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.

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Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust.

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.

Etymology

Feldspar is derived from the German Feld, field, and Spat, a rock that does not contain ore. "Feldspathic" refers to materials that contain feldspar. The alternative spelling, felspar, has now largely fallen out of use.

Compositions

This group of minerals consists of framework or tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:

K-feldspar endmember KAlSi₃O₈

Albite endmember NaAlSi₃O₈

Anorthite endmember CaAl₂Si₂O₈

Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or more properly plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Albite is considered both a plagioclase and alkali feldspar. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar.

The alkali feldspars are as follows:

• orthoclase (monoclinic), — KAlSi₃O₈
• sanidine (monoclinic) —(K,Na)AlSi₃O₈
• microcline (triclinic) — KAlSi₃O₈
• anorthoclase (triclinic) — (Na,K)AlSi₃O₈

Sanidine is stable at the highest temperatures, and microcline at the lowest. Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can only be seen using an electron microscope.

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):

• albite (0 to 10) — NaAlSi₃O₈
• oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi₂O₈
• andesine (30 to 50) — NaAlSi₃O₈ — CaAl₂Si₂O₈
• labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si₂O₈
• bytownite (70 to 90) — (NaSi,CaAl)AlSi₂O₈
• anorthite (90 to 100) — CaAl₂Si₂O₈

Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.

The barium feldspars are monoclinic and comprise the following:

• celsian — BaAlSi₃O₈
• hyalophane — (K,Na,Ba)(Al,Si)₄O₈

Uses

• Feldspar is a common raw material in the production of ceramics and geopolymers.
• Feldspars are used for thermoluminescence dating and optical dating in earth sciences and archaeology
• Feldspar is an ingredient in Bon Ami US brand household cleaner.
• Feldspar is often an anti-caking agent used in powdered forms of non-dairy creamer.

In 2005, Italy was the top producer of feldspar with almost one-fifth world share followed by Turkey, China and Thailand, reports the International Monetary Fund.

References

• Bonewitz, Ronald Louis. (2005). Rock and Gem, New York: DK Publishing. ISBN 978-0-7566-3342-4

See also

• List of minerals