Download Minerals and Rocks lab info

 Lab manual for GEOG1120 MINERALS
To introduce minerals and rocks, we must understand a little bit of chemistry. The terms
ELEMENT, ATOM, and ION will not be discussed here. But they must be understood, so
look them up.
CRYSTALLINE SUBSTANCE refers to a solid material that has a definite atomic
structure and a definite chemical composition or formula.
ATOMIC STRUCTURE or CRYSTAL LATTICE refers to the regular, ordered
arrangement of atoms or ions that make up a crystalline substance. This arrangement is
the same for every specimen of a specific substance.
CHEMICAL FORMULA indicates elemental content of composition. For a given
crystalline substance, the formula is either constant or ranges within definite limits.
Note that each crystalline substance has its own particular formula and structure; its
formula indicates what kinds and what proportions of ions are present, and its structure
determines how ions are arranged in space.
MINERALS are naturally occurring, inorganic, crystalline substances.
ROCKS are naturally occurring, coherent aggregates of minerals.
CLASSIFICATION OF MINERALS
Although minerals have been studies for hundreds of years it was not until this century
that advances in chemical analysis and the discovery of x-rays made it possible for us to
understand and appreciated the order that exists in the mineral kingdom. The details of
these studies are too involved to cover in an introductory course such as this; however, it
is important to know that minerals are classified (grouped) on the basis of the kinds of
anion (negatively-charged atoms or atom groups) in their structures. Many of the
characteristics of minerals within a class are similar because their chemistries are similar.
A good example would be the carbonate minerals. All of these minerals will effervesce to
some degree with acids, giving off carbon dioxide gas. The major groups of related
minerals are called classes and are named as follows:
MINERAL CLASS
ANION
EXAMPLES
Native elements
no anions; minerals consist
of one element only
sulphur, graphite,
diamond, native copper
Sulphides
sulphur (S2-)
pyrite, galena,
chalcopyrite, sphalerite
1
Lab manual for GEOG1120 Oxides/hydroxides
oxygen (O2-) and/or
Hydroxyl (OH-)
limonite, hematite,
corundum
Halides
fluorine (F-) or chlorine (Cl-)
fluorite, halite, sylvite
Carbonates
carbonate (CO32-)
dolomite, calcite,
siderite, azurite
Sulphates
sulphate (SO42-)
gypsum, anhydrite,
barite
Phosphates
phosphate (PO43-)
apatite
Silicates
silicon-oxygen tetrahedral (SiO44-) olivine, quartz,
in a number of different linkages
feldspars, micas,
amphiboles, pyroxenes,
garnet, clay minerals, talc
In nature about one third of the known mineral species (about 2500 to 3000 minerals)
are silicates, and they make up about 95% of all the minerals in the earth’s crust. The
remaining 5% include all the other classes of minerals, which are termed as a group, nonsilicates.
A mineral is defined as a chemical element or compound that is a naturally occurring
crystalline solid and is formed as a result of inorganic processes. Each mineral has
characteristic physical properties that are easily determined and are therefore useful in
mineral identification. It will be important that you learn how to recognize the different
types of physical properties and identity the mineral from these characteristics. Your
knowledge of minerals will be tested with a lab test later in the semester; check
your schedule for exact date.
Minerals are the building blocks of rocks and rocks are the building blocks of Earth and
other rocky planets. We use the presence and composition of minerals and mineral
assemblages to figure out the geological history of a rock.
For sedimentary rocks, we can use the minerals to help understand where the
sediments that make up the rock came from, e.g., how far did the sediments travel, how
fast was the water flowing that carried them, and what were the original rocks that
eroded to create the sediments?
For igneous rocks, the minerals help us determine the pressures and temperatures that
formed the rocks, what the bulk composition of the rock was and whether there was
more than one melting and mixing event during the formation of the rock.
The minerals in metamorphic rocks can record the changes in pressure and temperature
during a rock’s history, which is useful in understanding the path the rock took during
its burial, heating and uplifting back to Earth’s surface.
2
Lab manual for GEOG1120 PHYSICAL PROPERTIES OF MINERALS
Minerals are chemical compounds and it is possible to identify them by x-ray or chemical
analyses. This, however, is neither practical nor necessary in an introductory study of
minerals. Most common minerals can be identified by observing certain readily
determined physical characteristics or properties such as colour, hardness, lustre,
specific gravity, etc. By making a few simple observations and tests on a specimen in
conjunction with an identification table or key, a common mineral can be readily
identified. Thus it is necessary for the student of elementary mineralogy to become
familiar with the various physical properties in order to successfully identify the
common minerals you will see in this course.
The physical properties of minerals are sometimes divided into two groups:
(a) General physical properties: those physical properties which all minerals exhibit,
such as colour, hardness, lustre, etc.
(b) Special or specific physical properties: those properties found only in a few
minerals such as taste, magnetism, odour, etc. A mineral may be identified on the
basis of its general physical properties alone but very often one of the special
properties may by the diagnostic one which makes identification possible.
GENERAL PHYSICAL PROPERTIES
The following list gives some of the physical properties commonly employed for
identifying minerals in both the laboratory and the field.
(a) Colour
Colour is one of the most obvious characteristics of a mineral and is determined by
examining a fresh surface in reflected light. For some minerals colour is diagnostic (eg,
galena [grey], azurite [blue], olivine [green]) whereas others (eg, quartz) may exhibit a
wide range of colours, due to either slight differences in chemical composition or minor
amounts of impurities in the minerals. Because fresh and weathered surfaces of a mineral
may have different colours (especially among minerals having a tendency to tarnish – see
below) it is important to note the type of surface being viewed when determining colour.
(b) Lustre
The quality and intensity of the light that is reflected from a fresh surface of a mineral is
its lustre. Frequently we describe the lustre of a mineral by comparing it to some familiar
substance. Two main categories of lustre are recognized in minerals: metallic and nonmetallic. There is no sharp division between them and minerals with lustre intermediate
between them are referred to as having sub-metallic lustre.
3
Lab manual for GEOG1120 Metallic lustre: minerals that look like a metal are said to have metallic lustre.
They are opaque or nearly so (see also diaphaneity).
Non-metallic lustre: several varieties are recognized.
Vitreous
having the lustre of glass or broken glass. This is a very common
type of lustre among rock forming minerals.
Resinous
lustre like that of resin
Silky
the lustre of silk; often displayed by minerals composed of fibrous
aggregates arranged in parallel fashion.
Pearly
the lustre of pearl; often displayed by minerals with layer structure
such as talc and the micas.
Dull or earthy some minerals, because of their weathered or porous surface,
scatter incident light so completely they lack lustre
(c) Hardness
The hardness (H) of a mineral is generally defined as its resistance to abrasion or
scratching. Hardness is related to the crystal structure of a mineral and the strength of
the bond between adjacent atoms. Hardness may be roughly determined by attempting
to scratch a mineral of unknown hardness with one of known hardness, or by using the
unknown mineral to try to scratch material of known hardness.
Mineral hardness is measured on a relative scale called the Mohs Scale of Hardness
(after German mineralogist Friedrich Mohs), which consists of ten common minerals
arranged in order of their increasing hardness as follows:
Hardness
1
2
3
4
5
6
7
8
9
10
Mineral
Talc
Gypsum
Calcite
Fluorite
Apatite
Orthoclase
Quartz
Topaz
Corundum
Diamond
(hardness of a fingernail is 2 to nearly 3)
(hardness of a penny is slightly less than 3)
(hardness of a needle, glass plate are 5.5)
(hardness of a streak plate is 6.5)
When using a glass plate to determine hardness do not hold it in your hand buy lay it flat
on the table surface and draw the mineral across the plate. Check to make sure that what
appears as a scratch on the glass is not some of the mineral, which has rubbed off on the
glass and can be wiped off with the finger.
4
Lab manual for GEOG1120 Generally hardness is a reliable physical property but variations may occur due to
variations in composition. Hardness may also vary with crystallographic direction in a
mineral. Since weathering affects hardness, all hardness tests should be made on a fresh
surface.
(d) Streak
Streak is the colour of the finely powdered mineral. It is obtained by crushing, filing or
scratching the mineral or by rubbing the specimen on a pieced of unglazed porcelain –
called a streak plate. (A streak plate cannot be used with all minerals since some
minerals are harder than the plate.)
Some minerals have a streak that is the same colour as the hand specimen; others have a
streak that differs in colour from the hand specimen and may be diagnostic for the
identification of that mineral.
(e) Tenacity
The resistance a mineral offers to breaking, crushing, bending or cutting is referred to as
tenacity. Some terms used to describe tenacity are:
Brittle
Malleable
Sectile
Flexible
Elastic
minerals which break into angular fragments when struck with a
hammer (eg, quartz)
minerals whose shape can be changed without breaking (eg, native
copper)
minerals that can be cut into thin shavings with a knife, the pieces
do not disintegrate to powder
minerals, which will bend without breaking, and remain bent after
the bending force is removed (eg, talc, thin plates of gypsum)
minerals, which will bend easily and spring back to their original
form when the force is released (eg, micas)
(f) Diaphaneity
The ability of a mineral to transmit light is referred to as diaphaneity and is usually
expressed as:
Transparent
Translucent
Opaque
objects are clearly visible when viewed through the mineral
mineral transmits some light (eg, along the edges) but objects
cannot be clearly seen through the mineral or they are distorted
minerals which allow no light to pass through, even on the
thinnest edge
5
Lab manual for GEOG1120 (g) Cleavage
Cleavage is the tendency of a mineral to break preferentially along certain planes yielding
a smooth lustrous surface. Minerals possess cleavage because the bonds, which hold the
atoms together are not of equal strength in all directions. Some minerals have no cleavage
(they show fracture) while others may exhibit one, two, three, four, or six cleavage
directions.
