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This article throws light upon the ten important minerals found in soil. The minerals are: 1. Haematite 2. Limonite or Bog Iron 3. Goethite 4. Gibbsite 5. Carbonate Group 6. Calcite 7. Dolomite 8. Siderite 9. Sulphate Group 10. Gypsum.
Mineral # 1. Haematite: Fe2O3:
It varies in colour from red to blackish red and has a reddish streak. It has a metallic lustre and hardness (H) of about 5. Its presence in rocks is indicative of quick chemical change.
Haematite alters to limonite, magnetite, pyrite and siderite. It occurs as a coating on the sand grains and acts as a cementing agent. It swells up on absorbing water to form hydrated iron oxide, i.e. limonite, 2Fe2O3.3H2O and goethite, FeO(OH).nH2O.
Mineral # 2. Limonite or Bog Iron: 2Fe2O3.3H2O:
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It is a hydrated ferric oxide, yellow to brown in colour and is of wide occurrence. It is the final product of most of iron minerals, and hence is resistant to any further change, except for absorption of water. It is an important for colouring and cementing agent in soils.
On dehydration limonite forms haematite. On reduction and carbonation, it yields soluble iron. Limonite is a common alteration product of pyrite, magnetite, hornblende and pyroxene.
Mineral # 3. Goethite: FeO (OH) nH2O:
Most materials called limonite are goethite with some adsorbed water. It is usually white but may be pink or grey in colour; its hardness is 5.3.
Mineral # 4. Gibbsite (Hydragillite): AI2O3.H2O:
It is the most common aluminium compound in soils. Its natural colour is white. It is abundantly found in highly weathered soils of inter-tropical zones, called Laterites (Oxisol). Its presence in soils suggests high degree of weathering and leaching under well-drained conditions.
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The red, yellow or brown colours in soils are due to the presence of goethite and haematite which occur as coatings on the surfaces of soil, particles (especially clay).
Mineral # 5. Carbonate Group:
The basic compounds, like Mg (OH)2 and Ca(OH)2, combine with CO2 or carbonic acid (H2CO3) to form carbonates.
Ca(OH)2 + CO2 = CaCO3 + H2O (calcite)
Ca(OH)2 + H2CO3 = CaCO3 + 2H2O (calcite)
Mineral # 6. Calcite—CaCO3:
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A white mineral (H-3), widely distributed in sedimentary rocks (like limestone), decomposes easily as:
CaCO3 + CO2 + H2O⇋Ca(HCO3)2 (soluble form).
Mineral # 7. Dolomite—Ca.Mg(CO3)2:
Dolomite is less readily decomposed than calcite; it is the chief source of Mg in soils.
Mineral # 8. Siderite—FeCO3:
It is an alteration product of other iron-bearing minerals (H-4) and may itself alter to haematite or limonite. It is an important mineral in water logged soils.
Mineral # 9. Sulphate Group:
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The sulphate is a complex group formed by the combination of 1 S and 4 oxygen ions, which further reacts with Ca to form calcium sulphate (anhydrite, CaSO4). On hydration it forms gypsum (CaSO4, 2H2O).
Mineral 10. Gypsum—CaSO4.2H2O:
It is a common mineral in desert soils and in sedimentary rocks (H-2). It is water soluble and is most easily leached. It precipitates out as mycelium from ground-waters rich in CaSO4 ions.
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