Mineral

Lore & Background

The concept of mineral is distinct from rock, which is any bulk solid geologic material that is relatively homogeneous at a large enough scale. A rock may consist of one type of mineral or may be an aggregate of two or more different types of minerals, spatially segregated into distinct phases. Some natural solid substances without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound occurs naturally with different crystal structures, each structure is considered a different mineral species; for example, quartz and stishovite are two different minerals consisting of the same compound, silicon dioxide.

Reader's Guide

The definition of mineral has been a topic of contention, particularly regarding biogenic substances. The IMA normally excludes compounds that occur only in living organisms, though some minerals are often biogenic (such as calcite) or chemically organic compounds (such as mellite). Living organisms often synthesize inorganic minerals (such as hydroxylapatite) that also occur in rocks.

The IMA requires a substance to be naturally occurring, solid, with an ordered atomic arrangement, and a fairly well-defined chemical composition. Exceptions include native mercury, which is still classified as a mineral despite crystallizing only below −39 °C. The IMA is also reluctant to accept minerals that occur only as nanoparticles, but has not defined a minimum crystal size. Biogeochemical cycles have contributed to the formation of minerals for billions of years, with microorganisms capable of precipitating metals from solution and catalyzing mineral dissolution.