The number of cleavage planes as well as the angle at which they intersect are diagnostic
for any mineral which possesses cleavage.
Types of cleavage
one cleavage plane; usually called basal cleavage; perfect cleavage in one direction
as in micas
ii) two cleavage planes; usually called prismatic cleavage; in feldspars and pyroxene
(augite) cleavage surfaces intersect at about 90°; in amphibole (hornblende)
cleavage planes intersect at about 60° and 120°
iii) three cleavage planes; two types are recognized in this group:
a. cubic cleavage; minerals with three cleavage planes intersecting at 90°
b. rhombohedral (or rhombic) cleavage; minerals with three cleavage planes
which do not intersect at 90°; minerals break into six-sided prism
with each side having the shape of a parallelogram
iv) a few minerals have more than three planes of cleavage; four sets of parallel
cleavage surfaces in the form of an octahedron produce octahedral cleavage (eg,
fluorite)
i)
Figure 1 Common types of cleavages in
minerals. Also, refer to the displays in the
lab as you try to distinguish between the
different types of cleavage.
6
Lab manual for GEOG1120 The cleavage planes in some minerals are so well developed that they are easily detected.
In other cases cleavage planes may occur in step-like fashion and be so discontinuous as
to escape detection by casual inspection. However, if such a specimen is slowly rotated
in a good light source, in certain positions, parts of the specimen will reflect light in the
same way as large smooth cleavage surfaces.
(h) Fracture
In some minerals bonding between atoms is so uniform that there is no preferred
direction of breakage. Minerals without cleavage are said to fracture. Common types of
fractures are:
Conchoidal
Uneven or irregular
Fibrous or splintery
Even
fracture surface is smooth and rounded (like the surface of
a shell or conch) and frequently shows fine concentric
ridges; eg, natural glass (obsidian) and flint
fracture surface is irregular and rough; many minerals
exhibit this type of fracture
fracture surface is roughened by splinters or fibres
fracture surface, though rough with numerous small
elevations and depressions, still approximates a plane
surface
(i) Specific Gravity
The specific gravity (G) of a mineral is a number, which represents the ratio of the
weight of the mineral to the weight of an equal volume of water. The specific gravity of a
mineral can be determined accurately with suitable equipment but for the purposes of
identifying minerals in the laboratory specific gravity can be determined with sufficient
accuracy simply by lifting the specimen in your hand.
Light coloured silicate minerals have an average specific gravity of 2.6 whereas minerals
with a metallic lustre range from 4.5 to 7.0 or 8.0.
(j) Crystal Form
When a mineral is allowed to grow in an unrestricted environment (ie, not crowded by
its neighbours) it will form a crystal bounded by smooth crystal faces. The geometric
form (cube, octahedron, prism, etc.) of the crystal is a reflection of the internal (atomic)
structure of the mineral and can be used to identify many mineral species.
It is important to remember that natural crystals may be distorted by the unequal
development of different crystal faces and such crystals will not show their diagnostic
crystal form. However, in identifying distorted crystals it should be kept in mind that
although the appearance is unusual, the interfacial angles remain the same as in perfect
crystals. The interfacial angle is obtained by measuring the angle between one crystal
face and the other extended.
7
Lab manual for GEOG1120 Figure 2 Common types of crystal forms of minerals.
(k) Crystalline Aggregates
Most mineral specimens are aggregates of imperfect crystals. This is largely due to
conditions of growth in which crowding seriously interferes with the normal
development of any single individual.
In aggregates of crystals the individual crystals may be elongate (eg, fibrous form of
asbestos) or occur as flattened plates (lamellae). Aggregates of small lamellae are often
termed micaceous (eg, one variety of hematite). A mineral, which comprises an aggregate
of many small equidimensional crystals is described as granular.
8
Lab manual for GEOG1120 SPECIFIC PHYSICAL PROPERTIES
(a) Magnetism
Minerals, which in their natural state are attracted to a magnet are said to be magnetic.
Magnetite is the only common mineral that is readily attracted by a small hand magnet.
Pyrrhotite is also magnetic.
Some specimens of magnetite act as natural magnets and will attract iron filings when
suspended. Such specimens will orient themselves with their long axis pointing N-S.
This variety of magnetite is called lodestone and was used in the earliest forms of
compasses.
(b) Double Refraction
If an object appears double when viewed through a transparent mineral that mineral is
said to have double refraction. This is due to the fact that the light passing through the
crystal or cleavage fragment is broken into two rays travelling with different velocities
and thus having different indices of refraction. Calcite is the only common mineral to
show double refraction.
(c) Feel
The feel of a mineral is the impression one gains by handling or rubbing the specimen.
Common terms used to describe feel are smooth, greasy, soapy, etc.
(d) Taste
Certain minerals are sufficiently soluble (eg, halite or common salt) to be identified by
taste.
(e) Odour
Certain minerals give off a characteristic odour when damp. Kaolinite, for example, has
an earthy or dank odour when moistened by exhaling on the mineral.
(f) Reaction with acid
Certain minerals (especially some of the carbonate group) will effervesce (bubble) when
treated with dilute hydrochloric acid.
9
Lab manual for GEOG1120 (g) Twinning
A few common minerals, especially members of the plagioclase feldspar group, have
parallel, thread-lines or narrow bands running across their cleavage surfaces. These lines
are twinning striations, which mark the boundary between several intergrown or
twinned crystals.
(h) Tarnish
Some metallic minerals have characteristic tarnish on a weathered surface. For example,
chalcopyrite displays a “peacock” tarnish showing purplish-blue colours.
10
Lab manual for GEOG1120 IDENTIFICATION OF MINERALS
This laboratory exercise is designed to acquaint you with the common rock forming
minerals and minerals of economic importance. It is essential that you become familiar
with the rock forming minerals, as you will encounter them again in subsequent labs on
igneous, sedimentary and metamorphic rocks.
Use the flow charts in Figures 3 to 5 in conjunction with Mineral Identification Tables
on pages 12-14 to help you determine the physical properties and name of each of the
unknown mineral samples in the lab.
The flow charts and the mineral identification tables will assist you in naming the
minerals in the study set. The charts are arranged in a systematic manner. First,
determine whether the mineral’s lustre is metallic or non-metallic. Then determine the
streak, the mineral’s hardness, number of cleavage planes and angles between them, if
possible and other properties as needed (magnetism, reaction with acid, etc).
Figure 3 Identification key for minerals with metallic lustre.
11
Lab manual for GEOG1120 Figure 4 Identification key for dark-coloured minerals with non-metallic lustre.
Figure 5 Identification key for light-coloured minerals with non-metallic lustre.
12
Lab manual for GEOG1120 Physical Properties of Some Common Minerals
Table 1 MINERALS WITH METALLIC LUSTRE
1. Hardness less than 2.5 (can be scratched with fingernail)
Streak
Cleavage/Fracture
H
S.
Other
Mineral
G.
(composition)
black perfect cleavage in one
1
2 dark grey black;
Graphite
direction
greasy feel, leaves a
C
mark on paper
grey
3 directions at right
2.5 7.5 lead grey; commonly
Galena
angles
in cubic crystals
PbS
yellow- *
1-5 3.6 occasionally in
Limonite
brown
- masses with metallic Fe(OH)n(H2O)
4.0 lustre; more
commonly as earthy
masses
2. Hardness between 2.5 and 5.5 (harder than a fingernail, softer than glass)
Streak
Cleavage/Fracture
H
S.
Other
Mineral
G.
(composition)
black *
3.5
4 brassy yellow; often
Chalcopyrite
-4
tarnished; similar to
CuFeS2
pyrite but not in
cubic crystals
yellow 6 directions; only a few 3.5
4 most are resinous;
Sphalerite
are usually present in a
only a few varieties
ZnS
single specimen
have submetallic
lustre; brown, yellow,
black and colourless
varieties
3. Hardness greater than 5.5 (harder than a knife)
Streak
Cleavage/Fracture
H
S.
Other
Mineral
G.
(composition)
dark
no cleavage
65 brassy yellow;
Pyrite
grey6.5
commonly in cubes or
FeS2
black
12-sided crystals with
to
striated faces; also in
black
granular masses
black *
6
5.2 iron black; strongly
Magnetite
magnetic
Fe3O4
red*
5.5 5.3 Only the rare black
Hematite
brown
variety has metallic
Fe2O3
6.5
lustre, specular or
earthy lustre
* This property is rarely seen or measurable.
13
Lab manual for GEOG1120 Table 2 MINERALS WITH NON-METALLIC LUSTRE
1. Hardness less than 2.5 (can be scratched with fingernail)
Streak
Cleavage/Fracture
H
S.
Other
Mineral
G.
(composition)
White perfect cleavage in one 2
2.3 vitreous to pearly;
Gypsum
direction, two other
colourless
CaSO42H2O
good
transparent variety is
or
selenite; fine-grained
massive is alabaster;
white, yellow, grey,
red, brown
colourless perfect cleavage in one 22.8 colourless mica; can
Muscovite
direction
2.5
be peeled into
KAl2(AlSi3O10)
transparent sheets
(OH)2
2. Hardness between 2.5 and 5.5 (harder than a fingernail, softer than glass)
Streak Cleavage/Fracture H S.G.
Other
Mineral
(composition)
green or perfect cleavage in
2.5 2.7- dark-coloured mica;
Biotite
brown one direction
-3
3.3 usually brown or
K(Fe, Mg)3
green; can be peeled
(AlSi3O10) (OH,
into thin sheets
F)2
*
1-5 3.6-4 commonly as earthy,
Limonite
powdery masses; and
FeO(OH)n(H2
as coatings on other
O)
yellowminerals
brown 6 directions; only a 3.5
4
resinous (may have
Sphalerite
few are usually
yellow streak) to
ZnS
present in a single
submetallic lustre;
specimen
colours include white,
colourless, black,
brown
3 directions at
2.5 2.2 colourless to white;
Halite
white
right angle
cubic crystals; salty
NaCl
taste
3 directions, not at
3
2.7 varicoloured; usually
Calcite
or
right angle
white or colourless;
CaCO3
rhombic or elongate
crystals; reacts with
HCl
colourless 4 directions
4
3.2 Cubic crystals;
Fluorite
varicoloured; light
CaF
green, purple, blue,
yellow; colourless
14
Lab manual for GEOG1120 3. Hardness between 5.5 and 6.5 (harder than knife, softer than streak plate)
Streak Cleavage/Fracture H S.G.
Other
Mineral
(composition)
red*
5.5 5.3 reddish-brown; often
Hematite
brown
found in earthy masses
Fe2O3
6.5
(softer)
2 directions at
6
2.5- colour variable
Orthoclase
white
right angle
2.6 (commonly salmonFeldspar
pink); pearly-glassy
KAlSi3O8
lustre
2 directions at
6
2.5- Pearly-glassy lustre;
Plagioclase
or
right angle
2.6 striations on some
Feldspar
cleavage surfaces;
Ca(Al2Si2O8)
colour varies with Ca
NaAlSi3O8
colour
and Na amounts
less
2 directions at 56°
5-6 3-3.4 amphibole with double
Hornblende
and 124°
chain structure; dark
Chain silicate
green to black;
with Ca, Na, Fe,
to
prismatic crystals
Mg, Al, OH
2 directions at 87°
5-6 3.2- pyroxene with single
Augite
pale
and 93°
3.3 chain structure; dark
Chain silicate
green
green to black;
with Ca, Na, Fe,
prismatic crystals
Mg, Al
4. Hardness greater than 6.5 (harder than a streak plate)
Streak Cleavage/Fracture H S.G.
Other
Mineral
(composition)
6.5 3.2- granular masses;
Olivine
mineral
-7
4.4 stubby prismatic
(Mg,Fe)2SiO4
crystals; transparent to
scratches
translucent; yellow to
olive green or brown to
streak
black; vitreous
conchoidal
plate
fracture
7
3.5- equidimensional 12Garnet family
4.3 sided crystals;
Complex Ca, Fe,
but
varicoloured; red,
Mg, Al, Cr, Mn
green, brown, black
silicate
doesn’t
7
2.65 Elongate 6-sided
Quartz
prisms; wide range of
SiO2
powder
colour; colourless,
pink, purple, black,
yellow, green
* This property is rarely seen or measurable.
15
Lab manual for GEOG1120 ROCKS
In the next series of labs you will become familiar with the different types of igneous,
sedimentary, and metamorphic rocks. You will become adept at recognizing different
types of rocks and be able to classify them according to their texture and mineral
composition. Your previous experience with identifying minerals will be essential in order
to recognize the most common types of rocks.
This section of the lab will be organized as follows:
Lab 3
Igneous rocks. You will learn how igneous rocks form and what they tell
us of the parent magma and subsequent cooling history. You will become
familiar with the most common and/or distinctive types of igneous rocks
in hand specimens.
Lab 4
Sedimentary rocks and weathering and sediments. Hand specimens of
the major types of sedimentary rocks (clastic and chemical) will be
provided. You will learn how to recognize the different categories of each
based on information on grain size and mineral composition, and to infer
from this the environment of deposition. You will also look at sediments
and the effects of weathering and erosion. You will try to infer
information concerning the source, mode of transport and depositional
environment.
Lab 5
Metamorphic Rocks. You will be able to see from the samples provided
how the processes of metamorphism change the nature, size, and
orientation of minerals in a rock. You will learn the dominant textures
and minerals of metamorphic rocks, and what these tell us of the amount
of heat and/or pressure that the parent rock has undergone.
16
Lab manual for GEOG1120 THE ROCK CYCLE
Rocks are naturally occurring aggregates of minerals. Different rock types can be
recognized on the basis of their physical properties, texture, and mineral composition.
Three fundamental rock categories are recognized:
Igneous rocks result from the cooling and crystallization of molten earth materials.
Sedimentary rocks result from the erosion, transport, deposition, and lithification of
detrital material, or from biological/chemical precipitation.
Metamorphic rocks result from the alteration of pre-existing rocks by heat, pressure,
and mechanical deformation.
With reference to the rock cycle illustrated below, we can see how each type of rock is
related to the others. It is important to realize that any rock can be derived from, and
give rise to, any other rock. The rock that you hold in your hand is not the final product;
given enough time, it will eventually be changed into something else.
Figure 6 The rock cycle.
17
Lab manual for GEOG1120 IGNEOUS ROCKS
A rock is any natural aggregate of minerals (eg, granite), glass (eg, obsidian), or organic
particles (eg, coal).
Magma is molten rock beneath Earth’s surface that has been forced to become fluid by
the intense heat within the planet. If magma flows onto the surface of the land, or onto
the seafloor, then it is termed lava.
Igneous rocks are of two types; rocks that form from cooling of magma beneath Earth’s
surface, intrusive coarse-grained rocks, and rocks that form from cooling of lava at
Earth’s surface, extrusive fine-grained rocks. Dike rocks are intermediate in grain size
between intrusive and extrusive rocks, but in this lab we will consider dike rocks as
intrusive.
Igneous rocks are classified and named by looking at their grain sizes, textures, and the
compositions both of the initial melt and the final cooled mineral assemblages. In order
to understand igneous rocks and their modes of formation, you should be aware of the
following terms and definitions.
TEXTURES OF IGNEOUS ROCKS
Texture is one of the most important attributes of igneous rocks and refers to the shape,
size, and arrangement of the mineral grains or crystals in the rocks.
(a) Glassy texture
If magma is cooled very rapidly, such as into seawater or glacial ice, there is no time for
any ions within the magma to form a crystalline structure. The result is a non-crystalline,
glassy volcanic rock called obsidian. Although obsidian is black in colour, it is usually a
felsic rock, derived from a granitic magma.
(b) Vesicular texture
A lava which contains abundant gases may cool while the gases are still bubbling out,
producing a rock which is also very porous or vesicular. Scoria and pumice are examples
of rocks with abundant vesicles. Silica-rich lavas are very viscous and gases within them
have a hard time bubbling out. Instead, the bubbles churn up the lava and when it
solidifies it forms pumice, a volcanic rock that is so porous, it floats. Like obsidian,
pumice is formed from glass particles.
18
Lab manual for GEOG1120 The size of the vesicles can range from 1 mm to several centimetres in diameter. Vesicles
that are filled with secondary mineral matter precipitated by percolating water are called
amygdules and the texture is amygdaloidal.
(c) Fragmental texture
A volcanic explosion results in materials (molten lava, ash and broken rock debris) being
thrown into the air and then settling back onto land or water. Rocks formed in this
manner have a pyroclastic texture. The fragments in pyroclastic rocks have irregular
shapes and sizes. The angular fragments are called pyroclasts, and are classified in order
of grain size:
i)
ash; particles < 2 mm; when heat melts the ash particles together, the resulting
rock is called a tuff.
ii) lapilli; particles 2-64 mm, pebble size
iii) volcanic bombs; particles > 64 mm; when lapilli and/or volcanic bombs are
lithified the resulting rock is a volcanic breccia.
(d) Crystalline texture
Crystalline textures are classified on the basis of the sixe of the particles that make up
the igneous rocks. Grain size is a direct result of the rate at which magma cools. For
example, if magma cools slowly, as in intrusive rocks, mineral crystals have time to grow
to a visible size and are called medium (1-2 mm) or coarse grained (> 2 mm). Rocks with
mineral crystals from 1 to 10 mm in diameter are called phaneritic (from the Greek word
for visible). Granite is an example of a phaneritic rock. If the grains are all the same size,
the rock is said to be equigranular.
If, on the other hand, magma is extruded onto the surface of Earth, where heat is
dissipated away from the crystallizing lava much more quickly, minerals do not have
time to grow very large, and the result is a more fine grained (< 1 mm) texture. These
volcanic rocks are called aphanitic. Basalt is a rock with aphanitic texture.
Some igneous rocks have two distinct sizes of crystals, reflecting a two stage cooling
history: one in which magma cooled slowly enough for some large crystals
(phenocrysts) to form, and a second stage when the rest of the magma solidified quickly
(groundmass). This texture is called porphyritic.
