Historical Geology/Silicate minerals

In this article we shall look at the important class of minerals known as silicates.

Silicate structures
By a silicate tetrahedron we shall mean an atom of silicon bonded with four equally spaced atoms of oxygen forming the four corners of a tetrahedron: that is, a pyramid having a triangular base.

Each tetrahedron can share each one of its oxygen atoms with one other tetrahedron, so that two tetrahedra can join together corner-to-corner (but not edge-to-edge or face to face). Hence each tetrahedron can be linked with up to four other tetrahedra, one for each corner of the tetrahedron; or three, two, one, or none. This gives us a wide variety of structures that can be built out of tetrahedra: three-dimensional lattices; two dimensional sheets, chains, double chains, rings, et cetera. The diagram to the right shows some of the possible structures. Note that the chain and double chain can be extended indefinitely in one direction and the sheet in two directions.

A silicate mineral (or silicate for short) is a mineral containing silicate structures; so silicate minerals can be classified according to their silicate structures as lattice silicates, sheet silicates, chain silicates, and so forth. Note that with the exception of quartz and its polymorphs, a silicate mineral will not consist entirely of such structures. Atoms of other elements must necessarily be involved so that the rings, chains, sheets or whatever form part of a three-dimensional crystal.

Most silicate structures can be described either by a descriptive English name such as "sheet silicate" or by a name which describes the same thing in Greek such as "phyllosilicate". Where a plain English term exists, I shall employ it in this text. The table below shows how the structures relate to the English and Greek names and gives examples of minerals important to this textbook.

Because tetrahedra link together by sharing the oxygen atoms at their corners, the structure formed by the tetrahedra is reflected in the chemical formula of a silicate. For example, quartz consists of nothing but tetrahedra linked together in a three-dimensional lattice. This means that every tetrahedron is linked to another at all four corners; which means that every oxygen atom is shared by two silicon atoms; which means that quartz has the formula SiO2. Similarly, if you look at the formula for zircon (ZrSiO4) you can see that it must be a nesosilicate.

Aluminosilicates
In some silicates, the structure is based not just on silicate tetrahedra but also on tetrahedra with a central atom of aluminum rather than silicon. Silicates which incorporate these aluminum-based tetrahedra as well as silicate tetrahedra are known as aluminosilicates.

Aluminum-based tetrahedra have different chemical properties from silicate tetrahedra. This means that you cannot have an aluminosilicate which differs from an ordinary silicate only by the substitution of atoms of aluminum for some of the atoms of silicon; there must necessarily be other differences. For example, it is chemically impossible to build a lattice just out of these two kinds of tetrahedra that is analogous to quartz; other atoms are required to balance the charge of the chemical formula. Hence lattice aluminosilicates such as feldspars do not have the formula (Si,Al)O2, which is impossible, but have more complicated formulas such as KAlSi3O8 and CaAl2Si2O8.

Felsic and mafic silicates
Silicate minerals which are high in silicon are called felsic minerals; the opposite of felsic is mafic; minerals which are very mafic are known, sensibly enough, as ultramafic. Note that this term only applies to minerals which are silicate minerals and so contain some silicate tetrahedra; no-one would call calcite (for example) an ultramafic mineral on the grounds that it contains no silicate tetrahedra at all.

Some generalizations can be made about the differences between felsic and mafic minerals: as we progress from felsic to mafic the minerals are more dense (because the atoms in them which aren't oxygen or silicon are heavier elements such as magnesium or iron); they have higher melting points; and when they do melt they are less viscous (that is, they flow more easily).

Note on vocabulary
In some texts, particularly older texts and British texts, you may see the words acidic, basic, and ultrabasic used instead of felsic, mafic, and ultramafic. These terms refer to an obsolete hypothesis of mineral formation, and I shall not use them; I mention them only for the benefit of those readers who might come across them in further reading.