Feldspars are a very common and diverse collection of minerals.
They can be divied into three main groups: the Alkali Feldspars,
Plagioclase, and Celsian groups. The Alkali Feldspars have a
basic formula of (K,Na)[AlSi3O8]. The Plagioclase Feldspars have
a basic formula of Na[AlSi3O8]-Ca[Al2Si2O8]. Celsian Feldspar is
Ba[Al2Si2O8]. Felspars that are a blend of both the alkali and
plagioclase formulas are also common. The ternary solid solution
within which all the Alkali and Plagioclase feldspars fit has as
its apices Albite (Na[AlSi3O8]), Orthoclase (K[AlSi3O8]), and
Anorthite (Ca[Al2Si2O8]). A particular feldspar can be
characterized as Anx-Aby-Orz.