sanidine

Sanidine is a potassium feldspar mineral with the chemical formula KAlSi3O8. It represents the high-temperature polymorph of the potassium feldspar group and is one of the end-members in the solid-solution series that includes orthoclase and microcline. Sanidine forms in high-temperature environments, crystallizing from silicic magmas and is commonly found in rhyolites and other felsic volcanic rocks as phenocrysts or large euhedral crystals in obsidian and related rocks. As magma cools, sanidine may transform to lower-temperature forms such as orthoclase or microcline.

In terms of occurrence, sanidine is typical of landscapes formed by high-temperature crystallization, including rhyolitic lava flows, ignimbrites, and other silica-rich volcanic rocks. It can also occur in metamorphic aureoles adjacent to intrusions where temperatures were sufficiently high. Sanidine is a framework silicate (tectosilicate) and, like other feldspars, forms a three-dimensional AlSi3O8 network with potassium occupying one-dimensional channels.

Physical properties commonly observed include a color range from colorless to pale pink, a vitreous luster, and a hardness near 6 on the Mohs scale. It displays two cleavages at or near 90 degrees and typically forms as relatively euhedral crystals or grains in volcanic rocks.

Sanidine is important in geochronology because of its high potassium content and tendency to retain argon, making it a favored mineral for radiometric dating methods such as K-Ar and 40Ar/39Ar dating of volcanic rocks.