plagioclase feldspar: see feldspar.

orthoclase feldspar: see feldspar.

feldspar or felspar, an abundant group of rock-forming minerals which constitute 60% of the earth's crust. Chemically the feldspars are silicates of aluminum, containing sodium, potassium, iron, calcium, or barium or combinations of these elements. Feldspar is found in association with all rock types, including granite, gneiss, basalt, and other crystalline rocks, and are essential constituents of most igneous rocks. Feldspars weather to yield a large part of the clay found in soils. Feldspar crystals are either monoclinic or triclinic (see crystal), and all show clean cleavage planes in two directions. Orthoclase feldspars have cleavage planes that intersect at right angles; triclinic feldspars, including the plagioclase feldspars (e.g., albite, anorthite, and labradorite) and microcline, have cleavage planes that form slightly oblique angles. Pure feldspar is colorless and transparent but the mineral is commonly opaque and found in a variety of colors. Orthoclase and microcline are called potassium or potash feldspars. They usually range from flesh color to brick red, although other colors are found, and are used in the making of porcelain and as a source of aluminum in making glass. Moonstone is a milky, bluish variety of orthoclase used as a gem, and a green variety of microcline known as amazonite, or Amazon stone, is used for ornamental purposes. The plagioclase feldspars are most commonly gray and occasionally red. Another form of feldspar, labradorite, exhibits a play of colors, which makes it valuable for decorative purposes.

Any of a group of aluminosilicate (containing aluminum and silicon) minerals that also contains calcium, sodium, or potassium. Feldspars are the most common minerals in the Earth's crust and are the major component in nearly all igneous rocks found on the Earth, on the Moon, and in some meteorites. They also are common in metamorphic and some sedimentary rocks. Their complex chemical and structural properties make them useful for interpreting the origins of rocks. Natural feldspars can be divided into alkali and plagioclase feldspars.

Learn more about feldspar with a free trial on Britannica.com.