Pseudobrookite, Sanidine Field of view: 1.41 mm Locality: New Mexico Highway 1 roadcut, Sierra County, New Mexico, USA Description: These black pseudobrookite c — Photo: Jerry Cone · Public domain
Sanidine
Also known as: Glassy feldspar.
The high-temperature potassium feldspar — glassy crystals of rhyolite and trachyte lavas.
Intro adapted from Wikipedia (CC BY-SA).
Sanidine is an IMA-recognized mineral species in the silicate class, with the chemical formula K(AlSi₃O₈).
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Feldspar structural group.
Key properties: Mohs hardness 6–6.5; specific gravity 2.56–2.62; typically colorless or white or gray; White streak; Vitreous, pearly on cleavage luster; optically biaxial.
IMA symbol Sa, grandfathered status — known before the IMA’s founding, named in 1808.
Most common in felsic volcanic and hypabyssal rocks as rhyolites, phonolites, trachytes; as spherulites in volcanic glass. Also from ultrapotassic mafic, high-temperature contact metamorphic (sanidinite facies).
It is found associated with Quartz, sodic plagioclase, muscovite, biotite, \hornblende, " magnetite.
Its name comes from the Greek for tablet or board, in allusion to the mineral's common habit.
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