Over the previous two tutorials, we covered 7 out of the 8 classes of minerals. That
leaves just one more, and it’s the most important class, including the majority of the minerals in
Earth’s crust and upper mantle. We are talking about silicates. All silicates are made of the
same basic building block, the silica tetrahedron, which is simply a silicate, or SiO44- anion. The
structure of the silica tetrahedra is a central silicon atom that is bonded to four oxygen atoms.
You can think of it as a pyramid, with a silicon atom in the center and an oxygen at each corner.
Silicate minerals are divided into subclasses based on their arrangement and linkage of silica
tetrahedra, which can range from a single, isolated tetrahedron bonded to a metal,
to a complex three-dimensional framework of silica tetrahedra. The ability of silica
tetrahedra to link together, or polymerize, to form a diverse array of intricate structures
stems from its unique mesodemic bonding, where each of the oxygen anions has one of its two
valence charges satisfied by bonding with its tetrahedron’s central silicon atom, thus leaving
a remaining charge of negative one at each corner of the tetrahedron that is available for bonding.
Because of this, a given silica tetrahedron can bond with up to four other silica tetrahedra,
or none at all. The structural complexity of the silicate minerals increases with the degree
of polymerization, and even though quartz, or SiO2, is about as simple as it gets chemically,
it is structurally very complex, with its three-dimensional framework of silica tetrahedra.
We will first discuss the silicates that do not polymerize at all, the orthosilicates.
Orthosilicates, also called nesosilicates, are the simplest subclass of silicates,
as the silica tetrahedra are not polymerized, meaning there is no sharing of oxygen atoms
between silica tetrahedra. Instead, the silica tetrahedra are linked by bonding with cations,
like magnesium, iron, calcium, and aluminum. Unlike silicon, which bonds to four oxygen
anions to form tetrahedra, larger cations like magnesium and iron bond, or coordinate
with 6 oxygen anions to form octahedra. Moreover, even larger cations, like the alkali metals and
calcium, coordinate with eight or more oxygen atoms, depending on the mineral’s structure.
Knowing that the coordination of an ion is dependent on its radius, it is easy to
predict which atoms can substitute for others in a crystal’s lattice. This brings us to the most
common type of orthosilicate minerals, olivine. Olivine has two octahedral sites that contain
medium-sized divalent cations, in addition to its tetrahedral silicon. In a typical olivine, the
octahedral sites contain a mixture of magnesium and ferrous iron, with more magnesium than iron.
Geologists call this mixing of different ions on a crystallographic site, solid solution. In the
previous tutorial, we discussed endmembers, which are minerals that represent the chemical extremes
of a group. In the olivine group, fayalite, or Fe2SiO4, is the iron endmember, and forsterite,
or Mg2SiO4, is the magnesium endmember, with complete solid solution existing between them.
Another important group of orthosilicates is garnet, which comes in a wide variety of
compositions with extensive solid solution. Its base formula is X3Y2(SiO4)3, where the X-site
is an eight-fold site that can be occupied by calcium, magnesium, ferrous iron, and manganese,
and the Y-site is a six-fold site that typically contains aluminum and ferric iron.
The last group of orthosilicates we will discuss are the aluminosilicates,
which have a chemical formula of Al2SiO5. Unlike the others,
the minerals in this group all have the exact same formula. Where they differ is their crystal
structure. Minerals with the same formula but different structures are called polymorphs,
with the temperature and pressure of formation influencing which polymorph crystallizes.
For the aluminosilicates, kyanite is the high-pressure phase, sillimanite is the
high temperature phase, and andalusite is the low temperature-low pressure phase.
Next are the disilicates, also called sorosilicates, which are composed of
pairs of silica tetrahedra that share one oxygen atom and have the basic structural unit Si2O76-.
Due to its similarity in size and charge with the silicon cation, aluminum sometimes substitutes in,
forming aluminum tetrahedra. When aluminum, which has a charge of 3+, substitutes for silicon,
which has a charge of 4+, the mineral must somehow balance out this gained negative charge. This can
be accomplished by picking up a hydrogen ion or substituting a lower charged cation, like sodium,
for a higher charged one, like calcium. The most common sorosilicates are in the epidote group,
which contain the minerals zoisite, epidote, and allanite. These minerals are chemically complex,
with three octahedral sites, typically containing aluminum and ferric iron,
and a distorted 7-to-11-fold site that contains larger cations like calcium. The mineral allanite
is a known for taking up rare earth elements like yttrium and cerium into its structure, where they
substitute for calcium in the large-cation site, making it a valuable ore material.
Ring silicates, also called cyclosilicates, are composed of silica tetrahedra that share two
oxygen atoms and have a silicon to oxygen ratio of 1 to 3. Their name is derived from
their structure, which forms rings of six silica tetrahedra stacked atop one another.
They are a small group, with only a handful of commonly-occurring minerals, including
the popular gemstones beryl and tourmaline, with beryl also being an important ore of beryllium.
Let’s consider the structure of beryl, which has the chemical formula Al2Be3Si6O18.
Between its stacked 6-membered rings of silica tetrahedra are sheets of laterally
linked 12-membered rings composed of alternating aluminum octahedra and beryllium tetrahedra that
cross-link the rings of silica tetrahedra. This creates an intricate three-dimensional framework,
like that of a framework silicate, a group we will discuss in the next tutorial. The stacked rings
of tetrahedra create long cylindrical channels that are commonly filled with large cations,
like sodium, potassium, and cesium, which creates a net positive charge that must be balanced.
This is achieved by substituting divalent cations,
like chromium and ferrous iron for aluminum in the octahedral site. The gemstones emerald and
aquamarine are varieties of beryl, containing trace amounts of chromium and iron respectively,
which are responsible for their color. That covers three of the subclasses of
silicates, but there are several more, so let’s move forward and check out the rest.