Although closely related, the feldspars fall into two sub-groups: (1) the alkali feldspars which are generally monoclinic and (2) the sodium and calcium feldspars which form a solid solution series, collectively known as the plagioclase feldspars, which are triclinic. Members of the plagioclase series thus range in composition from 100% albite (NaAlSi3O8), often abbreviated Ab, to 100% anorthite (CaAl2Si2O8), abbreviated An. Within the plagioclase series, a number of feldspar types are distinguished based on their percentage composition of albite/anorthite (Table 6).
KeywordsOscillatory Zoning Alkali Feldspar Plagioclase Feldspar Plagioclase Composition Lamellar Twinning
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