A | B |
C | D |
(K,Na)AlSi3O8 These alkali feldspars are stable at high temperatures and occur in felsic volcanic rocks and some contact metamorphic rocks. The name sanidine refers to potassic compositions (K/(K+Na) > 0.37), which are monoclinic. More sodic compositions are termed anorthoclase which crystallize with monoclinic symmetry but invert to triclinic upon cooling, which results in polysynthetic twinning. Both are colourless in plane light (A and C), and have maximum interference colours of first order white (B, D). Sanidine commonly forms simple Carlsbad twins (B). The polysynthetic twinning in anorthoclase (Albite and Pericline Laws) is superficially similar to that of microcline, but the twins are much finer in anorthoclase (D), and these two feldspars do not form in the same environment, as anorthoclase is a volcanic product and microcline occurs in slowly cooled plutonic rocks. A and B are from a welded tuff from White River Canyon, Nevada, and C and D are from a rhyolite from an unknown location. All views are 2.2 mm across. A and C ppl, B and D x-nicols. |
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