sanidin

Sanidine is a potassium feldspar mineral with the chemical formula KAlSi3O8. It is the high-temperature polymorph of the alkali feldspar group, the others being orthoclase and microcline. Sanidine crystallizes in high-temperature environments and, as rocks cool, can transform to orthoclase and eventually to microcline.

Occurrence and formation: Sanidine is most commonly found in felsic volcanic rocks, such as rhyolites and ignimbrites, including obsidian, where it forms euhedral to subhedral crystals. It is also present in some plutonic rocks as late-stage or phenocryst crystals formed during the crystallization of felsic magmas. Its occurrence is often associated with high-temperature volcanic processes.

Physical properties: Sanidine ranges in color from colorless to white, with some samples showing pale yellow or pink tints. It has a vitreous luster, hardness of about 6 on the Mohs scale, and a specific gravity around 2.54–2.57. It exhibits two directions of cleavage at or near 90 degrees and a conchoidal to uneven fracture in coarser grains.

Geological significance and dating: Sanidine is important in geochronology because it can retain argon at high temperatures, making it suitable for potassium-argon and argon-argon dating of volcanic rocks. Its presence helps distinguish high-temperature feldspar in rock suites and complements studies of rock provenance and crystallization history alongside orthoclase and microcline.