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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 endmembers of the pyralspite garnets are almandine [Fe₃Al₂(SiO₄)₃], pyrope [Mg₃Al₂(SiO₄)₃] and spessartine [Mn₃Al₂(SiO₄)₃], A specific pyralspite garnet can have varying quantities of iron (Fe), magnesium (Mg) and manganese (Mn).
As another example, the plagioclase series of tectosilicate minerals within the feldspar group ranges from albite to anorthite (with respective compositions NaAlSi₃O₈ to CaAl₂Si₂O₈), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Thus, both albite and anorthite are endmembers of the plagioclase series. Albite is also the sodium endmember of the alkali feldspars whose series ranges from pure NaAlSi₃O₈ to pure KAlSi₃O₈.
(Solid solutions between KAlSi₃O₈ and anorthite endmembers are more rare.)