21 2.3 Mineral Groups — Physical Geology – 2nd Edition

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. 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.

Sulphides 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 sulphide 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)[1], 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. Note that it is called a phosphate, not a hydroxide, even though it has a hydroxyl ion.

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.

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:

Table 2.5 Provide group names for the following minerals
[Skip Table]
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

See Appendix 3 for Exercise 2.2 answers.

  1. The notations of two (or more) elements enclosed in parentheses with a comma between them: (Ca,Mg), indicates that both can be present, in varying proportions, but that there is still only one of them for each anion present. ↵

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Physical Geology by Cynthia Krutsinger is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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