Physical Geology

Silicate Minerals

silicate tetrahedra bonding

silicate mineral structures: simple, double, ring, chain, double chain, sheet, 3-D

The Silicate Mineral Family

There are over 3000 different minerals known. However, only about 20 of them are very common.

Likewise, of the 87 or so naturally occurring elements, only 12 are found in the earth’s crust in amounts greater than 1%. These twelve make-up 99% of the mass of the crust.

Oxygen and Silicon make-up more than 70% of the crust.

Oxygen is very reactive with other elements - needs two extra electrons- and it bonds as an O2- anion to form compounds called oxides.

Silicate Minerals

The silicate complex anion (SiO4)4-

Four oxygen atoms tightly bonded to a silicon atom. The oxygens arrange themselves around the central silicon to form a four sided pyramid - a tetrahedron.

The four oxygen ions each share an electron with the silicon, leaving each oxygen looking to share one additional electron.

How silicate anions can bond:

1. Bonding with cations

The oxygen ions can share their extra electrons by bonding with cations.

*Remember: Cations are atoms that have a deficit of electrons relative to protons and are abbreviated with a positive subscript.

e.g. two Mg2+ cations can combine with an SiO4 to give Mg2SiO4 ? (olivine)

2. Bonding with other tetrahedra

The oxygen ions in the silica tetrahedra can also share their electrons by bonding with additional silica tetrahedra.

Thus, an oxygen ion attached to one silica tetrahedra could bond to another silicon atom that itself is surrounded by three other oxygens. Another way to think about this is to picture two silicate tetrahedra that are sharing one oxygen between them.

Of course, if a silica tetrahedron can share one oxygen with another tetrahedron, then it could share two, three, or even all four. This is the key to understanding the structures of silicate minerals found in the different silicate mineral families. The three dimensional shape of the crystal structure of the minerals changes in very predictable ways as you increase the number of oxygens that are shared between tetrahedra. As a result, long chains, sheets and 3-D networks of tetrahedra can be built.

Note: Two tetrahedra can only have one common oxygen ion shared between them. In 3D, this means that tetrahedra can only join at their apexes. Sharing two oxygens (the tetrahedra would then be joined along an edge) is an unstable condition and is not found in nature.

Silicates can cleave or fracture.

Isolated tetraheda:

The Olivine Group (Mg,Fe)2SiO4

Olivine minerals are usually pale green and glassy looking. Olivine is one of the earth’s most abundant minerals ? a principle constituent of the upper mantle and oceanic crust. It is also common in meteorites and in moon rocks. Crystals of pure Mg olivine are cut into the gemstone peridot. Ionic substitution between Fe2+ and Mg2+ gives olivine a range of chemical compositions and specific mineral types.

Note: in an isolated tetrahedra, none of the O2 atoms are shared by tetrahedra. Olivine - 2 ions of either Mg+2 or Fe+2 are attached

The Garnet Group A3B2(SiO4)3

A and B can be a range of substituted ions.

A: Mg2+, Fe2+,Ca2+, Mn2+

B: Al3+, Fe3+,Cr3+

Garnet mineral forms beautiful dodecahedral crystals that are common in metamorphic rocks. Because of its relatively high hardness (6.57) it is used as an industrial abrasive.

Continuous chain silicates

Two related families.

Single Chains - Pyroxene mineral family (SiO3)n2-

Pyroxene minerals are long chains of tetrahedra, each of which is sharing two oxygen atoms with adjacent tetrahedra. Pyroxene minerals are dark minerals that are very common in igneous rocks in the crust and mantle.

Each single chain has a net excess of negative charges balanced by positive ions that hold them together.

Double Chains - Amphibole mineral family (Si4O11)n6-

Amphiboles can be imagined as long chains of connected silicate rings. These are dark minerals common in igneous rocks.

Chain silicates tend to have two directions of cleavage because the individual chains of tetrahedra can be separated much more easily than the chains themselves can be broken.

Sheet silicates

Clays, micas, and chlorites (Si4O10)n4-

These minerals are constructed of continuous sheets of interconnected tetrahedra.

Each silicate tetrahedron is connected to three others via three shared oxygens. The remaining unshared oxygen bonds with cations such as Al3+, Mg2+, and Fe2+. These cations connect individual silicate sheets together, creating a sandwich-like structure. Van der Waals bonds between each sandwich are much weaker than the bonds between the tetrahedra sheets or within the sheets, giving these minerals a well developed cleavage plane (explains why mica tears into thin sheets).

The ubiquitous clay mineral kaolinite is a sheet silicate with aluminum ion as the interconnecting cation.

Three dimensional silicate networks

Quartz SiO2

In quartz, each silica tetrahedron is connected to four others, all oxygens are shared, and there is no need for cations.

The structure of quartz is neither chain like or sheet like, rather it is a complex 3D network of tetrahedra. Quartz is a very common mineral in continental crust and it is found in a variety of forms.

Feldspar minerals

Feldspars are similar in structure to quartz with a complex 3D network of silicate tetrahedra, however, feldspars have Al3+ substituting for Si4+ in the tetrahedra. Because of this, the substituted tetrahedra require another cation to contribute the missing electron. This cation can be K+, Na+, or Ca2+.

Feldspar is the most common mineral in the earth’s crust, making up 60% of its mass. There are two main varieties of feldspar:

Orthoclase or Potassium (K) feldspar and plagioclase (NaCa) feldspar.