The group name for a set of anhydrous framework silicate minerals. The most common feldspar minerals belong to the ternary system of end members of albite [Ab: Na(Si3Al)O8], anorthite [An: Ca(Si2Al2)O8], and orthoclase [Or: K(Si3Al)O8]. Si and Al atoms are in tetrahedral sites of the aluminosilicate framework. Large monovalent or divalent cations (Na, K, Ca, Ba) occupy interstices of the framework to maintain charge balance. In general, cleavage along the {001} is perfect and good on the {010} planes. The feldspar group minerals are separated into two series, alkali (K- to Na-rich) feldspars and plagioclase (Na- to Ca-rich) feldspars. At high temperature, solid solutions are common in both alkali feldspar (between K and Na) series and plagioclase feldspar (between Na and Ca, and Si and Al) series, but not between K- and Ca-rich feldspars. During cooling, homogeneous compositions may exsolve to form perthite (alkali feldspar with plagioclase exsolution) or antiperthite (albite with “orthoclase” exsolution). Feldspar minerals readily weather near the surface of the earth to produce clay minerals. In addition, ground feldspar and quartz may be mixed with clays and/or clay minerals (e.g., kaolin) and fused to create porcelain and glazes, with the feldspar acting as a binder material.
Cf., alkali feldspar