2.2 Mineral Groups
Most minerals are made up of a cation (a positively charged ion) or several cations, plus an anion (a negatively charged ion (e.g., S2−)) or an anion complex (e.g., SO42−). For example, in the mineral hematite (Fe2O3), the cation is Fe3+ (iron) and the anion is O2− (oxygen). The two Fe3+ ions have an overall charge of +6 and that balances the overall charge of −6 from the three O2− ions.
We group minerals into classes on the basis of their predominant anion or anion complex. These include oxides, sulphides, carbonates, silicates, and others. Silicates are by far the predominant group in terms of their abundance within the crust and mantle, and will be discussed in more detail in Lab 3. Some examples of minerals from the different mineral groups are given in Table 2.1.
| Group | Examples |
|---|---|
| Oxides | Hematite (iron oxide Fe2O3), corundum (aluminum oxide Al2O3), water ice (H2O) |
| Sulphides | Galena (lead sulphide PbS), pyrite (iron sulphide FeS2), chalcopyrite (copper-iron sulphide CuFeS2) |
| Sulphates | Gypsum (calcium sulphate CaSO4·H2O), barite (barium sulphate BaSO4) (Note that sulphates are different from sulphides. Sulphates have the SO42− ion while sulphides have the S2- ion) |
| Halides | Fluorite (calcium flouride CaF2), halite (sodium chloride NaCl) (Halide minerals have halogen elements as their anion — the minerals in the second last column on the right side of the periodic table, including F, Cl, Br, etc. — see the periodic table in Appendix 1: List of Geologically Important Elements and the Periodic Table.) |
| Carbonates | Calcite (calcium carbonate CaCO3), dolomite (calcium-magnesium carbonate (Ca,Mg)CO3) |
| Phosphates | Apatite (Ca5(PO4)3(OH)), Turquoise (CuAl6(PO4)4(OH)8·5H2O) |
| Silicates | Quartz (SiO2), feldspar (sodium-aluminum silicate NaAlSi3O8), olivine (iron or magnesium silicate (Mg,Fe)2SiO4) (Note that in quartz the anion is oxygen, and while it could be argued, therefore, that quartz is an oxide, it is always classed with the silicates.) |
| Native minerals | Gold (Au), diamond (C), graphite (C), sulphur (S), copper (Cu) |
Oxide minerals have oxygen (O2−) as their anion, but they exclude those with oxygen complexes such as carbonate (CO32−), sulphate (SO42−), and silicate (SiO44−). The most important oxides are the iron oxides hematite and magnetite (Fe2O3 and Fe3O4, respectively). Both of these are common ores of iron. Corundum (Al2O3) is used as an abrasive, but can also be a gemstone in its ruby and sapphire varieties. Frozen water (H2O) is a mineral (an oxide), but liquid water is not because it doesn’t have a regular lattice.
Sulphides are minerals with the S2- anion, and they include galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), and molybdenite (MoS2), which are the most important ores of lead, zinc, and copper respectively. Pyrite (FeS2), or “fool’s gold”, is another important sulphide mineral.
Sulphates are minerals with the SO42- anion. In this lab, gypsum (CaSO4.2H2O) is the only sulphate mineral that will be examined. However, many other sulphate minerals exist, and these will be covered in future courses.
The halides are so named because the anions include the halogen elements chlorine, fluorine, bromine, etc. Examples are halite (NaCl) and fluorite (CaF2).
The carbonates include minerals in which the anion is the CO32- complex. The carbonate combines with +2 cations to form minerals such as calcite (CaCO3).
In phosphate minerals, the anion is the PO43- complex. An important phosphate mineral is apatite (Ca5(PO4)3(OH)), which is what your teeth are made of.
The silicate minerals include the elements silicon and oxygen in varying proportions ranging from Si : O2 to Si : O4. These are discussed at length in Lab 3.
Native minerals are single-element minerals, such as gold, copper, sulphur, and graphite.
Practice Exercise 2.2
We classify minerals according to the anion part of the mineral formula, and mineral formulas are always written with the anion part on the right. For example, for pyrite (FeS2), Fe2+ is the cation and S− is the anion. This helps us to know that it’s a sulphide, but it is not always that obvious. Hematite (Fe2O3) is an oxide; that’s easy, but anhydrite (CaSO4) is a sulphate because SO42− is the anion, not O. Along the same lines, calcite (CaCO3) is a carbonate, and olivine (Mg2SiO4) is a silicate. Minerals with only one element (such as S) are native minerals, while those with an anion from the halogen column of the periodic table (Cl, F, Br, etc.) are halides. Provide group names for the following minerals:
| Name | Formula | Group |
|---|---|---|
| sphalerite | ZnS | |
| magnetite | Fe3O4 | |
| pyroxene | MgSiO3 | |
| anglesite | PbSO4 | |
| sylvite | KCl | |
| silver | Ag | |
| fluorite | CaF2 | |
| ilmenite | FeTiO3 | |
| siderite | FeCO3 | |
| potassium feldspar | KAlSi3O8 | |
| sulphur | S | |
| xenotime | YPO4 |
