Adularia is a rare low temperature polymorph of the most common K-feldspar Orthoclase.
There are two form of feldspar; plagioclase feldspars and K-feldspars. The K-feldpars contain potassium. They are all solid solutions which contain varying proportions of potassium and sodium. There are several K-feldspar polymorphs; minerals which have the same chemical formulas but different atomic structures and crystal lattices.