Anendmember (alsoend-member orend member) inmineralogy is amineral that is at the extreme end of amineral series in terms of purity of its chemical composition. Minerals often can be described assolid solutions with varying compositions of somechemical elements, rather than as substances with an exact chemical formula. There may be two or more endmembers in a group or series of minerals.
For example,forsterite (Mg
2SiO
4) andfayalite (Fe
2SiO
4) are the two end-members of theolivinesolid-solution series, varying inMg2+
andFe2+
in their chemical composition. So, the chemical formula of olivine can be better expressed asMg(2−x)FexSiO4 orMgxFe(2−x)SiO4.
As another example, thetectosilicatefeldspar can be described as a solid solution of the endmembersK-feldspar (KAlSi
3O
8),albite (NaAlSi
3O
8) andanorthite (CaAl
2Si
2O
8).[1] A specific feldspar can have varying quantities ofpotassium (K),sodium (Na) andcalcium (Ca).
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