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Feldspar is the most common mineral on Earth, constituting approximately 60% of the crust . It forms directly from cooling magma and is a major component of granite and most other igneous rocks .

The term feldspar actually covers a whole family of minerals , all of which consist of a framework of aluminum , oxygen , and silicon atoms plus an additive, usually potassium, sodium, or calcium. Feldspars vary in color from pink to gray, and are categorized by the additives they contain. Pure potassium feldspar is orthoclase (KAlSi3O8), pure sodium feldspar is albite (NaAlSi3O8), and pure calcium feldspar is anorthite (CaAl2Si3O8). A feldspar may contain both sodium and calcium or sodium and potassium. The sodiumcalcium feldspars form a continuum from albite to anorthite, the plagioclase feldspar series, which corresponds to the continuous branch of Bowen's reaction series . The sodiumpotassium feldspars form a continuum from albite to orthoclase that is termed the orthoclase or alkali feldspar series. Feldspars containing significant quantities of both calcium and potassium are not found, as such mixtures are not chemically stable in cooling magma and react to form other minerals.

Orthoclase feldspars cleave along two planes that are at right angles, and plagioclase feldspars cleave along two planes that are not quite at right angles. Feldspar nomenclature is based on these mechanical properties: ortho, plagio, and clase are the Greek for right, slanted, and breaking, respectively.

Feldspar is less chemically stable when exposed to water than quartz , the other major ingredient of granite. Granite exposed to weather therefore becomes crumbly as its feldspar decays, and mechanical forces (e.g., wind , running water) break the granite up into sand . Rough, rapid fragmentation liberates some feldspar before it has had time to decay chemically, so a sand's ratio of feldspar to quartz records the rate at which its source granite was fragmented. This information is used by geologists to deduce ancient patterns of mountain-building and erosion .

See also Weathering and weathering series

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feldspar (fĕl´spär, fĕld´–) or felspar (fĕl´spär), 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 usually 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. A milky variety of plagioclase feldspar oligoclase also is used as the gem moonstone, and a reddish or golden variety that exhibits flashes of reddish color is used as the gem sunstone. Another form of feldspar, labradorite, exhibits a play of colors, which makes it valuable for decorative purposes.

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feld·spar / ˈfel(d)ˌspär/ • n. an abundant rock-forming mineral typically occurring as colorless or pale-colored crystals and consisting of aluminosilicates of potassium, sodium, and calcium.

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feldspar Group of common rock-forming minerals that all contain aluminium, silicon and oxygen, but with varying proportions of potassium, sodium and calcium. They are essential constituents of igneous rocks. Hardness 6–6.5; r.d. 2.5–2.8.

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