Classification of minerals

A. Ferro magnesium minerals:

a. Pyroxenes and amphiboles:

• Are two groups of ferromagnesian minerals (heavy group) the structure of which consists of long chains of linked silica tetrahedral.
• The pyroxenes consist of a single chain (2 oxygen shared in each tetrahedron) whereas amphiboles consist of a double chains (alternately 2 and 3 oxygen atoms shared successive tetrahedral). These chain silicates are sometimes referred to inosilicates.
• The pyroxene group of minerals comprised of different minerals namely enstatite, hypersthene, diopside and augite.
• The amphibole group of minerals is common in acidic rocks and it can be represented by the isomorphous series between tremolite, actinolite olivine and hornblende.

b. Micas

• Primarily originated from the parent rock from which the soil is derived.
• The most common well ordered micas are muscovite, paragonite, biotite and phlogopite (trioctahedral).
• Among the ordered micas, biotite weathers more rapidly than muscovite.

B. Non-Ferro magnesium minerals

a. Feldspars

• Are anhydrous aluminosilicates of K, Na and Ca and occasionally of other large cations such as Ba.
• Structure consists of tetrahedral which are attracted by sharing each oxygen atom between neighbouring tetrahedran. The tetrahedral contain mainly Silicons with sufficient Al substitution.
• Potassium feldspars KA1Si3O8) include orthoclase, microcline, adularia and sanidine.
• Plagioclase feldspars: a series consisting of a solid solution of albite (NaA1Si3O8) high in sodium and anorthite (CaA12Si2O8) high in calcium.

b. Quartz

• Very densely packed and occurs in a high degree of purity
• Strongly resistant to weathering.
• Most abundant mineral next to feldspars

c. Serpentine, a hydrous magnesium silicate occurs more commonly as a secondary product.

d. Garnets are characteristic of metamorphic rocks and are very hard and most resistant to weathering.

C. Silicate minerals

a. Ortho/ Neosilicates:

• Composed of single tetrahedral linked together by Mg or Fe.
• g., zircon makes the mineral comparatively hard. On the other hand, the looser packing of oxygen in olivine makes the mineral weather faster.

b. Inosilicates

• The inosilicate group has in its structure single-chain (pyroxenes) and double chain (amphiboles) silica tetrahedral linked together by Ca, Mg, or Fe.
• Because of the presence of many weak spots provided by the Ca-O, Mg-O, or Fe-O bonds, these minerals tend to weather rapidly.

c. Phyllosilicates

• Linkages of silica tetrahedral and Alumina octahedral sheets by mutually shared oxygen atoms.
• Some of the minerals, e.g., biotite and muscovite, are relatively susceptible to weathering, whereas others, like clay minerals, are resistant weathering

d. Tectosilicates

• Are considered solid solution minerals with a framework of silica tetrahedral, in which the cavities are occupied by Na, Ca, and so on.
• The minerals in this group may also vary considerably in their resistance to weathering, e.g., leucite and plagioclase versus potash fertilizers.

D. Non-silicate minerals

a. Oxides:

• Hematite (Fe2O3)
• Limonite (Fe2O3, 3H2O)
• Goethite (FeO (OH) H2O)
• Gibbsite (Al2O3H2O)

b. Carbonates:

• Calcite (CaCO3)
• Dolomite (CaMgCO3)

c. Sulphates:

• Gypsum (CaSO4.2H2O)

d. Phosphates:

• Apatite (Rock phosphate Ca3 (PO4)2 – primary source of phosphorus