Download Igneous Rock Textures.

IGNEOUS ROCK
TEXTURES
Prepared by Dr. F. Clark,
Department of Earth and Atmospheric
Sciences, University of Alberta
Oct. 05
ROCK TEXTURES DEFINED
The use of the term “texture” as applied to rocks is not at
all intuitive. One might expect rock textures to include
possibilities such as smooth, rough, silky, hard, soft,
waxy, or other such properties, but this is not the case.
Whether applied to igneous or other rocks, the term
texture embraces the size, shape, and arrangement of
mineral grains [the general term we use rather than
“crystals”, although mineral grains are crystals, by
definition]. It is generally by the texture that a geologist
will first recognize whether a rock is igneous,
sedimentary, or metamorphic, before they get down to
the business of identifying the rock or naming it.
IGNEOUS ROCK TEXTURES PRINCIPLE
The fundamental principle behind igneous rock textures is
that grain size is controlled by cooling rate. Thus, rapid
cooling at the Earth’s surface of extrusive molten
material, or lava, results in the growth of smaller
crystals, or prevents crystal growth altogether.
Conversely, slow cooling within the Earth’s crust of
intrusive molten material, called magma, results in the
growth of fewer but larger crystals, because atoms are
able to migrate through the liquid to attach themselves
to crystals that have already begun to form. The many
igneous rock textures are simply variations on or
modifications of this principle.
Igneous Extrusive, or Volcanic, Rocks
These rocks, formed by cooling and solidification (not
always crystallization) of lava, are typically fine-grained,
to the extent that detailed analysis of the mineralogy of
these rocks is only possible with the high magnification
of a petrographic microscope, or even higher resolution
techniques. As we shall see in the file on igneous rock
classification, the colour of the rock is an important clue
to its bulk or average composition, which controls its
mineralogy, and other clues may tell us what minerals
are present.
Aphanitic
Crystals are
uniformly finegrained and
interlocking,
with individual
crystals
invisible to the
unaided eye.
This is the generic, base texture for most igneous extrusive rocks, the
fine-grained nature being a result of rapid cooling that prevents growth
of large crystals. Even with a hand lens, virtually nothing can be
identified, and such rocks can be very frustrating to deal with.
Porphyritic
Large, evident
crystals called
phenocrysts
[red arrows]
are surrounded
by an aphanitic
matrix or
groundmass
[blue arrows].
This texture represents two stage cooling. Slower cooling of magma
within the crust leads to growth of the phenocrysts, whose early
growth leads to the development of well-formed faces. Once erupted
as lava, the remaining liquid crystallizes as the aphanitic groundmass.
Vesicular
Vesicles are
simply bubbles
produced by
gases escaping
from lava as it
solidifies.
The vesicles [red arrows] are in a full range of sizes, enclosed in an
aphanitic groundmass [blue arrows]. The few slightly larger crystals
[green arrows] are of the mineral olivine, a common constituent of
Hawaiian basalt volcanic rocks such as this one.
Vesicles
and
Xenoliths
This sample from
the previous slide
shows a more
complex face. The
red, blue, and
green arrows are
the same as in the
previous slide.
Volatiles dissolved in magma under high pressure within the Earth
escape when lava is erupted, like CO2 from soda pop. A xenolith
[literally, “foreign rock”, purple arrow] is a fragment of a previously
crystallized rock incorporated in the magma as it nears the surface.
Scoria – a Highly Vesicular Basalt
If basalt, which is a mafic [high Fe+Mg content, low silica content]
volcanic rock, is highly vesicular, then we have scoria, which for lack of
a better way to describe it, resembles the guts or interior of an Aero
chocolate bar. Note how the iron content in this chemically unstable
mafic rock weathers to produce a rust-coloured surface [brown arrows],
versus the greenish grey fresh surface [cut surface; green arrows].
Pumice
This is an
extremely
vesicular felsic
[low Fe+Mg, high
silica content]
volcanic rock. This
material will
actually float on
water because it
has so many
bubbles or
vesicles.
Gases do not escape easily from high viscosity [stiff] felsic lavas, and
so a light-coloured volcanic froth is produced. Pumice is used in the
cosmetic industry as an exfoliant, effective at removing calluses due to
the abrasive nature of the glass-hard frothy surface.
