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Minerals of Igneous Rocks
1 – The Ferromagnesian Minerals
(Contain iron (Fe) and magnesium
(Mg)
[May or may not contain aluminum (Al)]
Olivine
(Fe,Mg)2SiO4
Olivine is always some shade of green.
Lighter green indicates greater Mg
content (and lower density), darker color
more Fe (and higher density).
Large crystals uncommon, almost always
occurring as dominant or sole mineral in
a rock, as small granular crystals.
No cleavage – conchoidal fracture (like
glass).
Coin is ~2.5cm (1”) in diameter. Included for scale
Pyroxene
(Augite, most commonly)
(Ca,Na)(Fe,Mg,Al)(Si,Al)2O6
Pyroxene can be green (usually dark green) or black
depending on the exact composition.
There are two directions of cleavage that meet at 90°,
though one or both may be difficult to see.
The luster is usually less bright and the surface texture
less splintery than amphibiole, with which pyroxene is
easily confused.
Pyroxene generally occurs in igneous rocks with another
mineral – a feldspar.
Amphibole
(Hornblende, most commonly)
Ca2Na(Fe,Mg)4(Al,Fe,Ti)3Si8O22(O,OH)2
Amphibole may be dark green
but is more typically black.
There are two directions of
cleavage that do not meet at
90°, The cleavage tends to step
rather than continuing across a
smooth face. This gives the
mineral a splintery appearance.
The luster is usually quite bright
and reflective – sparkly in fact.
Amphibole generally occurs in
igneous rocks with another
mineral – a feldspar.
Biotite
(black mica)
K(Fe,Mg)3(AlSi3O10)(OH)2
Biotite is a “black” mica, but
usually appears dark brown if
the “book” is thin enough to
see through.
There is one perfect direction of
cleavage that allowt thin sheets
to be peeled off of a crystal.
The luster is quite bright and
reflective.
Biotite is not common in
igneous rocks and when it
occurs feldspar and quartz are
usually the dominant minerals.
2 – The Non-Ferromagnesian Minerals
(Contain little or no iron (Fe) or magnesium (Mg)
[Usually contain aluminum (Al)]
Muscovite
(white mica)
KAl2(AlSi3O10)(OH)2
Muscovite is a “white” mica,
but may appears brown if the
“book” is thick. There may also
be some Fe substituted for Al,
giving it some compositional
similarity to biotite and making
it darker.
As with all micas thin sheets to
can be peeled off. This
character has allowed it to be
substituted for glass,
particularly in high-temperature
oven doors, for example.
The luster is quite bright and
reflective. Muscovite on
mountain stream beds has
fooled many children (including
me) into mistaking it for gold.
Muscovite occurs uncommonly
in the same sorts of igneous
rocks as biotite, with the same
other minerals. Which (if
either) occurs depends on the
availability of Fe.
Plagioclase Feldspars
(Ca Plagioclase)
(Ca,Na)Al2(Al2Si2O8)
(Ca and Na freely substitute in plagioclase. If Ca
predominates we call it a Ca plag or Ca feldspar)
The feldspars are a very important group of
minerals, occurring in almost every igneous rock
many metamorphic rocks, and some sedimentary
rocks.
Feldspars are the most common mineral in the
Earth’s crust. Because the “backbone” of every
feldspar (and all other common igneous minerals)
is a silicon oxygen molecule with the formula SiO4,
that makes oxygen the most common element in
the crust. (In the mantle too). Silicon is second
most abundant, followed by Al, Fe, Mg, and then
the other cations you’ve seen in the mineral
formulas.
The feldspars have two directions of cleavage at
about 90°, one of which is usually easier to spot
than the other. Plagioclase often has obvious
parallel lines on the obvious cleavage face called
“striations”.
The most common color for all feldspars is white.
Often, however, Ca-rich plag is dark gray to black,
like the one shown. For our purposes we will
consider this a diagnostic color, though it really
isn’t.
Plagioclase Feldspars
(Na Plagioclase)
(Na,Ca)(AlSi3O8)
(Ca and Na freely substitute in plagioclase. If Na
predominates we call it a Na plag or Na feldspar)
Sodium plag is typically white. Because K and Na
are both +1 cations there can be some substitution
of K for Na, giving the mineral a flesh-pink tinge.
Before we leave the plagioclase feldspars we need
to make sure to understand the composition. This
mineral group is called a “solid solution” – there
can be any proportion of Ca or Na in the crystal
structure – 100%Ca/0%Na, 99%Ca/1%Na …
0%Ca/100Na. The series is arbitrarily broken into
segments and names are applied to various ranges
of composition, but these are not truly distinct
minerals, only convenient names to pin onto a
thing without borders.
Potassium Feldspars
(Kspar)
K(AlSi3O8)
Potassium (K) feldspar comes in several forms and
can be several colors, most commonly white.
We are going to pretend in this class that pink is
the characteristic color because it is fairly common.
Remember that it isn’t really the diagnostic color,
we are just pretending.
Notice that the specimen shown has what appear
to be striations across it. These are not actually
striations as you can tell from a couple of things.
First, they are not all continuous across the
specimen as striations are, and second, the edges
(and the “zones” themselves) are not parallel to
each other. The lighter bands are actually K-rich
(Na) plagioclase that has intergrown with the
Kspar.
Quartz
(SiO2)
Quartz can have many colors depending on
impurities, but in igneous rocks it is usually
perfectly clear. Of course if the crystal is
embedded inside a rock and you are looking at it
from the surface you won’t be able to see through
it, so it will look clear only if it is a very thin piece.
The thicker it is the darker gray it will look. Still, it
is never as dark as a ferromagnesian mineral and
so you shouldn’t confuse it with those.
The give-away for quartz on the broken face of a
rock is that it doesn’t cleave. It’s crystal faces are
never evident (for reasons we will get into) and it
breaks with a conchoidal fracture (like glass). So in
a rock you will see lots of feldspars and other
minerals with flat cleavage faces sparkling at you.
The quartz will be the stuff with the non-flat
broken edges.
The picture shows the crystal faces of a well-grown
crystal (right) and the fracture of a broken piece
(left)