Often the very last of the magma to crystallize in a magma chamber is very fluid due to
the concentration of volatiles in the residual magma. In this water-rich magma, minerals
may grow to very large sizes, and the resulting texture is said to be pegmatitic. Some
pegmatites contain minerals over a metre in diameter. They generally occur as veins or
dikes.
19
Lab manual for GEOG1120 COMPOSITION AND COLOUR OF IGNEOUS ROCKS
The single most important component for the classification of igneous rocks according to
composition is silica (SiO2) content. There are four main categories based on the
percentage of silica:
(a) Felsic rocks >65% SiO2. Felsic minerals are light coloured and rich in silicon and
aluminium. They include quartz, potassium feldspar (orthoclase), sodium-calcium
feldspar (plagioclase) and muscovite. The magmas that produce these rocks are high in
potassium, silicon, and sodium, and low in iron, magnesium, and calcium. Granites and
rhyolites are therefore characteristically light coloured. The finer grained volcanic rocks
(where individual grains cannot be seen) show a wide variation from white to pink to
buff.
(b) Intermediate rocks 53-65% SiO2. Quartz is not an essential mineral and is usually
<10%. There is more plagioclase than orthoclase. The ferromagnesian minerals are biotite
and hornblende and minor pyroxene may be present. Diorites and andesites are
characteristically intermediate in colour. Andesites can be shades of rust, grey, green, or
purple.
(c) Mafic rocks 45-53% SiO2. The predominant minerals are pyroxene and calcium-rich
plagioclase (dark grey-blue colour). The magmas that produce these rocks (gabbros and
basalts) are relatively high in iron, magnesium, and calcium, but deficient in silica. Rock
colouration is characteristically black or dark green.
(d) Ultramafic rocks <45% SiO2. The predominant minerals are pyroxene and olivine.
Calcium-rich plagioclase may be present in minor amounts but is not light coloured as it
is in felsic rocks. It has a dark grey-blue colour.
The mineral constituents of an igneous rock impart a characteristic colour to it. Hence,
rock colour is used as a first-order approximation in establishing the general
mineralogical composition of an igneous rock. The variations in silica-rich minerals and
ferromagnesian minerals result in differing colours of rocks. Figure 7 can be used as an
aid to visually estimate the percentage of dark minerals present (which is inversely
proportional to the silica content).
Felsic: <30% dark minerals
Intermediate: 30-60% dark minerals
Mafic: 50-60% dark minerals
Ultramafic: >60% dark minerals
20
Lab manual for GEOG1120 Figure 7 Comparison chart for visual percentage estimation (after Terry and Chilingar,
1955)
As you try to determine the mineralogical composition of the rocks, keep in mind that
you are trying to assess three major mineralogical criteria:
1. Presence or absence of quartz. Quartz is an essential mineral in felsic rocks and
an accessory mineral in intermediate or mafic rocks.
2. Composition of the feldspars. K-feldspar and Na-plagioclase are essential
minerals in felsic rocks but are rare or absent in intermediate and mafic rocks.
Calcium plagioclase is characteristic of mafic rocks.
3. Proportion and kinds of ferromagnesian minerals. Mafic rocks are rich in
ferromagnesian minerals, while felsic rocks are depleted in these minerals. Olivine
is generally restricted to mafic rocks. Pyroxenes and amphiboles are present in mafic
to intermediate rocks. Biotite is common in intermediate and felsic rocks.
21
Lab manual for GEOG1120 Hints on Mineral Identification in Hand Specimens
Identification of the major rock types may be difficult at first for the beginning student.
The following is a brief summary of some minerals and how they may appear in igneous
rocks. Use of a hand lens and a microscope generally are a great help.
Relatively few minerals make up most igneous rocks. To correctly identify the rock, you
must identify the major minerals. This is fairly easy for coarse-grained rocks, more
difficult (but not necessarily impossible) for fine-grained rocks, and impossible for glassy
rocks (unless phenocrysts are present).
Quartz
K-feldspar
Plagioclase
Muscovite
Biotite
Amphibole
Pyroxene
Olivine
Occurs as irregular, glassy grains commonly clear to smoky in colour. No
cleavage, but conchoidal fracture may be seen on some surfaces. A hand
lens is useful in distinguishing quartz from light coloured feldspars, which
appear milky or translucent.
Porcelain lustre. Commonly coloured pink, white, or grey. Cleavage in
two directions at right angles may be detected. Cleavage planes flash light
when specimen is rotated.
Usually grey or white in granite, dark bluish colour in gabbro. Striations
common. Two cleavage directions at right angles may be detected.
Brass-coloured flakes associated with quartz or K-feldspar. Perfect
cleavage in one direction. Sometimes glitter like small specks of pyrite.
Small black flakes. Perfect cleavage in one direction. Reflects light. Can be
distinguished from hornblende or pyroxene by scratching or crushing
with a steel probe.
Long, black crystals in a light coloured matrix. Cleavage at 60° and 120°.
Short, dull, greenish black minerals in darker rocks. Cleavage in two
directions at right angles. If grain size is very fine, it may be difficult to
distinguish between pyroxene and amphibole. If this is the case, it is
appropriate to put “presence of pyroxene and/or amphibole” in your
description.
Glassy, light green (sometimes yellow or light orange) stubby crystals
massed together. Conchoidal fracture on some grains.
NOTES:
Most dark-coloured igneous rocks are rich in calcium plagioclase and ferromagnesian (ironmagnesium) minerals such as pyroxene and olivine. The word mafic refers to such rocks.
Ultramafic igneous rocks are composed entirely of ferromagnesian minerals.
Light-coloured or felsic igneous rocks commonly contain potassium feldspar, sodium plagioclase,
and quartz.
Intermediate igneous rocks are neither dark nor light and generally contain light-coloured
minerals (feldspars, some quartz) and dark minerals such as hornblende or biotite.
Pink feldspar is usually potassium feldspar, white or grey feldspars may be either potassium
feldspar or plagioclase – if twinning striations are present, it is plagioclase.
Amphibole cleavages do not intersect at 90°, but pyroxene cleavages do, amphibole typically are
elongate and have a splintery appearance whereas pyroxenes look blocky.
22
Lab manual for GEOG1120 Figure 8 The names of common igneous rocks are based on the minerals and texture of a
rock. A mineral’s abundance in a rock is proportional to the thickness of its band
beneath the rock name.
Magma Composition and Tectonic Setting
The composition of igneous rocks can vary between two end members:
a light coloured, acid (silica-rich) syenite and granite/rhyolite
a dark coloured, basic (silica-poor) gabbro/basalt
These end members represent extremes in the compositional variation of the parent
magma, which in turn reflects tectonic setting in which melting took place.
Granitic magmas tend to form in collisional plate boundaries (subduction zones) where
oceanic crust and overlying sediments descend into the mantle, gradually heating up
until the melting point of silica-rich minerals is reached (around 600° C). This silica-rich
magma then rises to produce deep-seated granitic intrusions and rhyolitic lava flows at
the surface. Similarly, at the root of mountain belts where high pressures are found, the
temperature can rise up to the melting point of silica-rich minerals and melt some of the
deformed (metamorphosed) rocks to produce granitic magmas.
Andesitic magmas also form at collisional plate boundaries. The magmas at subduction
zones form partly from a mixture of seafloor sediments and partly from melting of
basaltic and felsic crust.
23
Lab manual for GEOG1120 Basaltic magmas are formed at divergent plate boundaries as a result of the melting of
the asthenosphere. The asthenosphere is made up of peridotite, an ultrabasic, ultramafic,
ferromagnesian-rich rock that melts at a much higher temperature (around 1000° C) and
subsequently rises to the surface and solidifies as basaltic oceanic crust.
PROCEDURE FOR IGNEOUS ROCK IDENTIFICATION
Examine the numbered samples and fill out the Igneous Rock Identification forms in
your lab handout using the method below, the hints on page 22 and Figure 8 on page 23.
1. Examine the rock and determine the colour. You can refer to the rock as:
a. felsic - few dark minerals, generally a light colour
b. intermediate - nearly 50% dark minerals
c. mafic - over 70% dark minerals, very dark colour
2. Examine the rock and determine the type of texture. You can then refer to the
rock as:
a. glassy – smooth or frothy
b. pyroclastic – ash, lapilli or breccia
c. crystalline – aphanitic, phaneritic or porphyritic; specify the type of
minerals that make up the phenocrysts, if present
d. vesicular
e. amygdaloidal
f. pegmatitic
3. Determine the approximate percentage of quartz:
a. 10-40% quartz; granite-rhyolite family
b. <10% quartz; diorite-andesite family
c. no quartz; gabbro-basalt family
4. Determine the approximate percentage and type of feldspar:
a. pink feldspar is almost invariably a potassium (K) feldspar
b. white or grey feldspar may be either potassium feldspar or plagioclase
c. if they feldspar has striations it is definitely plagioclase.
5. Determine the percentage and type of dark minerals in the rock.
6. Use Figure 8 on page 23 and determine the rock name.
Referring to the notes in this lab, place each of your numbered unknown samples in a
tectonic setting.
Intrusive bodies are coarse-grained because they were cooled deep within the crust. How
can intrusive rocks now be exposed at Earth’s surface?
