Basic definition
The general definition of a mineral encompasses the following criteria:
Naturally occurring
Stable at room temperature
Represented by a chemical formula
Usually abiogenic (not resulting from the activity of living organisms)
Ordered atomic arrangement
The first three general characteristics are less debated than the last two.[2] The first criterion means that a mineral has to form by a natural process, which excludes anthropogenic compounds. Stability at room temperature, in the simplest sense, is synonymous to the mineral being solid. More specifically, a compound has to be stable or metastable at 25 °C. Classical examples of exceptions to this rule include native mercury, which crystallizes at −39 °C, and water ice, which is solid only below 0 °C; as these two minerals were described prior to 1959, they were grandfathered by the International Mineralogical Association (IMA).[3][4] Modern advances have included extensive study of liquid crystals, which also extensively involve mineralogy. Minerals are chemical compounds, and as such they can be described by fixed or a variable formula. Many mineral groups and species are composed of a solid solution; pure substances are not usually found because of contamination or chemical substitution. For example, the olivine group is described by the variable formula (Mg, Fe)2SiO4, which is a solid solution of two end-member species, magnesium-rich forsterite and iron-rich fayalite, which are described by a fixed chemical formula. Mineral species themselves could have a variable compositions, such as the sulfide mackinawite, (Fe, Ni)9S8, which is mostly a ferrous sulfide, but has a very significant nickel impurity that is reflected in its formula.