Fall09-8, Crystal Chemistry II

Geos 306, Fall 2009, Lecture 8
Crystal Chemistry II

The chemical composition of a mineral is one of its fundamental properties. Individual mineral species are defined by their chemistry and crystal structure.
Since the 1600's it has been observed that the ideal chemical composition of a mineral could be expressed as integer amounts of various elements. For instance, SiO₂ or Mg₂SiO₄. In the crystallography component of the course, we will learn why the ideal composition can be written as intergers.

Charge Balance

The chemical formula of most minerals must display charge balance. This means that there is no excess of anions or cations. In particular, this holds for the oxide minerals. Some common oxide minerals include:
Charge balance is not so easily figured out for sulfide minerals. This is because there may be metal-metal or sulfur-sulfur bonds. For example, some nickel sulfides:

Chemical Variations

The chemical formulae given above are known as ideal formulae. They represent the composition of the ideal mineral. Actual minerals usually have minor amounts of other elements in them. For instance, Mg and Fe commonly substitute for each other. The chemical composition of olivine from San Carlos Arizona is commonly found to be (Mg_0.92Fe_0.08)₂SiO₄.
The elements that can commonly substitute for each other usually have similar radii and similar charges.
For instance, take the carbonates
All these minerals form crystals with the same crystal structures, called isostructural. Therefore, it is not unusual to find carbonates with this crystal structure that are mixtures of Ca, Mg, Mn, Fe. If there is more Ca than the others, then the mineral is called calcite, or if the Fe component is greater, then the mineral is called siderite. We say that these minerals form a series.
The chemical formula of the feldspar, albite, is NaAlSi₃O₈, while it is CaAl₂Si₂O₈ for anorthite. The radii of Ca and Na are similar, so they can easily substitute for each other, but their charges are different, Na⁺¹ versus Ca⁺². To have a mixed feldspar requires a coupled substitution, where Na⁺¹ + Si⁺⁴ = Ca⁺² + Al⁺³. Thus we can have a chemical composition for a feldspar such as: Ca_0.50Na_0.47K_0.03Al_1.50Si_2.50O₈. This feldspar is given the special name of labradorite.
No minerals are found in nature that have pure ideal compositions.

Solid Solutions

When crystals form from a melt they can have variable chemical compositions. For instance, the Mg-Fe olivines, or the Na-Ca feldspars.

Formula Weights

The weight of a mole of a mineral can be computed from its chemical formula and the weights of elements given in the periodic table.
For example, quartz, SiO₂. Weight = 28.086 + 2*15.999 = 60.084 g/mole.

Obtain formula from oxide weights

Microprobe experiments frequently provide chemical composition data in the form of oxide weight percents. This is an example of how to convert the oxide wieght percent data into a chemical formula

₂

₃

²⁺

₃

₂

₃

₁₂

Polymorphs

Knowledge of the chemical composition of a mineral is not sufficient to identify it. Different crystal structures may have the same chemistry. For instance, graphite and diamond are both composed of carbon. Minerals with the same chemical composition are known as polymorphs. Other well-known polymorphs include calcite and aragonite, or quartz, tridymite, cristobalite, coesite and stishovite. The different polymorphs have different pressure and temperature stability fields.

Reading:
Wenk and Bulakh, 143-144; 231-232; chapter 14, 18
K&H, chapter 5