Learn more about "Feldspar"

Feldspar

Etymology

Compositions

Albite endmember Sodium|NaAluminium|Alsilicon|Si₃Oxygen|O₈ Anorthite endmember Calcium|CaAluminium|Al₂silicon|Si₂Oxygen|O₈ 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),"The Mineral Orthoclase." Feldspar Amethyst Galleries, Inc. Retrieved on February 8 2008. — KAlSi₃O₈
- sanidine (monoclinic)"Sanidine Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8 2008. —(K,Na)AlSi₃O₈
- microcline (triclinic)"Microcline Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8 2008. — 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.Ralph, Jolyon & Ida. "Perthite." Perthite Profile on mindat.org Retrieved on February 8 2008. 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 Feldspars can form clay minerals through chemical weathering.

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 Category:Tectosilicates Category:German loanwords simple:Feldspar

Related Images

- Feldspar.
- Alkali feldspar perthite (7cm long X 3cm width).
- Labradorite.
- Feldspar output in 2005. Click the image for the details.

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