plagioclase series

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plagioclase series Isomorphous series of feldspars ranging in composition between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Between the two end-members there is a continuous solid solution series: albite, oligoclase, andesine, labradorite, bytownite, anorthite. Usually, the higher the temperature of formation the more likely the feldspar is to be calcium-rich, whereas at lower temperatures sodium-rich plagioclase tends to form. The preservation of electrical neutrality in the plagioclases is achieved by concomitant substitution of Ca2+ and Al3+ by Na+ and Si4+. See PLAGIOCLASE FELDSPAR.