A naturally occurring solid phase with a restricted chemical composition and a definite atomic arrangement. This is really an idealized definition insofar as some minerals (especially those that form at low temperature) have poorly defined atomic structures. A few solid geological materials (such as coal and obsidian) are not minerals because they have no ordered crystal structure and a completely variable composition. Rocks are usually made up of one or more minerals. In a thermodynamic sense, a mineral is a separate phase that is made up of one or more components. For example, the mineral olivine ((Fe,Mg)₂SiO₄) can be viewed as being made up of fayalite (Fe₂SiO₄) and forsterite (Mg₂SiO₄) in a solid solution.