In the previous tutorial we began learning about the most important class of minerals,
the silicates, and we examined three of the silicate subclasses,
those being orthosilicates, disilicates, and cyclosilicates. There are three more subclasses
to go over, so let’s talk about these now. The next group of silicates, called chain
silicates, or inosilicates, are divided into two subgroups: pyroxenes, which have single chains
of silica tetrahedra, and amphiboles, which have double chains. Pyroxenes are composed
of silica tetrahedra linked together in a zig-zag pattern, where one oxygen atom in each tetrahedron
is shared with the adjacent tetrahedron. They are similar to ring silicates, in that each silica
tetrahedron shares two of its oxygens with other silica tetrahedra, having a silicon to oxygen
ratio of 1 to 3. Pyroxenes have the general formula XY(Si,Al)2O6 where the Y-cation is
a medium-sized cation in 6-fold coordination, and the X cation is either a 6-fold or 8-fold cation,
depending on whether the crystal structure is orthorhombic or monoclinic. Orthorhombic
pyroxenes are called orthopyroxenes, and they have 6-fold cations on both their X and Y sites,
making them more symmetrical than monoclinic pyroxenes, or clinopyroxenes, which have 8-fold
cations, typically calcium, on their X-site. The orthopyroxene group only has two endmembers,
the iron-rich ferrosilite, and magnesium-rich enstatite, with complete solid solution between
them. The clinopyroxenes are more diverse and widespread, forming a sizeable fraction
of most mafic rocks. Like orthopyroxene, there is extensive solid solution among the clinopyroxenes.
Some endmember clinopyroxenes are the calcic, magnesium-rich diopside and iron-rich
hedenbergite, the sodic, aluminum-rich jadeite and iron-rich aegirine, and the lithium-bearing
spodumene. Gemstone-quality jadeite is used as jade and spodumene is an important ore of lithium.
The most abundant pyroxene is augite, which is a calcic pyroxene with an intermediate composition.
Double chain silicates, better known as amphiboles, combine two single chains
of tetrahedra by having every other silica tetrahedron share three of its oxygen atoms
with the adjacent tetrahedron, forming a ring-like structure. They have a silicon to oxygen ratio of
1 to 2.5 and always have extra anions, typically volatiles, like hydroxyl, chlorine, and fluorine.
Amphiboles can crystallize from a magma, but they usually form during the hydrothermal alteration of
pyroxenes, via the reaction pyroxene + quartz + water yields amphibole. There is an entire class
of metamorphic rocks called amphibolite because their mineralogy is dominated by amphiboles.
Amphiboles have the general formula, AB2C5(Si,Al)8O22W2, and are sometimes
called the “garbage can mineral”, because of their ability to take in so many different elements.
The A, B, and C sites are distinguished based on the size of cation they can accommodate,
and the W-site contains hydroxyl, chlorine, and fluorine. The A-site can either be vacant,
or contain the large cations potassium, sodium, and/or calcium, the B-site contains medium-large
cations, such as calcium, sodium, and ferrous iron, and the C-site is composed of strips
of octahedra that are sandwiched between the chains of tetrahedra, gluing them together. The
octahedra are occupied by the medium-sized cations magnesium, iron, and aluminum. Like the pyroxenes,
amphiboles are broken down into orthoamphiboles and clinoamphiboles, and of course, there is
extensive solid solution among their respective endmembers. Compared to clinoamphiboles,
orthoamphiboles are chemically simple, having a vacant A-site, and only magnesium and iron in the
B-and C-sites. Clinoamphiboles are classified based on the dominant cation in the B-site,
being either calcium amphiboles, sodium-calcium amphiboles, or sodium amphiboles.
Blueschist, which is a class of metamorphic rock common in subduction zones, gets its name from the
blue color of the sodium amphibole glaucophane (Na2[Mg3Al2]Si8O22(OH)2). The medium-grade
metamorphic rock, amphibolite, typically contains calcium amphiboles, such as hornblende.
Sheet silicates, or phyllosilicates, take double chain silicates one step further. Each
silica tetrahedron shares 3 oxygen atoms with the adjacent tetrahedron, forming an infinite
sheet of shared tetrahedra. Sheet silicates have a silicon to oxygen ratio of 2 to 5.
