certain atom within the original
structure (with formation of multiple grains):
 in-situ change of the original grain to an aggregate-of-grains of the same mineral;
 chemical reaction between the original (and free-standing) mineral grain and a gas,
fluid, or melt resulting in an aggregate of grains of a different composition;
 other similar processes and combinations (of the foregoing); Fig. 22;
 partial to complete solution combined with new growth within a mold (of the original
mineral grain); Fig. 23;
 the oxidation state of a certain atom has been changed with the formation of grains or
domains (or perhaps without the formation of domains or grains).
34
Figure 22. Pseudomorph of malachite after azurite, probably from the Bisbee Mining District,
Arizona (purchased). The azurite was probably a free-standing crystal in a vug in a hydrothermal
rock; note that the faces are obvious but rough; identity—malachite (aggregate) pseudomorphous
after azurite/mineraloid/unknown number of events.
Figure 23. Pseudomorph of ulexite after borax from the mine dump of the U. S. Borax Mine at
Boron, California; sample is from the mudstone of Kramer Lake; isolated samples occur here
consisting of molds in the mudstone incompletely filled with a mass of needle-like grains of
ulexite; identity—cluster of borax grains, growth type-1, within lake mud/lithification of the
mud/alteration of borax to an aggregate of ulexite grains, growth type4/pseudomorph/mechanical removal (open-pit mining)/mechanical removal of mudstone.
35
Mineralogists have always been (and continue to be) particularly fascinated by pseudomorphs.
However, the process involved in the formation of such pseudomorphs is not always unequivocal
or easily determined. It is suggested that such substances be noted as ‘pseudomorph or chemicalreplacement’ and further noted as to the process or processes of formation (when known). Note
that rarely glass may be the replacing material in the case of shock due to meteorite impact or
atomic blast (Smith and Brown, 1988, p. 187-189).
Replacement of a grain may be partial; if this is the case, a surface on the original grain may
exist beneath the replacing material and on the initial grain (Fig. 24). Might this be a type of
Figure 24. Spodumene grain from the Etta Mine, Black Hills, South Dakota. This sample is a
fragment of a larger (~5X) crystal; main surface (euhedral, growth type-1, probably {110}) and
left surface (subhedral, growth type-1, probably {011}); these surfaces are crystal surfaces. The
lower surface consists of irregular cleavage {110} surfaces; surface to the right is an artificially
cut surface. Note the slightly darker, irregular patches on the main crystal face—this is attached
alteration product (clay). The main surface on this grain is one of the smoothest at this locality;
on many of the grains, the {110} surfaces were splintery.
dissolution surface where the alteration products remain? Note that Schwartz (1937) describes
the alteration of spodumene to clay and implies that this is common for the newly exposed
spodumene crystals. The samples collected (Fig. 24) had been exposed for several years; hence
the clay, if it had existed on these grains, could have been washed away by rainfall.
Crystallized gel (new)
This material started as a gel. Then, after deposition, it nucleated and crystallized into an
aggregate (typically) of small grains. Many samples of chert would be so classified.
It is herein suggested that ‘core-quartz’ which is present in many pegmatite miarolitic cavities
was originally emplaced as a silica gel. Note that London, 2008 (p. 286-288) does not favor this
model and does not include it in his four possibilities, but several persons working on pegmatites
are discussing this as a model (for example, Taylor, 2006). The crystallization of this material
36
resulted in an aggregate of quartz grains with a grain size (maximum dimension) ranging
between approximately 5 and 20 cm (larger and smaller grains have been observed). Because of
the large grain size, many of the surfaces formed by mechanical modification of the core quartz
are dominated by one of the poor cleavages in quartz—probably {10ī1}.
Grain-metamict.
Such grains have had the structure destroyed by radioactivity and are amorphous. Some of these
grains are pseudomorphs in that the internal structure is destroyed, but the original crystal growth
surfaces survive (Fig. 25). Such grains are rare (usually because of the rare and radioactive
Figure 25. Metamict grain (amorphous, in pegmatite rock) pseudomorphous after allanite from
an undocumented pegmatite body (probably Precambrian in age) in Pacoima Canyon, Los
Angles Co., California. Grain is identified as metamict allanite from the pegmatite occurrence,
blade-like form, crumbly and pitchy nature, lack of a {100} cleavage, alteration of adjoining
rock, and occurrence of reported allanite in pegmatite bodies in the area; identification—allanite
growth type-1/alteration (conversion to glass by decay particles)/mineraloid-metamict
allanite/pseudomorph.
elements involved) but, in terms of the abundance of the original mineral-crystals, this is
probably the single most abundant type of pseudomorph.
