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. Minerals often can be described as solid solutions with varying compositions of some chemical 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) and fayalite (Fe
₂SiO
₄) 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)Fe_xSiO₄ or Mg_xFe_(2−x)SiO₄.

As another example, the tectosilicate feldspar can be described as a solid solution of the endmembers K-feldspar (KAlSi
₃O
₈), albite (NaAlSi
₃O
₈) and anorthite (CaAl
₂Si
₂O
₈). A specific feldspar can have varying quantities of potassium (K), sodium (Na) and calcium (Ca).