Endmember

An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition.For example, forsterite (Mg2SiO4) and fayalite (Fe2SiO4) are the two end-members of the olivine solid-solution series, varying in Mg2+ and Fe2+ in their chemical composition.So, the chemical formula of olivine can be better expressed as Mg(2−x)FexSiO4 or MgxFe(2−x)SiO4.As another example, the tectosilicate feldspar can be described as a solid solution of the endmembers K-feldspar (KAlSi3O8), albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).[1] A specific feldspar can have varying quantities of potassium (K), sodium (Na) and calcium (Ca).
mineralogymineralsolid solutionschemical elementsforsteritefayaliteolivinesolid-solutionfeldsparK-feldsparalbiteanorthitepotassiumsodiumcalciumLarvikiteWayback Machine