24
Lab manual for GEOG1120 SEDIMENTARY ROCKS AND WEATHERING AND SEDIMENTS
INTRODUCTION
Sedimentary rocks are produced on the surface of Earth by a chain of processes that
includes weathering, erosion, transportation, deposition, and diagenesis.
Sedimentary rocks are important for several reasons:
1. Although the volume of sedimentary rocks in Earth’s crust is small relative to
igneous and metamorphic rocks, they cover over 2/3 of Earth’s surface.
2. Sedimentary rocks contain virtually the entire record of Earth’s past life in the
form of plant and animal fossils. Igneous and metamorphic rocks, on the other
hand, form at temperatures too high and in environments not suited for living
organisms or for the preservation of fossil remains.
3. Sedimentary rocks are economically important because they contain almost all
of our fossil fuels (petroleum, natural gas, and coal), and many important ore
deposits (uranium, copper, lead, zinc, iron, manganese).
The purpose of this lab is to provide you with some insight into the processes that
produce sedimentary rocks and to help you identify the common sedimentary rocks.
Your lab assignment will consist of examining unknown rocks and recording their
textural and compositional characteristics.
There are three fundamental types of sedimentary rocks, based on textural features and
particle size:
Inorganic detrital
Chemical precipitates
Biogenic (organic)
derived from particles (clasts) of minerals and rocks that
have been separated from the parent material largely by
processes of physical weathering.
derived from the dissolved products of chemical
weathering, which have precipitated from solution by
inorganic processes (evaporates, travertine).
particles are derived from life activities of organisms (eg,
shell fragments, coal, chalk, chert).
We will look at the textures and compositions of rocks within each of these groups.
25
Lab manual for GEOG1120 FORMATION OF SEDIMENTARY ROCKS
The four steps required to make sedimentary rocks:
1.
2.
3.
4.
Weathering produces loose particles (sediment) from previously existing rocks.
Transportation moves those particles to an area where they can be collected.
Deposition causes the particles to settle to the bottom of the collecting basin.
Lithification transforms the sediment into a cohesive, solid rock again, via
various processes that may include compaction, cementation, and
recrystallization.
Figure 9 The processes involved to form sediments and sedimentary rocks.
26
Lab manual for GEOG1120 INORGANIC DETRITAL (CLASTIC) ROCKS
Textures of Clastic Rocks
Texture in sedimentary rocks is important because it provides important clues
concerning the distance that the sediment has been transported and the environment in
which it was deposited. The factors that define the texture of clastic rocks are grain size,
rounding, sorting, compaction, and cementation.
(a) Grain Size
The size of particles in clastic rocks ranges from large blocks several metres in diameter
to fine dust. A simple classification of clastic rocks is based on grain size:
Gravel
Sand
Silt or Clay
> 2 mm in diameter. Rocks, which contain clasts in this size
category are termed conglomerates (rounded clasts) or breccias
(angular clasts).
1/16 to 2 mm diameter. Rocks, which contain clasts in this size
category are termed sandstones.
< 1/16 mm in diameter. Rocks with grains in this size category are
called mudstones or if fissile (split easily) shales. Includes grains
from 1/256 to 1/16 mm (silt) and grains less than 1/256 mm (clay).
(b) Rounding
Rounding refers to the smoothness of the grains, which is a result of the amount of
abrasion that the grains have undergone during transport.
Sediment moved by ice or by the direct action of gravity is commonly angular, whereas
particles carried by wind or water are rounded by continual abrasion.
Also, large, angular boulders indicate a nearby source area in a mountainous terrain, as
significant transport by streams rapidly rounds off the rock corners and wears down the
boulder.
Figure 10 Terms used to define roundness of sediment. It is best to observe this
characteristic with a hand lens or microscope.
27
Lab manual for GEOG1120 (c) Sorting
Sorting is a measure of the range of grain sizes present in the rock. It is a very important
textural characteristic because it can provide clues concerning the history of
transportation and the environment in which the sediment accumulated.
Well sorted material comprises grains of one dominant size, and usually of one type
of composition. This usually reflects a uniformity of current flow, usually by wind
or streams.
Poorly sorted material contains grains of several different grain sizes. Glaciers and
mudflows typically deposit fine and coarse materials together, hence they are
poor sorting agents.
Figure 11 Terms used to describe sorting in sediments.
(d) Compaction
Compaction occurs as sediments are built up and the pressure of the overlying beds
squeezes water and air out of the loose sediment, often causing partial dissolution and
reprecipitation of minerals on the contacts between the grains. This helps to bind the
sediments together, one way by which unconsolidated sediment is converted to rock.
(e) Cementation
Cementation is the process by which mineral grains or shell fragments are bound
together by minerals precipitated from groundwater in pore spaces between the clasts.
The most common natural cements are calcite, quartz, hematite, and limonite.
A rock’s reddish or brown colour is usually enough to identify the iron oxide minerals
(hematite and limonite) as cements. Calcite cement is easily identified using a drop of acid.
Silica cement results in a very hard rock from which individual grains are dislodged only
with some difficulty.
28
Lab manual for GEOG1120 Composition of Clastic Rocks
The majority of clasts in clastic sedimentary rocks are composed of a few common
constituents, most of which you have seen in previous labs. These are:
Minerals
quartz, feldspars, less common silicates (eg, micas, garnet), clay
minerals (eg, kaolinite), iron oxides (magnetite, hematite)
Lithic clasts
fragments of other rocks
Biogenic clasts organically produced fragments such as shells or plant remains
Maturity of Clastic Rocks
Maturity is a term that is used to describe the stability of the sediments in a clastic rock,
which may be related in part to the distance the sediment has been transported or the
time that has elapsed between weathering and deposition. The rounding, sorting, and
mineral composition of clastic rocks all can be used to describe the maturity of a
sediment.
Chemically mature rocks are those, which contain large quantities of quartz or clay.
Quartz is the most stable of all silicate minerals, whereas the ferromagnesians (like
olivine, pyroxene, biotite) having higher crystallization temperatures, alter rapidly to
clay minerals under surface weathering conditions. The dominant presence of quartz or
clay in a sediment, therefore, suggests that considerable time has passed in order for the
other less stable silicate minerals to break down.
Mechanical maturity is exhibited by sediments which have undergone considerable
abrasion during transportation, or have been deposited a long distance from their source.
Thus a well-rounded, well-sorted sandstone is considered mechanically mature, whereas
a poorly-sorted, angular breccia is considered mechanically immature.
Mudstone and quartz sandstone are the most mature of all clastic rocks, being well
sorted and composed largely of stable minerals (clay and quartz).
The concept of maturity is not easily applied to, nor useful for, classifying chemical
sedimentary rocks.
29
Lab manual for GEOG1120 Grain size class
and diameter
gravel (> 2 mm)
sand (1/16 – 2 mm)
Composition
quartz, feldspar,
rock fragments, and
clay minerals
mostly quartz
feldspar >25%
mostly lithic
fragments
>50% silt (1/2561/16 mm) and <50%
clay (<1/256 mm)
>50% clay (<1/256
mm) and <50% silt
(1/256-1/16 mm)
fine quartz grains,
clays
Texture
rounded grains
angular grains
all types of
rounding and
sorting
usually poorly to
moderately sorted
usually poorly
sorted
nonfissile
(compact)
fissile (splits easily)
Rock name
CONGLOMERATE
BRECCIA
QUARTZ
SANDSTONE
ARKOSIC
SANDSTONE
LITHIC
SANDSTONE
SILTSTONE
SHALEY
SILTSTONE
MUDSTONE
clay, organic
nonfissile
particles (may be
coloured by
impurities)
such as carbon
fissile
>90% clay (<1/256
CLAYSTONE or
(black), reduced
mm)
SHALE
iron (green), or
oxidized iron (red)
Table 3 Common clastic rocks according to composition and texture.
30
Lab manual for GEOG1120 CHEMICAL ROCKS
Textures of Chemical Precipitates
(a) Crystalline texture
Crystalline texture is in marked contrast to the clastic textures already described. The
minerals precipitated from seawater, groundwater, or lakes from this texture, which
consists of a network of interlocking crystals similar to the texture in some igneous
rocks. However, we can assign a similar size classification to chemical rocks as we do to
clastic rocks, substituting the word crystalline for grained. Thus you have the following:
coarse crystalline
medium crystalline
fine or microcrystalline
cryptocrystalline
> 2mm
1/16 – 2 mm
1/256 – 1/16 mm
< 1/256 mm
Deposits from springs and cave-formed rocks are commonly microcrystalline with a
banded appearance that results from chemical variations and impurities during
deposition.
(b) Skeletal texture (fossiliferous)
Skeletal texture is formed by the accumulation of skeletal parts of invertebrate marine
life. Calcium carbonate is removed from seawater by marine organisms to make their
shells and other hard parts. When the organisms die, their shell material settles to the
sea floor and may be concentrated as shell fragments on a beach or near a reef. The
texture of the resulting rock is similar to a classic texture, but the material is unique in
that it consists of the skeletal fragments of organisms that originate in the environment
of deposition. Skeletal texture predominates in many limestones.
