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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 with varying compositions of some , 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 () and fayalite () are the two end-members of the olivine solid solution series, varying in and in their chemical composition. So, the chemical formula of olivine can be better expressed as or .
As another example, the tectosilicate feldspar can be described as a solid solution of the endmembers Orthoclase (), albite () and anorthite (). What is Feldspar? Industrial Minerals Association. Retrieved on February 9, 2008. A specific feldspar can have varying quantities of potassium (K), sodium (Na) and calcium (Ca).
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