Geos 306, Lecture 7
Crystal Chemistry I
- The chemical composition of a mineral is one of its fundamental properties. Individual mineral species are defined by their chemistry and crystal structure.
- The complete list of all known mineral, approved by the Nomenclature Commision of the International Mineralogical Association, can be found at:
IMA Mineral List.
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Since the 1700'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 integers.
Charge Balance
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The chemical formula for most minerals displays 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:
| Mineral name | Chemical Formula |
| quartz | SiO2 |
| albite | NaAlSi3O8 |
| forsterite | Mg2SiO4 |
| diopside | CaMgSi2O6 |
| hematite | Fe2O3 |
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Assigning valences and creating charge balanced chemical formulae are not so easy for sulfide minerals. This is often because there may be metal-metal or sulfur-sulfur bonds. For example, some nickel sulfides:
| Mineral name | Chemical Formula |
| millerite | NiS |
| vaesite | NiS2 |
| heazlewoodite | Ni3S2 |
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
(Mg0.92Fe0.08)2SiO4
so this is the mineral species forsterite, ideally Mg2SiO4
- The elements that can commonly substitute for each other usually have similar radii and similar charges.
- For instance, take the carbonate calcite group minerals:
| Mineral name | Chemical Formula |
| calcite | CaCO3 |
| magnesite | MgCO3 |
| rhodochrosite | MnCO3 |
| siderite | FeCO3 |
All these minerals form crystals with the same crystal structures, and the group of minerals are said to be 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 other metals, 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: Ca0.50Na0.47K0.03Al1.50Si2.50O8.
This particular feldspar is given the special varietal name of labradorite. The series is known as plagioclase.
- No minerals are found in nature that have pure ideal compositions.
It is generally believed that there are trace amounts of just about all the elements in every mineral sample.
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 weight percent data into a chemical formula.
The experiment produces the first two columns, and you should be able to compute the rest of the table.
|
Oxide component
|
Weight percent (g)
|
Weight (g/mole)
|
moles
|
Normalized proportion
|
|
FeO
|
43.30
|
71.84
|
43.30/71.84=0.60
|
3
|
|
Al2O3
|
20.49
|
101.96
|
20.49/101.96=0.20
|
1
|
|
SiO2
|
36.21
|
60.08
|
36.21/60.08=0.60
|
3
|
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The formula is then 3FeO + Al2O3 + 3SiO2 = Fe2+3Al2Si3O12.
This is the formula for the garnet mineral known as almandine.
Polymorphs
- Knowledge of the chemical composition of a mineral is not sufficient to identify it. Different minerals can exhibit different crystal structures and yet may have the same chemistry.
For instance, graphite and diamond are both composed of carbon. Distinct minerals with the same chemical composition are known as polymorphs.
Other well-known polymorphs include the pair of CaCO3 minerals calcite and aragonite, or SiO2 quartz, tridymite, cristobalite, coesite and stishovite.
The different polymorphs have different pressure and temperature stability fields.
The silica phase diagram
Reading:
Wenk and Bulakh, 143-144; 231-232; chapter 14, 18
K&H, chapter 5
