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.
[edit] References
- ^ a b c d Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on February 9, 2008.
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