Terminology of “potassium feldspar” and “alkali feldspar”

Abstract

The usage of “potassium feldspar” and “alkali feldspar” has been confused in earth sciences for a long time. The term “alkali feldspar” is for the solid solution series between the two components of KAlSi3O8 （potassium feldspar） and NaAlSi3O8 （sodium feldspar）, including more or less CaAl2Si2O8（ lime feldspar） as the third component. The term “potassium feldspar” is in practice for KAlSi3O8 as an end member of the feldspar solid solution. At present, the scientific or appropriate usage of the two terms without confusion is needed in earth sciences. This review outlines the history of hitherto confused usage of the two terms, and interprets how important the appropriate usage of the two terms is in earth sciences, especially in mineralogical and petrological sciences. We recommend that perthite should be termed not “potassium feldspar” but alkali feldspar, and that the term potassium feldspar should be applied to alkali feldspar at least with KAlSi3O8 content ≥ 90 mol%.