Grain-domain Containing (new)
These are currently only known and described for some of the plagioclase feldspars; they consist
of formerly homogeneous grains (at high temperatures) which have exsolved at lower
temperatures into ‘domains’ where the slightly different structures and compositions occupy
poorly-bounded and difficult to delimit volumes (peristerite, Bøggild, and Hüttenlocher
37
intergrowths). The ‘play of colors’ ‘iridescence’ typical of certain labradorite samples is due to
Bøggild intergrowths; the play of colors results from the diffraction of light on passing from one
composition and structural domain to another. It is suspected that with further and closer studies,
other minerals will be found to have such features.
THREE EXAMPLES
This section contains two examples of how this information about mineral grains may be used to
develop ideas about the history of formation of a mineral or rock sample.
Pegmatite-Example 1
Figure 26 shows a sample from a pegmatite body. The K-feldspar and the tourmaline both
Figure 26. The image is of a pegmatite rock consisting of K-feldspar (Kf), quartz (Qz), and a
single tourmaline grain (black) from an undocumented pegmatite body in the Lakeview
Mountains, Riverside Co., California. See text for description.
competed for space (growth type-4) but the tourmaline outgrew the K-feldspar and developed
crystal surfaces (growth type-1) into a cavity filled either with fluid or melt. The feldspar crystal
surfaces were probably modified by reaction with the fluid. The quartz was emplaced (?) around
the existing crystal surfaces (growth type-3), and it now consists of an aggregate of centimetersized grains of quartz with growth type-4 surfaces. It is suggested that the quartz was initially
emplaced as a gel and then crystallized. The later quartz did not affect the tourmaline (surfaces
are pristine), but may have interacted with the K-feldspar to a small degree. This represents,
then, the edge of a miarolitic cavity which was later filled by quartz (so called core-quartz). At
least three other examples of this set of features have been observed in the Southern California
pegmatite district.
Quartz in Rhyolite (Crystal Tuff)-Example 2
38
On examining the five rhyolite crystal tuff samples (see section on rhyolite crystal tuffs and
Figure 19) a feature was observed in the five samples—the rounding and modification of the
crystal surfaces of the quartz. Once is an oddity; twice is interesting; three times is a feature
requiring explanation. The suggested gross steps in the formation of the quartz grains were:
 growth type-1 of quartz at >573o as initial grains in a cooling rhyolitic melt;
 partial dissolution of the grains;
 rapid eruption of the melt to preserve the dissolution surfaces.
A suggestion for how this may have occurred follows:
1. a relatively dry rhyolitic melt ascends toward the surface;
2. it cools sufficiently to nucleate and grow β-quartz (and feldspar);
3. at a depth of several kilometers the melt encounters deep surface waters;
4. the water is absorbed and lowers the liquidus temperature;
5. minor dissolution of the quartz (and feldspar) grains occurs;
6. convective circulation of the melt or crystal settling within the magma body
moves the grains to different depths and environments resulting in uneven and
different degrees of solution;
7. continued assent toward the surface brings the melt to a level allowing
vesiculation;
8. vesiculation drives a final rapid assent and eruption which preserves early,
partially dissolved quartz (and feldspar) grains.
If this is the general case for crystal tuffs, then, the reason for the formation of obsidian domes
and flows is the lack of step 2 and 3 (insufficient cooling to nucleate quartz and the lack of an
encounter with water); hence, the hot rhyolitic melt ascends all-the-way to the surface. Note that
Anderson and others (2000) conclude, based on phenocryst compositional studies of the Bishop
Tuff, that crystal settling (crystal mixing) occurred.
