When molten magma solidifies, different elements present in them freely arrange in
accordance with the attractive forces and geometric form. Silica tetrahedron is the
fundamental building blocks for the formation of different minerals. (SiO2). Different silicate minerals are ortho silicates, ino-silicates, phyllosilicates and tectosilicates. There are nonsilicate minerals also. These are different oxides, carbonates, sulphates, phosphates etc.
Minerals that are original components of rocks are called primary minerals.
(feldspar, mica, etc.). Minerals that are formed from changes in primary minerals and rocks are called secondary minerals (clay minerals). Those minerals that are chief constituents of rocks are called as essential minerals (Feldspars, pyroxenes micas etc) and those which are present in small quantities, whose presence or absence will not alter the properties of rocks are called accessory minerals (tourmaline, magnetite etc).
Ferro magnesium minerals
Pyroxenes and amphiboles: The 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, of which augite is the most important minerals in soils and it is found in basic rocks. The amphibole group of minerals are common in acidic rocks and it can be represented by the isomorphous series between tremolite actinolite olivine and hornblende. Hornblende weathers fairly rapidly. Olivine (olive-green) minerals from an isomorphous series between foresterite (Mg2 SiO4) and fayalite (Fe2SiO4).Pyroxenes are more basic in character and therefore it weathers more rapidly than amphiboles.
Micas: Micas occur extensively in soils. They are primarily originated from the
parent rock from which the soil is derived. Generally soils are inherited from well-ordered
and imperfectly ordered micas. Well-ordered micas are derived from sedimentary rocks.
The most common well ordered micas are muscovite, paragonite, biotite and phlogopite
(trioctahedral). The imperfectly ordered micas contain less potassium and more water as
compared to well-ordered micas and this type of micas are most abundant in the clay
fraction of soils. Among the ordered micas, biotite weathers more rapidly than muscovite.
In imperfectly ordered micas, many of the illite-type specimens as well as the disordered
micas of soils exhibits some mixed-layering with phases of vermiculite, smectite group of
minerals, chlorite and intergrades of several of these species.
Feldspars: Feldspars are anhydrous aluminosilicates of K, Na and Ca and occasionally of
other large cations such as Ba. The feldspar structure consists of tetrahedral which are
attracted by sharing each oxygen atom between neighbouring tetrahedran. The tetrahedral
contain mainly Silicons with sufficient Al substitution. It belongs to the group of minerals
that are light in weight. There are two groups of feldspars:
(i) potassium feldspars
(KA1Si3O8) include orthoclase, microcline, adularia and sanidine. Orthoclase and
microcline are more common in the plutonic and metamorphic rocks. The potassium
feldspars occur commonly in the silts and sands of soils and also abundant in clay-size,
(ii) plagioclase feldspars- a series consisting of a solid solution of albite (NaA1Si3O8) high in sodium and anorthite (CaA12Si2O8) high in calcium. Plagioclase weathers more rapidly than orthoclase