Amygdaloidal – With Filled Vesicles
It may be that a vesicular rock, such as this basalt [red arrows point to
vesicles set in the aphanitic groundmass highlighted by blue arrows],
has fluids circulating through the vesicles, from which minerals may
precipitate or crystallize. These deposits, with rounded outlines
reflecting their origin as bubble-filling, are called amygdules [purple
arrows]. Phenocrysts would be angular in outline.
Glassy
Very rapid cooling
may prevent
crystal growth
altogether, and we
get natural
volcanic glass,
called obsidian.
Note the
conchoidal
fracture [green
arrows] we would
expect for glass.
Note the contrast between the black fresh surface [yellow arrows] and
the rusty brown weathered surface [red arrows], which reveals the
minor iron content that stains the glass black. This is a black igneous
rock whose colour index [% mafic minerals] is nevertheless zero.
Glassy
This texture is
more likely in
felsic [high silica
content, not
necessarily as
quartz] lavas,
which are viscous
or stiff. This sharp
natural glass can
be fashioned into
effective weapon
tips.
Obsidian is an exception to the definition of rocks, which among other
things are aggregates of one or more minerals, which by definition are
crystalline solids. Glass is an amorphous solid, which means it lacks the
ordered arrangement of atoms that characterizes crystals.
Igneous Intrusive, or Plutonic, Rocks
The coarse crystal size associated with slow cooling means
that the hand specimen properties of minerals can be
easily applied and exploited to identify the minerals
present in the rock. This may extend as far as being able
to recognize cleavage intersection angles in the few
crystals that may be favourably oriented in the sample
under consideration.
Phaneritic – With Evident Crystals
Igneous intrusive rocks have evident crystals [the Greek word phaneros
means visible or evident] that one can easily distinguish with the
unaided eye, even if one doesn’t have the skill to identify what minerals
they are. In these samples, one sees grey glassy quartz, black biotite
and amphibole, and cream-coloured potassium feldspar.
Phaneritic – the Igneous Intrusive Staple
Although these samples have smaller crystals than the previous two,
the individual crystals or mineral grains are still readily distinguished
without magnification. The coarse crystal size makes many igneous
intrusive rocks quite attractive, and they are also durable and
reasonably stable chemically. This makes them good choices for grave
markers and facing stone for buildings.
Phaneritic – Seeing Cleavage on Grains
The coarse grain size of phaneritic intrusive rocks allows us to see
cleavage faces developed on many grains. Essentially, when the rock is
stressed and breaks when samples are taken, many of the randomly
oriented crystals have their cleavage directions more or less parallel to
the face of the sample, and so they break along cleavage. This gives
excellent reflections [blue arrows] from potassium feldspar in this case.
Phaneritic
and
Porphyritic
Porphyritic
textures are
classically
developed in
extrusive rocks,
but it is not
restricted to
them.
This mafic intrusive rock is called a gabbro, a mixture of pyroxene and
plagioclase feldspar. The bulk of the rock is phaneritic, with grains
approximately 1-2 mm across. However, there is a megacryst, a large
pyroxene crystal, whose extent is indicated by the yellow arrows.
Phaneritic and Porphyritic
The same specimen as in the previous slide, we take advantage of the
development of cleavage to highlight the megacryst [width indicated by
yellow arrow], which is somewhat obscure in the previous image. The
cleavage direction meets the surface of the specimen at something like
30 degrees, but the crystal still breaks along cleavage in a series of
several steps, which reflect at the same time.
Pegmatite
This term applies
to extremely
coarse-grained
igneous intrusive
rocks, usually of a
felsic composition.
Some restrict the
term to rocks with
a grain size
exceeding 2 cm,
but others allow
more latitude.
In the late stages of cooling, volatiles tend to be concentrated in the
magma. This lowers magma viscosity, accounting for the abnormally
large crystals. In this specimen, there are crystals of an unusual
turquoise variety of potassium feldspar called amazonite [red arrows].
Pegmatite
Finer crystals
on the upper
surface of the
specimen may
represent more
rapid cooling at
the exterior of
the intrusion;
field data could
confirm this.
The large crystals [see cleavage face, yellow arrow] may be gem
quality crystals, often of minerals that are significant sources of
unusual elements such as lithium, fluorine, and boron. Pegmatites are
thus attractive as well as economically significant.