2.4: Mineral Groups

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Most minerals are made up of a cation (a positively charged ion) or several cations and an anion (a negatively charged ion (e.g., S^2–)) or an anion complex (e.g., SO₄^2–)_.For example, in the mineral hematite (Fe₂O₃), the cation is Fe₃⁺(iron) and the anion is O^2– (oxygen). We group minerals into classes on the basis of their predominant anion or anion group. These include oxides, sulfides, carbonates, silicates, and others. Silicates are by far the predominant group in terms of their abundance within the crust and mantle. (They will be discussed in Section 2.4). Some examples of minerals from the different mineral groups are given in Table 2.3.1.

*Table 2.3.1: The main mineral groups and some examples of minerals in each group*
Group	Examples
Oxides	Hematite (iron oxide Fe₂O₃), corundum (aluminum oxide Al₂O₃), water ice (H₂O)
Sulfides	Galena (lead sulfide PbS), pyrite (iron sulfide FeS₂), chalcopyrite (copper-iron sulfide CuFeS₂)
Sulfates	Gypsum (calcium sulphate CaSO₄·H₂O), barite (barium sulphate BaSO₄) (Note that sulphates are different from sulfides. Sulphates have the SO₄^–2 ion while sulfides have the S^–2 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 Appendix 1.)
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)

Mineral Groups

Oxide minerals have oxygen (O^2–) as their anion, but they exclude those with oxygen complexes such as carbonate (CO₃^2–), sulphate (SO₄^2–), and silicate (SiO₄^4–). The most important oxides are the iron oxides hematite and magnetite (Fe₂O₃ and Fe₃O₄, respectively). Both of these are important ores of iron. Corundum (Al₂O₃) is an abrasive, but can also be a gemstone in its ruby and sapphire varieties. If the oxygen is also combined with hydrogen to form the hydroxyl anion (OH^–) the mineral is known as a hydroxide. Some important hydroxides are limonite and bauxite, which are ores of iron and aluminium respectively. Frozen water (H₂O) is a mineral (an oxide), but liquid water is not because it doesn’t have a regular lattice.

sulfides are minerals with the S^–2 anion, and they include galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS₂), and molybdenite (MoS₂), which are the most important ores of lead, zinc, copper, and molybdenum respectively. Some other sulfide minerals are pyrite (FeS₂), bornite (Cu₅FeS₄), stibnite (Sb₂S₃), and arsenopyrite (FeAsS).

Sulphates are minerals with the SO₄^–2 anion, and these include anhydrite (CaSO₄) and its cousin gypsum (CaSO₄.2H₂O) and the sulphates of barium and strontium: barite (BaSO₄) and celestite (SrSO₄). In all of these minerals, the cation has a +2 charge, which balances the –2 charge on the sulphate ion.

The halides are so named because the anions include the halogen elements chlorine, fluorine, bromine, etc. Examples are halite (NaCl), cryolite (Na₃AlF₆), and fluorite (CaF₂).

The carbonates include minerals in which the anion is the CO₃^–2 complex. The carbonate combines with +2 cations to form minerals such as calcite (CaCO₃), magnesite (MgCO₃), dolomite ((Ca,Mg)CO₃), and siderite (FeCO₃). The copper minerals malachite and azurite are also carbonates.

In phosphate minerals, the anion is the PO₄^–3 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 Section 2.4.

Native minerals are single-element minerals, such as gold, copper, sulphur, and graphite.

Exercise 2.2 Mineral Groups

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 sulfide, but it is not always that obvious. Hematite (Fe₂O₃) is an oxide; that’s easy, but anhydrite (CaSO₄) is a sulphate because SO₄^2– 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
feldspar	KAlSi₃O₈
sulphur	S
xenotime	YPO₄

Contributors

Physical Geology by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca.