Minerals are defined as naturally occurring, homogenous, solid substances with a specific chemical composition and ordered atomic structure (crystalline), generally formed through inorganic processes. Over 5,000 minerals are known to exist worldwide, of which more than 300 have been found in Iceland.
Minerals may be classified in a number of ways, for example by distinguishing between primary (rock-forming) minerals, i.e., minerals such as olivine and plagioclase (a kind of feldspar) that form as magma cools and rock comes into being, and secondary minerals such as zeolites, which are formed as already existing rocks undergo alteration and secondary precipitation in vesicles, cracks, and fissures.
Minerals are also classified on the basis of their crystal structure and chemical composition. The IINH makes use of the Nickel-Strunz Classification System (2001), where minerals are divided into 10 main classes on the basis of their composition, which are further organised into a number of divisions, subdivisions and groups on the basis of crystal structure and chemical composition. Some groups are organised into families. Certain minerals can be further divided into a number of varieties. Minerals found together in rock, formed under the same conditions, and in equilibrium with each other are known as mineral assemblages.