feldspar

[Partial translation of obsolete German Feldspath : Feld, field (from Middle High German veld , from Old High German feld) + Spath, spar.]

Any of a group of aluminosilicate minerals whose crystal structures are composed of corner-sharing [AlO₄] and [SiO₄] tetrahedra linked in an infinite three-dimensional array, with charge-balancing cations primarily sodium (Na), potassium (K), and calcium (Ca)] occupying large, irregular cavities in the framework of the tetrahedra. Collectively, the feldspars constitute about 60% of the outer 8–10 mi (13–17 km) of the Earth's crust. They are nearly ubiquitous igneous and metamorphic rocks, and are a primary constituent of arkosic sediments derived from them. The importance of the many feldspars that occur so widely in igneous, metamorphic, and some sedimentary rocks cannot be underestimated, especially from the viewpoint of a petrologist attempting to unravel earth history. See also Arkose; Mineralogy; Petrology; Silicate minerals.

With weathering, feldspars form commercially important clay materials. Economically, feldspars are valued as raw material for the ceramic and glass industries, as fluxes in iron smelting, and as constituents of scouring powders. Occasionally their luster or colors qualify them as semiprecious gemstones. Some decorative building and monument stones are predominantly composed of weather-resistant feldspars. See also Clay minerals; Igneous rocks; Metamorphic rocks.

The general formula AT₄O₈ characterizes the chemistry of feldspars, where T (for tetrahedrally coordinated atom) represents aluminum (Al) or silicon (Si). The A atom is Ca²⁺ or barium (Ba²⁺) for the [Al₂Si₂O₈]²⁻ alkaline-earth feldspars and Na⁺ or K⁺ for the [AlSi₃O₈]⁻ alkali feldspar series of solid solutions and mixed crystals.

Knowledge of a feldspar's composition and its crystal structure is indispensable to an understanding of its properties. However, it is the distribution of the Al and Si atoms among the available tetrahedral sites in each chemical species that is essential to a complete classification scheme, and is of great importance in unraveling clues to the crystallization and thermal history of many igneous and metamorphic rocks.

Alkali feldspars are assigned to the polymorphs of KAlSi₃O₈ and NaAlSi₃O₈ in accordance with their symmetry and the Al content of their tetrahedral sites. See also Albite; Microcline; Orthoclase.

Anorthoclase is a triclinic solid solution of composition Or₃₇Ab₆₃-Or₀Ab₁₀₀ containing up to 10 mol % anorthite, or more. See also Anorthoclase.

Plagioclase feldspars containing significant amounts of exsolved K-rich feldspar are called antiperthites. It is only in once-molten rocks quenched at very high temperatures that the full range of so-called high plagioclases exist as simple solid solutions. With very slow cooling over millions of years, complex textures develop in most feldspar crystals as a coupled NaSi, CaAl ordering. See also Andesine; Bytownite; Labradorite; Oligoclase; Solid solution.

The variable properties of feldspars are determined by their structure, symmetry, chemical composition, and crystallization and subsequent history of phase transformation, exsolution, and alternation or deformation. Very few feldspars are transparent and colorless; many are white or milky due to internal reflections of light from inclusions, exsolution interfaces, and fracture or cleavage surfaces. Plagioclases are slightly harder (6–6.5) on Mohs scale than K-rich feldspars (6). Feldspars are brittle and, when broken, cleave along the (001) and (010) crystallographic planes.

A crystalline mineral of aluminum silicate with potassium, sodium, barium, or calcium—NaAlSi3O8 or KA_lSi3O₈. Feldspar melts over a range of 1,100° to 2,000° F (593.5° to 1093.5° C). An important constituent of dental porcelain.

For more information on feldspar, visit Britannica.com.

A group of igneous minerals, all of which are softer than quartz, having the chemical composition of calcium silicates, potassium silicates, or sodium-aluminum silicates.

see plagioclase feldspar.

