Solid solution

Minerals such as quartz have a discrete composition and thus one chemical formula (in this case SiO₂). Other minerals can have a range of compositions between two or three end-member chemical formulae. Olivine, for example, has an end-member composition called forsterite (FeSiO₄). By gradually replacing Fe atoms with Mg atoms, a continuous range of compositions can be produced ending at the Mg end-member, fayallite (MgSiO₄). The range of compositions is called a solid solution.