Endmember (mineralogy)
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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 tectosilicate feldspar^[1] can be described as compositions of the endmembers K-feldspar [KAlSi₃O₈ ^[1], albite [NaAlSi₃O₈^[1] and anorthite [CaAl₂Si₂O₈ ^[1]. A specific feldspar can have varying quantities of potassium (K), sodium (Na) and calcium (Ca).

The plagioclase series of minerals within the feldspar group ranges from endmembers albite to anorthite, where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. 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.)

Yet another example is apatite, a group of phosphate minerals having hydroxylapatite, fluorapatite, and chlorapatite as endmembers. The formula of the admixture of the three most common species is written as Ca₅(PO₄)₃(OH, F, Cl), and the formulae of the individual minerals are written as Ca₅(PO₄)₃(OH), Ca₅(PO₄)₃F and Ca₅(PO₄)₃Cl, respectively.

References

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