K-feldspar

K-feldspar is widespread and common in a wide variety of igneous and metamorphic rocks and in some sedimentary rocks. Its chemistry and atomic arrangement are variable, so optical properties vary as well. The three principle varieties of K-feldspar are microcline (low temperature), orthoclase (medium temperature), sanidine (high temperature). Other, rare, related feldspar varieties include adularia and anorthoclase. Microcline is found in silicic to intermediate plutonic rocks (not volcanics) and some metamorphic rocks. Orthoclase is common in many silicic plutonic rocks, less commonly in volcanic rocks. It is also found in metamorphic and some sedimentary rocks. Sanidine is the common K-feldspar in silicic volcanic rocks.