Spodumene Crystal-Example 3
Sample ‘A’ from Figure 1 has multiple phantom surfaces (Fig. 27) that indicates
39
Figure 27. Spodumene (sample A from Fig. 1) showing healing of the lower (c-axis; left)
termination of the grain and multiple, internal phantom surfaces. Image has been manipulated to
emphasize the phantom surfaces; note also, the shadow (base of image) shows the variation in
the amount of transmitted light due to the different internal (former) surfaces; interrupted growth
type-1/natural fracture (left end)/growth type-1.
at least two things about the growth of the grain:
1. All of the phantom surfaces (22+) are near duplicates of the upper (c-axis; right)
termination. This indicates that the grain was originally longer than its present length and
that the lower termination is a healed fracture (which is visible at the left of the grain).
2. These phantom surfaces indicate multiple pauses and spurts of growth. Question-What
caused these multiple growths? Such grains occur during the pocket stage of events for
the pegmatite body. It is suggested that these growth spurts are due to multiple releases of
fluid from the pegmatite body during this (near) final event—a pegmatite burp, if you
will. Each release of fluid results in a pause in growth and the continuation of growth
occurs when the fluid pressure rebuilds, either by:
a. a slight collapse of the still hot, and hence plastic body;
b. or by a further release of fluid from the final melt in the lowest part of the
miarolitic cavity.
DISCUSSION
Other Surfaces
Most classification schemes eventually need an ‘other’ section; for most surely, there will be
examples that will not fit—Mother Nature has billions of years and a multitude of environments
in which to develop odd things.
40
Crystals
Given the ideas developed in this article, it is necessary to formally redefine the term ‘crystal’,
but this probably should be rejected, modified, or accepted by a Mineralogical Society of
America committee. The problem centers on growth-interference surfaces where two or more
grains are competing for space; that is, where to draw a boundary when there is a gradation from
irregular, (definitely) non-crystal surfaces to poorly developed ‘crystal’ faces. As well, the terms
euhedral, subhedral, and anhedral will need to be redefined. It is suggested that:
 euhedral—type-1, type-2, or type 4-1 surfaces that are moderately to very-well
developed; also see Figures 1, 2, 3, 4, 5, 10;
 subhedral—type-1, type-2, or type 4-1 surfaces that are poorly to moderately
developed, but are identifiable as structurally controlled growth surfaces; also see
Figures 1 (some) and 25A (end of grain);
 anhedral—a general term for any irregular surface that cannot yet, or cannot be
identified as to type of surface; also any grain lacking for most any reason
growth-type-1, type-2, or type 4-1 surfaces; also see Figures 12, 13, 15.
Mixed and Complex Surfaces
Mineral grains often have mixed or complex surfaces and many grain surfaces may have
multiple (see figures 1 and 14) or different surfaces. This classification system would be used in
the following ways for this situation—place the different surfaces in order of formation separated
by ‘/’. For example--chemical-replacement/growth type-3—would be such a use. If, in addition,
the sample was a pseudomorph the classification would be—chemical-replacement/growth type3/pseudomorph.
Surfaces on Grains from Pegmatite Rocks
Many of the most usable examples of growth-surfaces occur in pegmatite rocks—the large grains
in parts of pegmatite bodies result in large surfaces between grains. Such large samples of
mineral grains are excellent as teaching samples for mineralogy classes and are available
relatively cheaply. Such material was extensively collected from the pegmatites of Southern
California and incorporated in my mineralogy classes.
Large, fine, crystals are found in the miarolitic cavities of some pegmatite bodies (for example,
see the illustrations in the ‘Mineralogy’ section of London, 2008 and figure 1 herein). Such
crystals when well-developed or rare can be quite expensive (100’s to 10000’s of dollars), but
broken, non-gemmy, or common-minerals like schorl (Fe-rich tourmaline), feldspar, spodumene,
and quartz grains can be purchased at reasonable prices.
Grains in Cavities
Cavities are the most common source of grains with well developed growth type-1 surfaces.
However, cavities are more subject to alteration by changing conditions than solid rock; hence
modification of growth type-1 surfaces is common in cavities. In the case of miarolitic cavities in
41
pegmatite bodies and cavities in hydrothermal rocks; these cavities have formed well below the
surface and are subject to a considerable change of PTX conditions over geologic time. Cavities
in sedimentary rocks tend to be readily exposed to water and atmospheric gases because the
rocks tend to be permeable.
