As we know, minerals are the individual units of rocks forming the crust of the earth. Thus rocks constitute the storehouse of minerals. On the basis of usefulness, all the minerals can be grouped into (i) Economic minerals and (ii) Rock-forming minerals.
The rock-forming minerals form a greater proportion of the rocks and are produced by normal mineral forming processes. Within the three major categories of rocks, the different rock-types are characterised by their own typical mineral assemblages. Silicates are by far the most important group of rock-forming minerals.
Most of them and all the important ones can be grouped into ‘groups’ or ‘families’ having members of physical, chemical as well as optical similarities. Non-silicate minerals are much less important as rock- forming minerals. Of the 2000 known mineral species about 50 are rock-formers of which only about two dozens are most common.
The rock-forming minerals may be classified as follows:
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1. Group-forming silicates)
} Silicate Minerals.
2. Non-family silicates
3. Non-silicate minerals.
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1 and 2. The silicate minerals are classified into various groups on the basis of the structural groups they contain, i.e., according to their atomic-structure. Their fundamental unit in building of silicate-minerals is the Si04 group in which the silicon atom is situated at the centre of a tetrahedron, whose corners are occupied by four oxygen atoms. The various structures may be regarded as being derived from a tetrahedral unit of Si04) by linking them together with the elimination of an oxygen atom at each linkage. There are mainly seven types of silicate structure, viz.
(a) Neso-silicates:
The structure is that of independent (SiO4) tetrahedron. e.g., Olivine, Zircon, Garnet etc.
(b) Soro silicates:
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Two tetrahedra sharing one oxygen, i.e. Si207, e.g., Melilite.
(c) Cydo-silicates:
These are closed rings of tetrahedra, haring two oxygens, i.e., the ratio between ‘Si’ and ‘O’ is 1:3.
There are three types of rings also as
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(i) each of 3-tetrahedra sharing an oxygen atom, e.g, Benitoite,
(ii) each of 4-tetrahedra sharing an oxygen atom, e.g., Axinite.
(iii) each of 6-tetrahedra sharing an oxygen atom, e.g., Beryl.
(d) Ino-silicates:
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These are also known as chain structure and are of two types:
(i) Single-chain structure:
In this case two oxygen atoms of the silicon-tetrahedron are snared. Si: 0=1: 3. These constitute continuous single chains of tetrahedral, e.g., Pyroxene.
(ii) Double-chain structure:
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These are continuous double chains of tetrahedral alternately sharing two and three oxygen. The ratio of silicon to oxygen is 4: 11, e.g., Amphibole. These are also known as Band structure.
(e) Phyllosilicates:
These are continuous sheets of tetrahedral sharing three oxygen’s. Ratio of silicon to oxygen is 4: 10, e.g., Mica. Also known as sheet structure.
(f) Tekto silicates:
These are three-dimensional frame-work of tetrahedral with all four oxygen atoms shared. Also known as Frame work structure. Ratio of silicon to oxygen is 1:2, e.g., Quartz, Felpspar etc.
Accordingly the following major silicate families have been, recognised:
(a) Olivine group. (b) Silica group.
(c) Felspathoid group. (d) Pyroxene group.
(e) Amphibole group. (f) Mica group.
Other silicate groups which are less abundant are
1. Garnet group. 2. Epidote group.
3. Zeolite group. 4. Kaoline group.
5. Alumino-silicates.
The other silicate minerals which do not form groups are as follows.
(i) Zircon, (ii) Sphene, (iii) Topaz, (iv) Staurolite, (v) Beryl, (vi) Cordierite, (vii) Tourmaline, (viii) Talc, (ix) Serpentine. (X) dumortierite etc.
3. Non-silicate minerals:
These are oxides, hydroxides, carbonates, sulphides, sulphates etc. Among the non-silicates the oxides are most important since many of them occur as minor accessories of rocks.
Minerals like quartz, feldspar, mica, amphiboles, pyroxenes and olivines are the common constituents of igneous rocks of different igneous types.
In case of metamorphic rocks minerals like kyanite, sillimanite, staurolite, andalusite, chlorite, garnet etc. are commonly found.
As the sedimentary rocks are formed due to the consolidation of the weathering products of the pre-existing rocks, they may contain any mineral or assemblage of minerals. Study of the specific minerals which are commonly treated as rock-forming minerals are dealt separately.