Feldspar

[feld-spahr, fel-] /ˈfɛldˌspɑr, ˈfɛl-/

noun
1.
any of a group of minerals, principally aluminosilicates of potassium, sodium, and calcium, characterized by two cleavages at nearly right angles: one of the most important constituents of igneous rocks.
/ˈfɛldˌspɑː; ˈfɛlˌspɑː/
noun
1.
any of a group of hard rock-forming minerals consisting of aluminium silicates of potassium, sodium, calcium, or barium: the principal constituents of igneous rocks. The group includes orthoclase, microcline, and the plagioclase minerals
n.

1785, earlier feldspath (1757), from German Feldspath (Modern German Feldspat), from Feld “field” (see field (n.)) + spath “spar, non-metallic mineral, gypsum” (see spar (n.2)); spelling influenced by English spar “mineral.”
feldspar
(fěld’spär’, fěl’-)
Any of a group of abundant monoclinic or triclinic minerals having the general formula MAl(Al,Si)3O8, where M is either potassium (K), sodium (Na), or calcium (Ca) or less commonly barium (Ba), rubidium (Rb), strontium (Sr), or iron (Fe). Feldspars range from white, pink, or brown to grayish blue in color. They occur in igneous, sedimentary, and metamorphic rocks and make up more than 60 percent of the Earth’s crust. When they decompose, feldspars form clay or the mineral kaolinite.