The sheet of silica tetrahedra, referred to simply as the tetrahedral, or ‘T” sheet is stacked on top
of an octahedral, or “O” sheet, which is a plane of divalent and trivalent cations like iron,
magnesium, and aluminum. The T and O sheets stack together to form layers,
which are bonded through strong, molecular bonds. They can stack in one of two ways;
a T-O, or “open-faced sandwich” configuration, with one T-sheet bonded to one O-sheet, or a
T-O-T configuration, where a single octahedral sheet is sandwiched by two tetrahedral sheets. The
sheet silicates are a diverse group of minerals that include the clay minerals, the micas, the
serpentine group, the chlorite group, and talc. The differences between the groups of sheet
silicates are the types of bonding that hold together the sheeted layers.
For example, in the mineral talc, Mg3Si2O5(OH)4, they are held together by van der Waals forces,
which are relatively weak. It is no coincidence that talc, with a Mohs hardness of 1 is the
softest mineral; so soft in fact, that it is used to make baby powder. On the other hand,
the micas are held together by weak ionic bonds that arise from electrostatic interactions
between large, interlayer cations, like potassium, and the negatively charged T-O-T layers.
This structure is called T-O-T + C, with the “C” standing for cation. The difference between
these two groups are the tetrahedral cations; in talc, only Si4+ occupies the tetrahedral sites,
while in the micas, one Si4+ swaps for Al3+, creating a net negative charge among the layers,
and allowing for stronger electrostatic bonds between the layers, with the large cations acting
as the glue. The chlorite minerals have a similar structure, with negatively charged layers due to
aluminum substitution; however, instead of being glued together by cations, they have an additional
positively charged octahedral sheet between layers. This structure is called T-O-T + O.
The last group has a T-O structure, and like talc, the layers are neutral, but are in this
case held together by stronger hydrogen bonds. It turns out that most of the sheet silicates
have a similar hardness, between about 2 and 3, except for the T-O-T group, which are the
softest of all the minerals. The weak bonding between layers gives all the sheet silicates
perfect basal cleavage, causing them to peel apart like the plies in a roll of toilet paper.
We will now move on to the most abundant subclass of minerals in the Earth’s crust,
the framework silicates, or tectosilicates. These minerals have a backbone of silica tetrahedra
that share all four of their oxygens with neighboring silica tetrahedra. The framework
silicates have a silicon to oxygen ratio of 1 to 2, with aluminum often substituting for silicon in
the tetrahedral sites, as in the feldspar mineral anorthite, where half its tetrahedral sites are
occupied by aluminum. The complete polymerization of tetrahedra creates a complex, rigid structure
with large spaces in between, allowing for a wide range of interstitial large cations.
The basic chemical formula for feldspars is (Ca,Na,K)(Si,Al)4O8,
and can be separated into three endmembers: anorthite, CaAl2Si2O8,
albite, NaAlSi3O8, and potassium feldspar, KAlSi3O8. There is complete solid solution
between albite and anorthite, with intermediate compositions being called plagioclase. There is
also complete solid solution between albite and potassium feldspar, being called alkali feldspar.
For two isostructural minerals to have solid solution, their cations must meet one of the
two following criteria: One, they have a similar ionic radius, or two, they have the same charge.
Since potassium and calcium do not meet these criteria, solid solution does not occur between
anorthite and potassium feldspar. Plagioclase is the most abundant mineral in the crust,
being a main component of both the continental and oceanic crust. Another important framework
silicate is quartz, which is composed entirely of silica tetrahedra, having
the formula SiO2. The silicon-oxygen covalent bond is one of the strongest single bonds,
making quartz and the other framework silicates very hard. Quartz is actually a polymorph of SiO2,
meaning its structure changes with pressure and temperature. Tridymite and cristobalite are the
high-temperature polymorphs, with coesite and stishovite being the high-pressure polymorphs.
Stishovite can only be formed at pressures greater than 10 gigapascals and is often
used to identify ancient asteroid impacts. With that we conclude our introduction to
the silicates, and to the classes of minerals in general. But how do these minerals form and come
together to produce rocks? Let’s move forward and begin to answer precisely this question.