This article is about a mineral. For the Malcolm in the Middle character see list of characters

feldspar Potassium feldspar crystals in a granite, eastern Sierra Nevada, Rock Creek Canyon, California. Scale bar is 2.0 cm. General Category tectosilicate Chemical formula KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8 Identification Color pink, white, gray, brown Crystal system triclinic or monoclinic Twinning tartan, carlsbad, etc Cleavage two or three Fracture along cleavage planes Mohs scale hardness 6 Luster vitreous Diaphaneity opaque Birefringence first order Pleochroism none Other characteristics exsolution lamellae common Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). Collected by Apollo 16 from the Lunar Highlands near Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, DC, United States. (Unknown scale.) Compositional phase diagram of the different minerals that constitute the feldspar solid solution. Feldspar. Alkali feldspar perthite (7cm long X 3cm width). Labradorite. Feldspar output in 2005. Click the image for the details.

Feldspars (KAlSi₃O₈ - NaAlSi₃O₈ - CaAl₂Si₂O₈) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.^[1]

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock.^[2] Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite.^[3] Feldspars are also found in many types of sedimentary rock.^[4]

Contents 1 Etymology 2 Compositions 2.1 Alkali Feldspars 2.2 Plagioclase Feldspars 2.3 Barium Feldspars 3 Uses 4 See also 5 References

Etymology

Feldspar is derived from the German Feld, "field", and Spath, "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.^[5]

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:

Potassium-Feldspar (K-spar) endmember KAlSi₃O₈^[1]

Albite endmember NaAlSi₃O₈^[1]

Anorthite endmember CaAl₂Si₂O₈^[1]

Solid solutions between K-feldspar and albite are called alkali feldspar.^[1] Solid solutions between albite and anorthite are called plagioclase,^[1] 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.

Alkali Feldspars

The alkali feldspars are as follows:

• orthoclase (monoclinic),^[6] — KAlSi₃O₈
• sanidine (monoclinic)^[7] —(K,Na)AlSi₃O₈
• microcline (triclinic)^[8] — KAlSi₃O₈
• anorthoclase (triclinic) — (Na,K)AlSi₃O₈

Sanidine is stable at the highest temperatures, and microcline at the lowest.^[7][6] 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.^[9] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Plagioclase Feldspars

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.

Barium Feldspars

The barium feldspars are monoclinic and comprise the following:

• celsian — BaAl₂Si₂O₈
• hyalophane — (K,Na,Ba)(Al,Si)₄O₈

Feldspars can form clay minerals through chemical weathering.^[10]

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 one of several abrasive ingredients in Bon Ami, a brand of household cleaner in the USA.

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

See also

Wikimedia Commons has media related to: Feldspar

• List of minerals

References

1. ^ ^{a b c d e f} Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18, 2007.
2. ^ "Metamorphic Rocks." Metamorphic Rocks Information. Retrieved on July 18, 2007
3. ^ "Gem, Rock, and Mineral Postage Stamps Featuring Anorthosite." Anorthosite Retrieved on February 8, 2008.
4. ^ "Weathering and Sedimentary Rocks." Geology. Retrieved on July 18, 2007.
5. ^ Harper, Douglas. "feldspar". Online Etymology Dictionary. http://www.etymonline.com/index.php?term=feldspar. Retrieved 2008-02-08. 
6. ^ ^{a b} "The Mineral Orthoclase." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
7. ^ ^{a b} "Sanidine Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
8. ^ "Microcline Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.
9. ^ Ralph, Jolyon & Ida. "Perthite." Perthite Profile on mindat.org Retrieved on February 8, 2008.
10. ^ Nelson, Stephen A. (Fall 2008). "Weathering & Clay Minerals". Professor's lecture notes (EENS 211, Mineralogy). Tulane University. http://www.tulane.edu/~sanelson/eens211/weathering&clayminerals.htm. Retrieved 2008-11-13. 

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

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