Identification of rocks forming silicate and ore minerals
Theory:- Although about 4000 minerals are known to exist, only about 8 of them are common. These common rock forming minerals are the major constituents of igneous, sedimentary and metamorphic rocks. They constitute more that 99% of Earth's crust. You must be able to identify these 8 minerals, and you must know the mineral group to which each one belongs.
The following table lists the common rock-forming minerals…
Properties:- Properties Such as Hardness, Density and cleavage are often impossible to determine in these specimens. However, colour, lustre and streak are usually sufficient for identification of common ore minerals.
Rock-forming minerals:-
Minerals are the building blocks of rocks. Geologists define a mineral as:
A naturally occurring, inorganic, solid, crystalline substance which has a fixed structure and a chemical composition which is either fixed or which may vary within certain defined limits.
Some minerals have a definite fixed composition, e.g. quartz is always SiO2, and calcite is always CaCO3. However, other minerals exhibit a range of compositions between two or more compounds called end-members. For example, plagioclase feldspar has a composition that ranges between end-members anorthite (CaAl2Si2O8) and albite (NaAlSi3O8), so its chemical formula is written as (Ca, Na)(Al, Si)AlSi2O8.
There are also minerals which form both by inorganic and organic processes. For example, calcite (CaCO3) is a common vein mineral in rocks, and also a shell-forming material in many life forms. Calcite of organic origin conforms to the above definition except for the requirement that it be inorganic. This is an inconsistency in the definition of a mineral that is generally overlooked.
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How can a mineral be identified?
A particular mineral can be identified by its unique crystal structure and chemistry. Geologists working in the field, however, don't usually have access to the sophisticated laboratory techniques needed to determine these properties. More commonly, they use Properties which can be observed with the naked eye (or with a hand lens) or determined with simple tools (e.g. a pocket knife).
Useful physical properties for identifying a mineral include its cleavage / fracture, colour, crystal habit / mode of occurrence, hardness, lustre, specific gravity, streak and transparency.