(c) Oolitic texture
Oolitic texture resembles sand, but the individual grains have concentric layers. This
layering is produced when calcium carbonate, precipitated on the sea floor, is agitated by
wave or current action and accumulates around a tiny shell fragment or grain of silt. As
the particle moves to and fro, thin spherical layers are built up by accretion. Close
examination of an oolitic texture reveals a concentric structure around each nucleus, and
generally a minor amount of associated shell debris.
31
Lab manual for GEOG1120 Composition of Chemical Rocks
Chemical sedimentary rocks are divided into four rock groups, based on their
composition and mode of formation.
(a) Carbonates
Carbonates consist of 50% or more carbonate minerals (calcite and dolomite), and
comprise the most abundant group of chemical sedimentary rocks. Their formation is
closely linked to the growth and death of organisms. Many aquatic organisms are able to
extract calcium carbonate dissolved in the water to form a protective shell or skeleton.
When these organisms die their shells or skeletons accumulate as sediments, which later
become cemented by calcite.
Carbonate rocks, therefore, are often rich in fossils and the fossils may provide
information on the depositional environment of the host rock (eg, presence of reef fossils
in a limestone suggests that the rock was formed in a shallow marine environment). A
sedimentary rock containing abundant visible fossils may have the adjective
“fossiliferous” attached to it, as in a fossiliferous limestone.
Examples of carbonate rocks are limestones and dolostones. Limestone that has been
precipitated from fresh water, such as in springs or geysers, is called travertine.
(b) Evaporites
Evaporites are composed of minerals, which precipitated out of a solution (eg, seawater)
because of solar evaporation. The most common examples of evaporate minerals include
halite (NaCl), sylvite (KCl), gypsum (CaSO42H2O), and anhydrite (CaSO4). Even the
carbonate minerals calcite and dolomite can form by evaporation, although
biochemically produced carbonate is much more abundant.
The presence of evaporates usually indicates an environment in which evaporation
exceeds precipitation, or in which seawater is prevented from mixing with the open
ocean such as might occur in a shallow or restricted sea.
(c) Siliceous rocks
Siliceous rocks are composed mainly of polymorphs of microcrystalline quartz (SiO2),
formed when silica dissolved in seawater precipitates. These include chert (light colour),
flint (dark), jasper (red), and agate (banded). Chert commonly occurs as nodules in
limestone. Like other varieties of quartz, siliceous rocks are characterized by a hardness of
7 and conchoidal fracture.
32
Lab manual for GEOG1120 Siliceous sedimentary rocks may have either an inorganic or biochemical origin.
Migrating groundwater may deposit large amounts of chert in the pore spaces of
limestone. Some cherts are thought to be derived from layers of silica in sediments
underlying high diatom or radiolarian abundance in the ocean (these are microscopic
organisms that extract silica from seawater to build their protective skeletons).
(d) Iron formations
Iron formations are a group of rocks that formed in the Precambrian Era when Earth’s
atmosphere was much lower in oxygen that it is today. They are chemical precipitates of
hematite, magnetite, and limonite, and may have a layered, oolitic, or compact
structure.
Rock group
Composition
Texture & diagnostic
features
medium to coarse crystalline =
crystalline; white, grey
microcrystalline = micrite
(lime mud); commonly black
Calcite (CaCO3)
aggregate of oolites = oolitic
Carbonate
abundant fossils in calcite
matrix = fossiliferous
banded calcite = travertine;
effervesces with HCl; may be
colour banded
Dolomite (CaMg[CO3]2) textural varieties similar to
limestone; powder effervesces
weakly with cold dilute HCl
Siliceous
Silica (SiO2)
cryptocrystalline, dense, with
conchoidal fracture
Gypsum (CaSO42H2O) fine to coarse crystalline; can
be scratched with fingernail;
Evaporite
white, buff
Halite (NaCl)
cubic cleavage; massive,
granular, compact, colourless,
white
Iron formation Hematite, magnetite,
variable; may be crystalline,
limonite
oolitic, earthy, or banded
Table 4 Chemical sedimentary rocks by composition and texture.
Rock
name
Limestone
Dolostone
Chert
Gypsum
Rock Salt
Ironstone
33
Lab manual for GEOG1120 BIOGENIC (ORGANIC) ROCKS
Organic rocks are not rocks in the strict sense because they are composed primarily of
organic material and not minerals. These deposits form in special environments in which
either the rate of accumulation of organic debris is very high, or the environment is such
that the decomposition of organic matter is very slow. Such environments may include
swamps, marshes, or stagnant lagoons.
Peat and coal are derived from plant debris that accumulated at various times during
Earth’s history. With each successive stage in coal formation, higher temperatures and
pressures drive off impurities and volatiles, and result in increasing carbon content,
hardness, and lustre.
peat
(partially
altered plant
material)
Rock group
Carbonate
Organic
burial

lignite
(soft,
brown
coal)
Composition
Calcite (CaCO3)
mostly carbon (C)
further burial

bituminous
(soft, black
coal)
metamorphism

Texture & diagnostic features
bioclastic; calcareous shell
fragments in a massive or crystalline
matrix; effervesces with HCl
bioclastic; fossils and fossil
fragments loosely cemented =
Coquina
earthy, (bioclastic); shells of
microscopic organisms, clay-sized,
soft; white; effervesces with HCl
fibrous, (bioclastic); earthy plant
remains; brown; soft, porous
dense, black, varying degrees of
lustre and hardness
anthracite
(hard, black
coal)
Rock name
Limestone
Chalk
Peat
Coal
(lignite,
bituminous,
anthracite)
Table 5 Biogenic (organic) sedimentary rocks by composition and texture.
SEDIMENTARY STRUCTURES
Features called sedimentary structures that formed during transportation or deposition
often reveal the environments in which clastic rocks were deposited.
34
Lab manual for GEOG1120 (a) Bedding or Stratification
The most common sedimentary structure is a bed, or stratum, a horizontal layer of
sediment that represents a deposition under similar conditions over a period of time.
A bed differs from those above and below it in ways that may be subtle (slight change in
grain shape, size, or sorting) or obvious (eg, change from green to red or black colour, or
from mudstone to conglomerate).
Many beds are homogenous from top to bottom,
but in some cases there is a systematic change in
grain size, from coarse at the bottom to fine-grained at
the top. This is called a graded bed, and results
when a mixture of different sediments is
deposited all at one time, such as in a turbidity
current. The coarsest material, being heaviest, is
deposited first, followed, by increasingly smaller
grains, and finally the very fine suspended
material. A graded bed is useful to geologists
studying highly deformed rocks, because it can
be used to tell which side of the bed was
originally the top and which was the bottom.
Figure 12 A typical graded bed.
(b) Cross-bedding (cross-stratification)
Most strata are deposited in nearly horizontal sheets. However, some stratification is not
horizontal at the time of deposition, and is referred to as cross-bedding. For example,
sediment transported in a single direction by water or air currents commonly forms
bedding planes that represent the slip faces of dunes or ripples.
The following diagrams illustrate the difference in appearance between horizontal and
cross-bedding.
Figure 13 Horizontal stratification.
Schematic vertical profile of a deposit of
horizontal strata.
Figure 14 Cross-bedding. Nonhorizontal layering that is found in
dunes and sand ripples.
35
Lab manual for GEOG1120 (c) Ripple Marks
When sand grains are deposited in the presence of either air or water currents, they
frequently develop a surface with undulating features, called ripple marks.
Ripple marks may be preserved either on the upper surface of the stratum, which
exhibits the cross-bedding, or on the lower surface of the overlying bed (which itself may
be either horizontally- or cross-stratified). Asymmetric ripple marks are especially useful
for determining paleocurrent directions, as the direction of the current is at right angles
to the crest of the ripple mark.
Figure 15 Asymmetrical ripple marks form
when the current flow is always in the
same direction, and thus have a steep
downstream side and a gently sloped
upstream side.
Figure 16 Symmetrical ripple marks form
where currents move back and forth, as
on a tidal flat or nearshore beach face.
Both slipfaces on symmetrical ripple
marks are equally steep.
(d) Sole Marks
Sole marks are a general term for sedimentary structures commonly found on the bases of
beds formed by scouring or dragging of material in the lower part of the depositional
flow. Tool marks are a class of sole marks, which may form on bedding planes when
shells, sticks, pebbles, or bones slide, roll or bounce along a surface. Tool marks appear as
holes or linear drag marks. Flutes are scoop-shaped or V-shaped depressions scoured
into a surface by the erosional, winnowing action of currents. Overlying beds will form
natural casts of these depressions; such structures are called flute casts. As is the case
with ripple marks, flutes and flute casts may be used to indicate paleocurrent direction.
(e) Mud Cracks and Raindrop Impressions
Many sedimentary rocks also contain structures that formed shortly after deposition of
the sediments that compose them. For example, desiccation cracks often form while
moist deposits of mud dry and shrink. These structures are often preserved when an
overlying bed of a different lithology penetrates the cracks, creating a network of mudcrack polygons that are distinct in colour or grain size from the surrounding material.
Raindrop impressions may form on terrestrial surfaces, usually on a soft material such as
mud. The result is numerous small depressions on the upper surface of the bed.
36
Lab manual for GEOG1120 (f) Trace Fossils
Animals make tracks, trails, and burrows, which can also be preserved in sedimentary
rocks. Such traces of former life are called trace fossils.
Figure 17 The three types of trace fossils of trilobites.