Cavities form in a variety of ways and in a variety of rocks; for example:
 volcanic, igneous rocks have fluid-formed (vesicles, vugs, etc.) and fractureformed cavities; grains in such cavities may come from two sources
o deposited some-time after cavity formation (as secondary minerals) and
the minerals are formed-deposited by other than igneous processes;
zeolites (Fig. 28A), agate, opal, carbonates are common minerals of this
sort;
Figure 28A.
42
Figure 28B. Examples of cavities in volcanic rock with secondary minerals. (A) Zeolite grains
(growth type-1 and -4) in basalt in a large, fracture-cavity; most of the grains are stilbite; Lane
Co., near Eugene, Oregon (Kleck, 1972). (B) Searlesite (a rare borosilicate) as milky-glassy
grains (growth type-1) as isolated clusters of grains in a large, irregular fluid-formed cavity,
Saddleback Basalt, mine dumps, U. S. Borax Mine, Kern Co., California (see Wise and Kleck,
1988, p. 135; Morgan and Erd, 1969, p. 168).
o deposited as a final stage of cavity formation (as primary minerals); note
that there is a complete gradation of rocks from coarse-grained pegmatite
rock to obsidian that have fluid-formed cavities and these types of grains.
The fluid-formed cavities in volcanic rocks are typically small and
abundant, and ones which have small, but visible grains (mm-sized) are
moderately common in certain rocks; such grains are not spectacular
except under magnification (see for example, Kleck, 1970 and Kleck,
1986); Fig. 29;
43
Figure 29. Image is of a sawn slab of basaltic-andesite showing ‘lenticular’ vesicles in the
interior of a plug-dome, Summit Rock, Klamath Co., Oregon (Kleck, 1970); note, the cavities are
believed to have formed by vesiculation after the rock had semi-solidified. Grains (growth type1, euhedral), mm-sized, primary minerals (plagioclase, orthopyroxene, ilmenite, and others) are
attached to the walls of the cavities. Grains in the cavities are best viewed at 15X with a
stereomicroscope.

classic miarolitic cavities (from cm to m in size) form in moderately deep,
intrusive igneous rocks, but shallow enough to allow the separation of a fluid (or a
water-rich melt), and are generally at less than ~10 km deep; Fig. 30;
44
Figure 30. Small miarolitic cavity in pegmatite rock (granitic) containing grains of K-feldspar,
quartz, muscovite, clay; Heriot Mountain, Pala Pegmatite District, San Diego Co., California.

solution cavities in soluble rock:
o water soluble sedimentary rocks such as limestone, phosphate-rock (Fig.
31), salt-rock;
Figure 31. Solution cavities in the Phosphoria Formation (a phosphate rock), Dead Indian Grade,
just north of Cody, Wyoming. The cavities were formed by the solution of large, fossil
brachiopods. Grains of calcite occur as crusts of mm- to cm-sized grains and as cm-sized,
isolated crystals (several of which are visible in the cavities).

o in hydrothermal deposits; note that hot water under high pressure can
dissolve a great variety of materials; as well, some hydrothermal deposited
material tends to be vuggy.
fracture cavities may form in almost any rock due to faulting (Fig. 32) or to the
presence of rubble zones associated with lava flows.
45
Figure 32. Fragment of chert covered with dolomite and chalcopyrite (fracture-formed cavity);
sample is from a mine dump in the Tri-State Lead-zinc District near Joplin, Missouri.
Ponds (for purposes of this discussion) can be considered to be ‘a sort of a large cavity’. These
are interesting when one or more compounds reach saturation (such as in desert playa lakes).This
requires somewhat of a stretch of visualization—in some wet environments, saturated
compounds may ‘push aside’ a fine-grained matrix and grow crystals (see figure 4); as well, this
growth may incorporate grains producing type-2 growth. Both of these can be considered to form
in ‘a sort-of-pond’ environment.
