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.

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, 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.


Orthoclase feldspar

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).

In hand specimen (a term referring to a piece of rock or mineral able to be easily handled) the nearly constant physical characteristics possessed by a mineral can be used in its identification. It is important to realise that although a particular mineral may be found in several apparently differing forms (e.g. colour, habit), its fundamental physical properties will always be the same.

Useful physical properties for identifying a mineral include its cleavage / fracture, colour, crystal habit / mode of occurrence, hardness, lustre, specific gravity, streak and transparency.

Metallic lustre