a monoclinic feldspar, KAlSiO, that is dimorphic with microcline; potassium aluminum silicate
See orthoclase in American Heritage Dictionary 4
(ôrˈthə-klāsˌ, -klāzˌ)
noun
A variety of feldspar, essentially potassium aluminum silicate, KAlSi3O8, characterized by a monoclinic crystalline structure and found in igneous or granitic rock. Also called potash feldspar.