Volcanic rocks pass through changing temperatures and pressures quickly; hence there is less
time for chemical change to take place. However, volcanic bodies tend to be badly fractured,
hence, surface water and atmospheric gasses may have ready access to some cavities with
resulting alteration. In addition, there are different environments at the base of, the center of, and
the surface of the volcanic body:
 the base of the volcanic body may be in contact with wet material below; hence the
presence of hot water may contribute to the formation of water containing minerals
(amphiboles or micas as primary minerals; zeolites (see Kleck, 1972) or chalcedony as
secondary minerals); as well, the primary minerals may be altered in the water-rich
environment producing clays;
 the interior of some volcanic bodies (generally larger or thicker bodies) tends to be
sufficiently isolated such that the grains in the cavities survive with little change (except
in the case of cavities intersected by fractures); these minerals tend to be the silicate and
oxide minerals common to the rock its self (see Kleck, 1970);
 at the surface of volcanic bodies, there is contact with the atmosphere, hence oxidation
of the primary minerals in cavities is common.
If the volcanic material is not of recent age and is exposed to other than surface conditions, many
things may affect the cavities—for example, this is the case of the copper-silver bearing lavas of
the Keweenaw Peninsula of Michigan (Fig. 33).
46
Figure 33. Copper bearing lava from the Keweenaw Peninsula, just north of Houghton,
Michigan; sample is from the abandoned mine dump of the Wolverine Mine. The basalt is
altered, but the vesicles remain and contain native copper and epidote.
Cavities in sedimentary rocks are many times formed in a permeable material, and the rocks may
reside in a multitude of environments before discovery. Because of the permeability, water and
atmospheric gases can have ready access and may have access over geologic time. In many cases
the cavities themselves are due to solution and are followed by deposition of cavity minerals. As
a result, cavity minerals in sedimentary rocks can have a complex history.
Perhaps, the only type of rock cavity that commonly has a relatively simple history is the
interior, non-fractured cavities in young volcanic rocks.
Some Comments on the Nature of Selected Minerals
Feldspar (in crystal tuffs)
Feldspar grains are generally coated with ash (fine glass-fragments) and are translucent. This
common coating interferes with an examination of the details of the grain surfaces. However,
both the crystal form of or the lack of growth type-1 surfaces on the grain is usually discernable,
and rounding due to dissolution can be inferred. Cleavage surfaces are moderately common.
47
Some are coated with ash and some are clean; this probably indicates the timing of the
cleaving—pre-eruption cleavages are coated and those created by the eruption are clean.
Some of the grains in the Mazama Tuff (of Crater Lake, OR) had an odd coating. It first
appeared that the grain had a ‘pebbled’ surface. After further examination, it appears that the
surface has small, oval ridges of glass which seem to be the remnants of attached glass bubbles.
This has only been observed for this specific material.
Β-quartz (in crystal tuffs)
Quartz grains generally have surfaces free of ash and are transparent; grains are commonly
etched. Growth type-1 surfaces are common but may be modified by solution or melting. Two
other surfaces are (in combination) about as common; these are fractures (probably due to
temperature shock and/or to the β to α transition) and irregular twin-boundaries--rarely does an
individual grain lack one or both of these surfaces.
Mica (in a variety of environments)
Mica has an excellent and easy cleavage which is parallel to a common crystal face. It is
difficult, in many instances, to visually distinguish which of these is responsible for a given
surface (even in cavities).
Graphite (in tactite)
For the study of graphite, approximately 30 samples of graphite in tactite were prepared by
dissolving the calcite with dilute HCl; one sample of graphite (out of ~100 examined) was a
complex, rosette composed of excellent euhedral grains with all crystal surfaces very well
developed. The contrast between this one graphite sample and the others was remarkable:
 bent, simple (single or 2-3 grain clusters) vs. a rosette of 10+ grains;
 (c)-{0001} surfaces bright and smooth, but commonly bent and having steps parallel to
the prism vs. (c) bright, nearly perfectly smooth, and lacking steps;
 (m)-{10ī0} faces absent (rounded, irregular) vs. (m) euhedral, bright prism faces.
It is suggested that this complex of grains grew on the surface of a solution channel or fracture
that was later filled with calcite.
CREATION OF SUITABLE GRAINS FOR STUDY
Grains suitable for study may occur (or be created) several ways: 1) weathering; 2) growth in
cavities; 3) naturally or artificially crushed or broken rocks; 4) naturally or artificially separated
by chemical or physical properties.