Trace fossils can provide a lot of information on animal behaviour and allows a better
understanding and reconstruction of past life forms.
SEDIMENTARY ENVIRONMENTS
Sediments are deposited in many different environments. They have been studied in
modern situations so that characteristic sediments, sedimentary structures, and fossils
are known for each one. The information gained from grain characteristics, sedimentary
structures, and fossils can be used to infer what ancient environments were like in
comparison to modern ones.
The term sedimentary environment refers to the place where the sediment is deposited,
and to the physical, chemical, and biological conditions that exist in that place. The
following brief summary describes some of the important characteristics of the
sedimentary rocks formed in each environment.
(a) Alluvial Fans
They are stream and sediment gravity flow deposits that accumulate near a mountain
front in a dry basin. They typically contain poorly sorted coarse arkosic sandstones,
gravel and boulders. These sediments may form in coarsely cross-bedded, lens-like
channel deposits. Fine grained sand and silt may be deposited near the margins of the
fan.
37
Lab manual for GEOG1120 (b) River Channel and Floodplains
Rivers deposit elongate lenses of conglomerate or sandstone in channels. Floodplains are
nearly flat expanses across which the rivers of the world typically meander before
reaching the sea. A considerable amount of sediment is deposited on these plains. Rocks
formed in a floodplain environment are commonly channels of cross-bedded sandstone
deposited on the point bar of a meander and enclosed in shale beds, which represent the
finer horizontally deposited flood deposits.
(c) Lakes
They are characterized by thin-bedded shales, perhaps containing fish fossils. If the lake
dries up periodically the shales may show mud cracks and evaporite interbeds.
(d) Aeolian Environments
Wind transports and deposits material in these environments. Wind is an effective
sorting agent and will selectively transport sand and dust, leaving gravel behind. Dustsized particles are lifted high in the atmosphere and may be transported thousands of
miles to where they accumulate as a blanket of loess. Windblown sand commonly
accumulates in dunes that are characterized by well-sorted, fine grains. The dominant
sedimentary structure in aeolian environments is large-scale cross-bedding.
(e) Glaciers
They transport but do not effectively sort material. Thus glacial environments are
characterized by unsorted, unstratified accumulations of angular boulders, gravels, sand,
and fine silt, called glacial till. There are also fluvial and lacustrine deposits associated
with glaciations.
(f) Deltas
These are large accumulations of sediment that are deposited at the mouths of rivers. A
delta is one of the most significant sedimentary environments and includes a number of
subenvironments such as stream channels, floodplains, beaches, bars, and tidal flats. The
deltaic deposit as a whole consists of a thick accumulation of sand, silt and mud, which
fines outwards and downwards (contrast this fining-downward appearance with graded
bedding, found in deep ocean environments).
(g) Shoreline environments
They are characterized by beaches, bars, and spits that commonly develop along low
coasts. They may contain quiet water lagoons and tidal flats. The sediment in these
features is well washed by wave action and is typically clean, well-sorted quartz sand.
Behind the bars and adjacent to the beaches, fine silt and mud are often deposited in tidal
flats.
38
Lab manual for GEOG1120 (h) Organic Reef
Reefs are solid structures comprising shells and secretions of marine organisms. The reef
framework is typically built by corals and algae, but many other types of organisms
contribute to the reef community. Together, these organisms produce a highly
fossiliferous limestone.
(i) Shallow Marine Environments
These are found along the continental margins, and in the past were even more extensive
than today. Sediments deposited in shallow marine waters form extensive layers of wellsorted sand, shale, and limestone, which typically occur in a cyclical sequence as a result
of shifting environments from changes in sea-level.
(j) Deep Ocean Environments
If they are adjacent to the continents, they receive a considerable amount of sediment
transported from the continental margins by turbidity currents. Deep sea deposits are thus
characterized by a sequence of graded beds.
Figure 18 The main sedimentary depositional environments.
39
Lab manual for GEOG1120 METAMORPHIC ROCKS
INTRODUCTION
During this lab period you will study a third major group of rocks, the metamorphic
rocks. You will learn the minerals and mineral assemblages found in metamorphic rocks
and how different metamorphic textures form, and you will see how earth scientists use
metamorphic rocks to decipher aspects of Earth’s history.
Metamorphic rocks are important because:
They are common in regions that have undergone significant deformation.
Geologists are interested in these regions because they may mark the positions of plate
collisions that took place millions of years ago. As with intrusive igneous rocks, erosion
strips off as much as thousands of metres of overlying strata to reveal these rocks that
formed deep in Earth’s crust. Vast areas of metamorphic rock form the cores of every
continent and are present in linear mountain belts, such as the Appalachians, Rockies,
Himalayas, and Alps, which formed as a result of the collision of lithospheric plates. Thus
metamorphic rocks reveal the processes by which continents and mountains form. They
also record the conditions and processes that occur within Earth, and thus provide clues
to the nature of parts of the crust that we have never seen.
A number of economically valuable products are either directly or indirectly related
to metamorphic processes. In some cases, these products are the rocks themselves, such
as marble, slate, or talk; in other cases valuable minerals such as tin and tungsten ores
can be extracted from metamorphic rocks. Diamond, the hardest mineral in the world, is
the product of intense metamorphism of carbon deep in Earth’s crust.
CHARACTERISTICS OF METAMORPHIC ROCKS
The process of metamorphism is similar to the processes that form sedimentary rocks in
that they observed changes in mineral composition and texture are the result of the
rock’s response to different conditions of temperature and pressure. While the processes of
chemical weathering proceed under conditions of reduced (surface) temperature and
pressure, metamorphic changes occur because of increased temperatures and/or
pressures. These conditions are most often found at depth in Earth’s crust.
The original rock may be sedimentary, igneous, or an older metamorphic rock (although
igneous and metamorphic rocks are generally less affected than sedimentary rocks). In
the process, original textural features such as bedding may be obliterated, replaced by a
texture, which reflects that metamorphic processes involved.
The effects of metamorphism include:
40
Lab manual for GEOG1120 (a) Chemical recombination and growth of new minerals, with or without the
addition of new elements from circulating fluids and gases.
(b) Deformation and rotation of the constituent mineral grains.
(c) Recrystallization of minerals to form larger grains.
The net result of any or all of these effects are rocks of greater crystallinity, increased
hardness, and new structural features that commonly exhibit the effects of flow or other
expressions of deformation.
SCALES OF METAMORPHISM
There are two main scales at which metamorphic processes occur:
(a) Contact Metamorphism
Occurs locally beneath lava flows, adjacent to igneous intrusions or along fracture that
are in contact with hot gases. Although extremely high temperatures may occur,
metamorphism always occurs below the melting point of rocks. If melting occurs, the
process in considered igneous. The intensity of contact metamorphism is greatest at the
contact between host rock and magma, and decreases rapidly with distance from the
magma. Thus zones of contact metamorphism (known as aureoles) are relatively
narrow. This aureole or halo of metamorphism is characterized by the formation of hightemperature minerals close to the contact zone and progressively lower temperature
minerals as the distance from the contact zone increases. The size of the aureole depends
on the temperature and size of the pluton, mineralogy of the host rock, and the presence
or absence of fluids.
(b) Regional Metamorphism
Occurs over extremely large areas and is associated with mountain building. As rocks are
buried deep within the crust, they are subjected to changes in temperature, pressure, and
chemical conditions. They reach equilibrium with the new conditions by developing in
the solid state a new set of minerals that often shows a degree of preferential orientation
or foliation.
METAMORPHIC GRADE
The grade of metamorphism reflects the extent to which a metamorphosed rock is
different from the parent rock from which it is derived. The higher the temperatures and
pressures to which the parent rock is exposed, the higher the grade of the
metamorphosed rock. Mineral assemblages and textures reflect the metamorphic grade
41
Lab manual for GEOG1120 reached while the chemical composition remains essentially unchanged with the
exception of water and gasses. The grain size of metamorphic rocks commonly, but not
always, increases with grade.
The temperature range in which metamorphism occurs is between 100° and 800° C. The
range of pressures acting on rocks is more or less unlimited, as pressures increase linearly
with depth. However, for the purposes of classification, we use a range of 1-10 kilobars (a
bar is equal to standard atmospheric pressure). Thus, one kilobar is equal to one
thousand times normal atmospheric pressure.
Combining these two parameters, we assign a metamorphic grade to a regionally
metamorphosed rock, which tells us the intensity of the temperatures and pressures that
have been applied to the rock.
Metamorphic Grade
Low
Intermediate
High
Temperature
100-300° C
300-500° C
500-800° C
Pressure
1-3 kilobars
3-6 kilobars
6-10 kilobars
Table 6 Metamorphic pressure and temperature conditions as related to grade.
In some cases, temperature and pressure do not increase concurrently. For example,
contact metamorphism often occurs under conditions of high temperature and low
pressures, such as beneath a lava flow. And at subduction zones metamorphism results
from high pressures and (relatively) low temperatures.
Figure 19 Some of the environments where metamorphic rocks form.
42
Lab manual for GEOG1120 TEXTURES OF METAMORPHIC ROCKS
In terms of their appearance, low grade metamorphic rocks look like their parent rock or
protolith but with mineral alteration giving them a different colour (like greenschist).
