end-member

In mineralogy, an end-member refers to a pure chemical compound that is part of a series of minerals formed through solid solution. These end-members are hypothetical or actual pure substances that represent the extremes of a compositional range within a mineral group.12

For example, in the olivine series, forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄) are the end-members. This means that the olivine minerals found in nature can have compositions that are a mix of magnesium and iron in varying proportions, with pure forsterite and pure fayalite representing the two ends of this compositional spectrum. Most minerals exhibit such compositional variation due to the substitution of one ion for another within their crystal structure.213