Sanidine and high albite constitute a solid solution series with intermediate compositions termed anorthoclase.

In addition to its presence in the groundmass of felsic rocks, sanidine is a common phenocryst in rhyolites and, to a lesser extent, rhyodacites.

A typical natural composition is:[8] At elevated temperature, a complete solid solution exists between sanidine and albite.

[9] The crystal structure of ideal potassium feldspar has four sets of tetrahedral sites, each capable of accepting either an aluminum or a silicon ion.

Each transition requires exchange of ions between tetrahedral sites, which takes place at measurable rates only at high temperature.