Transcript igneous rock textures

IGNEOUS ROCK TEXTURES
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 finegrained 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 lightcoloured 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.