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Endmember (mineralogy)

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.)

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.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Endmember_(mineralogy)". A list of authors is available in Wikipedia.
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