However, high grade metamorphic rocks more closely resemble igneous rocks than
sedimentary rocks. Both igneous and high grade metamorphic rocks tend to have
crystalline textures, the difference being that metamorphic rocks develop their crystalline
texture as existing minerals recrystallize to form new mineral phases without passing
through the liquid state.
Like igneous and sedimentary rocks, metamorphic rocks are classified on the basis of
texture and composition. Two main groups of textures are recognized, foliated and nonfoliated.
Foliation results from recrystallization and the growth of new minerals oriented in such
a way as to produce a layered, or leafy effect. Non-foliated texture develops by
recrystallization of rocks composed predominantly of one mineral, such as sandstone
(quartz) or limestone (calcite).
FOLIATION
Foliation is a planar element in metamorphic rocks. It develops in response to directed
stress and results in the preferred orientation of platy minerals such as micas,
amphiboles, and others along lines of least stress. The direction of foliation is
perpendicular to the direction of the stress that acted upon the rock. When foliation is
defined by mica grains, it is called cleavage. Different types of foliation develop with
increasing metamorphic grade:
(a) Slatey Cleavage
The tendency of some rocks to fracture along nearly perfect, flat parallel planes. Under
low grade metamorphism, very fine-grained platy minerals (such as micas, talc, and
chlorite) commonly develop in slate. This metamorphic feature is not to be confused with
bedding planes, which are a sedimentary structure.
(b) Phyllitic Texture
A parallel (but wavy or wrinkled) foliation of fine-grained (occasionally medium
grained) platy minerals (mainly micas, chlorite, and graphite), exhibiting a silky or
metallic lustre. It is developed best in phyllite, which is the product of a relatively low
grade of metamorphism.
43
Lab manual for GEOG1120 (c) Schistosity
A parallel to subparallel foliation of medium to coarse grained platy minerals (mainly
micas and chlorite). It is commonly developed in schists, which are the product of
intermediate to high grades of metamorphism.
(d) Gneissosity
A parallel to subparallel foliation of medium to coarse grained minerals in alternating
bands of different composition. Ferromagnesian minerals such as hornblende or
pyroxene usually form the dark bands and quartz and feldspars usually form the light
bands. Most gneisses are the product of intermediate to high grade metamorphism.
NON-FOLIATED TEXTURES
Non-foliated textures typically result from the growth of existing minerals to form a
more interlocking structure. They do not contain parallel planes of minerals; however,
they may have stretched fossils or other grains that formed in response to directed
metamorphic stresses. Examples of metamorphic rocks having non-foliated textures are
marble and quartzite.
Metamorphic rocks, which contain mineral grains all of one size are said to exhibit an
equigranular, or granoblastic, texture. The grains are evenly shaped (sugary) in texture
and typically without foliation.
Hornfelsic texture is a fine-grained sugary texture found in dark coloured hornfelses
(rocks formed at the innermost part of contact metamorphic aureoles).
OTHER TEXTURES
(a) Porphyroblastic Texture
Some metamorphic rocks contain large crystals (porphyroblasts) set in a finer-grained
groundmass, analogous to phenocrysts in igneous porphyries. It may be present in
foliated or non-foliated metamorphic rocks. Often the porphyroblasts are non-platy
minerals such as garnet or magnetite.
(b) Lineation
Lineation may develop in non-foliated rocks. Pebbles and sand grains that were
originally spherical may be stretched by the directed stress within the rock. Deformation
of limestone will often produce streaks of organic debris. These linear features are not
true foliation, though they often give a banded appearance to the rock. An example of
this feature can be seen in a stretched-pebble conglomerate.
44
Lab manual for GEOG1120 (c) Folds and Crenulations
Folds (bends) or crenulations (parallel sets of very tiny folds up to 1 cm long) are also a
common feature of metamorphic rocks. Many gneisses contain these deformation features.
COMPOSITION OF METAMORPHIC ROCKS
Mineral Changes
Various mineralogical changes occur in rocks as they are metamorphosed. The new
minerals that appear represent a crystalline configuration that is more stable at the
increased temperature and/or pressure. The most common changes we observe are:
(a) Recrystallization
Occurs when small crystals of one mineral slowly convert to fewer, larger crystals or the
same mineral without melting of the rock. For example, microscopic muscovite crystals
in a slate can recrystallize to a larger size (but still microscopic) in phyllite and an even
larger size (macroscopic) in schist. These changes indicate increasing grade of
metamorphism.
(b) Neomorphism
Is the process by which minerals not only recrystallize, but also form different minerals
from the same chemical elements. In most, the atoms present in the parent rock are the
only ones that metamorphism can rearrange to create new minerals. The resulting rock is
therefore strongly controlled by the nature of the parent rock, or protolith.
The mineral composition of a metamorphic rock depends on the composition of the
parent material. Figure 20 illustrates the typical transition in mineralogy that occurs as a
clay-rich mudstone undergoes progressive neomorphism, and the metamorphic rocks
that result. A basal, which contains mostly mafic minerals, will produce a different suite
of minerals during metamorphism than will the shale.
Figure 20 Progressive metamorphism of a shale.
45
Lab manual for GEOG1120 Some minerals (quartz, feldspars, micas) that you have already seen in sedimentary and
igneous rocks are common in many types of metamorphic rocks. Others are important
indicator minerals because they reflect a specific chemical composition and/or intensity
of metamorphism. Some minerals (staurolite, cordierite, andalusite, sillimanite, kyanite)
originate only in metamorphic rocks.
(a) Chlorite (Mg, Fe, Al)12[(Si, Al)8O20](OH)16
Various shades of green; common in metamorphosed basalts and Mg-Al-rich rocks; silky
lustre; streak white or pale green; hardness = 2; commonly has micaceous cleavage;
tabular crystals or scales
(b) Garnet (alumino-silicates of Ca-Mg-Fe-Cr-Mn)
Red, yellow, green; vitreous to translucent; hardness = 6.5-7.5; no cleavage, subconchoidal
fracture; crystals are 12 sided or equidimensional
(c) Staurolite Fe2Al9O6(SiO4)4(OOH)2
Red-brown, black-brown, yellow-brown; dull lustre; hardness = 7-7.5; one cleavage
parallel to long axis, also conchoidal fracture; prismatic crystals
(d) Kyanite Al2SiO5 [Andalusite-Kyanite-Sillimanite series]
These three minerals or identical composition are stable at different pressure and
temperature conditions. Kyanite and sillimanite are elongate minerals while andalusite
tends to form shorter prismatic crystals.
Kyanite is blue-green or blue-white; pearly, translucent to transparent; hardness = 4-7
(varies from face to face, can be scratched with a knife parallel to the length but not
across it); bladed crystals
Rock name
Slate
Phyllite
Schist
Gneiss
Dominant minerals
quartz, feldspar, mica,
chlorite, clay
quartz, feldspar, mica,
chlorite
chlorite, biotite, muscovite,
garnet, hornblende,
staurolite, kyanite,
andalusite, sillimanite;
porphyroblasts common,
used in name
quartz, feldspar in light
coloured bands; dark bands
may be hornblende, augite,
garnet or biotite, kyanite,
andalusite, sillimanite
Texture/Structure
very fine grained; has slatey
cleaveage; dull sheen on
cleavage; black, grey, red,
purple, green
fine grained, dense; phyllitic
texture; conspicuous
sparkly sheen due to mica
fine to medium grained
with directed or schistose
texture
Derived from
mudstone, shale,
tuff
coarse grained with layered
or gneissose texture; often
blocky minerals lend it an
“eyed” appearance, ie
“augen gneiss”
any rock; often
plutonic igneous
(granite) or
sedimentary
clastic
mudstone, shale,
tuff
any rock; most
commonly
mudstone, shale,
tuff
Table 7 Foliated metamorphic rocks.
46
Lab manual for GEOG1120 Rock name
Greenstone
Dominant
minerals
chlorite,
epidote,
Na-plagioclase,
±calcite,
±actinolite,
±quartz
quartz, ±mica,
feldspars
Texture/Structure
Derived from
aphanitic to fine grained;
nondirected texture
mafic to
intermediate
volcanic rocks
(basalt, andesite);
low grade
metamorphism
quartz sandstone or
chert
fine to medium grained;
nondirected texture;
involves recrystallization
of existing quartz sand
grains resulting in a more
interlocking crystalline
structure, schistose if
mica present; difficult to
tell grade
calcite,
fine to coarse grained;
Marble
dolomite,
nondirected texture;
±mica
schistose if mica present;
difficult to tell grade
original pebbles
Stretched-pebble quartz,
[feldspars,
distinguishable, but
conglomerate or
strongly deformed
Metaconglomerate micas]
talc, amphibole fine to medium grained;
Soapstone
minerals
soapy feel; white to grey
carbon
soft, dark grey, with
Graphite
greasy feel
carbon
bright, hard coal; breaks
Anthracite Coal
with conchoidal fracture;
glassy lustre
hornblende,
medium to coarse
Amphibolite
plagioclase,
grained; nondirected
garnet, quartz texture; can be gneissose
pyroxene,
fine grained; grey, greyHornfels
plagioclase
green, black; result of
contact metamorphism
Table 8 Non-foliated metamorphic rocks.
Quartzite
limestone or
dolostone
conglomerate
peridotite
anthracite coal
bituminous coal
mafic igneous rocks
basalt or gabbro
47