Weathering
The fluids which promote weathering (over geologic time) naturally will use grain boundaries
(as well as fractures) as avenues of travel, hence may aid in the separation of individual grains.
48
In addition, different grains may respond to weathering in different ways and may free selected
grains for examination (while destroying others).
Cavities
Cavities are a well-known environment in which growth surfaces may occur. Miarolitic cavities
in pegmatite bodies are the most sought after cavities—because of the large, fine, rare crystals
which occur there. However, such cavities are quite rare, even in pegmatite districts where they
are known to occur; this is also the case because such samples can command significant prices
and are eagerly sought for sale.
However, most any cavity is potentially a place where crystals may grow and are worthwhile
seeking and examining. For example, many volcanic and shallow intrusive rocks have fluidformed cavities which may have small (mm grained) crystals. Such crystals, although small and
requiring magnification for observation, can be highly instructive.
Artificially Crushed, Broken, or Naturally Fragmented Rocks
Many breakage surfaces naturally follow grain boundaries--these commonly will be the weakest
part of the rock. Therefore, broken samples of rock should be examined to see if interesting
grains are freed and exposed by breakage; this can be also done by successively breaking rocks
into smaller and smaller fragments (while examining the crushed material at each stage of
crushing). In some cases and with care, a person can end with crushed rock composed of a
significant proportion of usable grains.
Naturally fragmented rocks, such as tuffs or pyroclastic material are a fertile type of material to
examine for mineral grains.
Separation by Chemical or Physical Properties
Grains which occur in soluble rock (water, acid, etc. soluble) can in some cases be separated—if
the grains of interest are not or are less soluble than the matrix; the calcite bearing tactites
described earlier are a case in point. The person interested in separation may have to experiment
with different acids, different concentrations, or different temperatures to achieve the necessary
separation.
Differences in density of grains have been used by miners for 100’s of years to separate gold.
Panning, with practice, allows the separation of grains with as little difference as 1 gm/cm3 in
density. High density liquids (if funds are available) are a more precise and high technology way
of accomplishing the same separation but are more expensive.
Slight to significant differences in magnetic properties are useful for separating some minerals;
note that settling the grains to be separated in a liquid (water or clear oil) in a glass cylinder (with
magnets attached to the outside) will decrease the density difference, hence the settling velocity
and allow the separation of weakly magnetic grains. If available (or if funds are available)
commercial magnetic separation devices allow separation of grains with just slight differences.
49
ACKNOWLEDGMENTS
Several persons have directly or indirectly contributed to the ideas expressed herein. I wish to
particularly acknowledge the input of Gene Foord (deceased) for his many hours of discussion
about the nature of pegmatite minerals. Matt Taylor and I had many arguments while standing on
pegmatite body outcrops; we argued with passion and loudly about what particular textures and
features really meant and still remained good friends. David London and I have, over several
years, discussed aspects of pegmatite genesis; although we disagree on several things, I am
grateful for and greatly respect his ideas. Several mine owners, particularly Blue Sheppard, Louis
Spaulding, and George Ashley (deceased), in San Diego County graciously allowed me to
examine and collect from pegmatite bodies exposed on their properties. Special thanks go to
Frederic Foit, my advanced mineralogy instructor from many years back, w ho read a very early
draft of this paper and pointed out several inconsistencies. Finally, a word of thanks to the many
geologists who have donated their time and expertise to lead GSA field trips to interesting
mineral localities.
REFERENCES CITED
Anderson, A. T., Davis, M. M., and Lu, F., 2000, Evolution of Bishop Tuff rhyolitic magma
based on melt and magnetite inclusions and zoned phenocrysts: Journal of Petrology, v. 41, p.
449-473.
Blatt, H., and Tracy, R. J., 1996, Petrology; Igneous, Sedimentary, and Metamorphic, 2nd edition,
W. H. Freeman and Company, New York.
Dana, E. S. and Ford, W. E., 1932, A Textbook of Mineralogy, 4th edition, John Wiley & Sons,
Inc., New York.
Deer, W. A., Howie, R. A., and Zussman, J., 1992, An Introduction to the Rock-Forming
Minerals, 2nd edition, John Wiley & Sons, New York.
