Alkali feldspars rank with quartz and plagioclase as the most common minerals in the earth's crust. Chemically they are represented by the formula (K,Na)AlSi3 O8. The chemical end member KAlSi3O8(Or) includes the minerals orthoclase , microcline , and K-rich sanidine ; and the end member NaAlSi3O8(Ab) is the mineral albite . In most natural specimens, there is also a small amount of the anorthite component CaAl2Si3O8 Traces of Rb, Tl, Pb, Ba, Sr, Fe3+, and Ge may also be present. Alkali feldspars occur extensively in a variety of geologic environments, and, owing to their compositional and structural complexity, are especially useful as petrologic indicators.
Most natural specimens are whitish, gray, or pinkish. They have two well-developed cleavages at or nearly at right angles to one another; relatively low density (2.55–2.63 gm/cm3); and monoclinic or triclinic crystal symmetry. In thin sections, alkali feldsparsare colorless and can be identified by their cleavage, low relief,...
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