DEFINITION OF FELDSPAR

A general name for a group of related framework

silicate minerals that form the most abundant constituent in

igneous rocks and form about 60 percent of the Earth’s crust.

They are common in most types of igneous rocks, except for

the ultramafic suite, and form major constituents of schists,

gneisses, migmatites, pegmatites, and in arenaceous sediments.

Most feldspars are white or clear to translucent, and

have a hardness of 6 on the Mohs hardness scale.

Most feldspars are members of a Ternary system with

three compositional end members. The compositions

NaAlSi3O8, KAlSi3O8, and CaAl2Si2O8 are referred to as sodium,

potassium, and calcium feldspar. There is a continuous

compositional range between the potassium and sodium

feldspars, with the intermediate varieties known as alkali

feldspars, including sanidine, anorthoclase, and albite. Likewise,

the continuous compositional series of feldspars between

sodium and calcium feldspar are known as the plagioclase

feldspars, with compositional variants including albite, oligoclase,

andesine, labradorite, bytownite, and anorthite. Celsian

feldspar has the composition BaAl2Si2O8.

Feldspars exhibit a variety of crystal forms and structural

states. Calcium feldspar (anorthite) and sodium feldspar

are triclinic, whereas potassium feldspar is monoclinic. Several

of the feldspars crystallize in different structural states and

may have different lattice obliquities depending on the temperature

of crystallization.

See also MINERALOGY.

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