Donaldson, C. H., 1985a, The rates of dissolution of olivine, plagioclase, and quartz in a basalt
melt: Mineralogical Magazine, v. 49, p. 683-693.
Donaldson, C. H., 1985b, A comment on crystal shapes resulting from dissolution in magmas:
Mineralogical Magazine, v. 49, p. 129-132.
Frondel, C., 1962, The System of Mineralogy, Volume III, Silica Minerals, 7th edition, John
Wiley & Sons, Inc., New York.
Gaspar, J. C. and Wyllie, P. J., 1983, Magnetite in carbonatites from the Jacupiranga Complex,
Brazil: American Mineralogist, v. 68, p. 195-213.
50
Heaney, P. J. and Veblen, D. R., 1991, Observations of the α-β phase transition in quartz: A
review of imaging and diffraction studies and some new results: American Mineralogist, v. 76, p.
1018-1032.
Hornig-Kjarsgaard, I., 1998, Rare earth elements in Sövitic carbonatites and their mineral
phases: Journal of Petrology, v. 39, p. 2105-2121.
Kleck, W. D., 1970, Cavity minerals at Summit Rock, Oregon: American Mineralogist, v. 55, p.
1396-1404.
Kleck, W. D., 1972, The geology of some zeolite deposits in the southern Willamette Valley,
Oregon: The Ore Bin (State of Oregon, Dept. of Geology and Mineral Industries), v. 34, p. 145152.
Kleck, W. D., 1986, Cavity minerals of the lava flow at Lemolo Lake quarry, Oregon: The
Fourteenth General Meeting of the International Mineralogical Association, Abs. with Program,
p. 143.
Kuo, L. and Kirkpatrick, R. J., 1985, Kinetics of the crystal dissolution in the system diopsideforsterite-silica: American Journal of Science, v. 285, p. 51-90.
London, D., 2008, Pegmatites: The Canadian Mineralogist, Special Publication 10.
Misch, P., 1971, Porphyroblasts and ‘crystallization force’: some textural criteria: Geological
Society of America Bulletin, v. 82, p. 245-252.
Morgan, V. and Erd, R. C., 1969, Minerals of the Kramer Borate District, California: Mineral
Information Service, California Division of Mines and Geology, v. 22, p. 143-153, 165-172.
Myson, B. and Richet, P., 2005, Silicate glasses and melts, properties and structure,
Developments in Geochemistry, 544 p., Elsevier, Amsterdam
Neuerberg, G. J., 1954, Allanite pegmatite, San Gabriel Mountains, Los Angeles County,
California: American Mineralogist, v. 39, p. 831-834.
Onions, C. T. editor, 1964, The Oxford Universal Dictionary, 3rd edition, Oxford University
Press, London.
Schwartz, G. M., 1937, Alteration of spodumene to kaolinite in the Etta Mine: American Journal
of Science, v. 33, p. 303-307.
Sinkankas, J., 1966, Mineralogy: A First Course: D. Van Nostrand Company, Inc., Princeton,
New Jersey.
Smith, J. V. and Brown, W. L., 1988, Feldspar Minerals, Volume 1, 2nd edition: Springer-Verlag,
Berlin.
51
Taylor, M. C., 2006, The gel model for the formation of gem-bearing pockets within granitic
pegmatites, and implication for gem synthesis: Gems and Gemology (abstract), v. 42, p. 110111.
Thatcher, V. S., editor in chief, 1952, The New Webster Encyclopedic Dictionary of the English
Language: Consolidated Book Publishers, Chicago.
Twenhofel, W. H., 1939, Principles of Sedimentation, McGraw-Hill Book Co., Inc., New York
Wilshire, H. G. and Trask, N. J., 1971, Structural and textural relationships of amphibole and
phlogopite in peridotite inclusions, Dish Hill, California: American Mineralogist, v. 56, p. 240255.
Wise, W. S. and Kleck, W. D., 1988, Sodic clay-zeolite assemblage in basalt at Boron,
California: Clay and Clay Minerals, v. 36. p. 131-136.
Yardley, B. W. D., 1974, Porphyroblasts and ‘crystallization force’: discussion of some
theoretical considerations: Geological Society of America Bulletin, v. 85, p. 61-62.
52