Question

write a short note on minerals which form rocks
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Answer 1

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.

This excludes man-made substances (e.g. synthetic diamonds), organic substances (e.g. chitin), and substances without a fixed composition which are classified as mineraloids (e.g. volcanic glass [obsidian]).
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, plagioclasefeldspar 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.