What is "endmember"

n. (context mineralogy English) Either of two minerals that occur in a range of solid solutions

An endmember in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity. 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, the tectosilicate feldspar can be described as compositions of the endmembers K-feldspar [KAlSiO], albite [NaAlSiO] and anorthite [CaAlSiO]. A specific feldspar can have varying quantities of potassium (K), sodium (Na) and calcium (Ca).