透长石是一种矿物名。是中酸性火山岩的常见矿物，形成于500℃以上。不溶于大多数酸，但是会溶于氢氟酸，形成于多种火山岩以及接触变质岩中。

Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi₃O₈). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures. At yet lower temperatures, microcline, a triclinic polymorph of potassium feldspar, is stable.