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This article is about a mineral. For the Malcolm in the Middle character see list of characters.
Feldspars (
KAlSi3O8 -
NaAlSi3O8 -
CaAl2Si2O8) are a group of rock-forming
tectosilicate minerals which make up as much as 60% of the
Earth's
crust.
Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic 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
Spath, "a
rock that does not contain
ore". "Feldspathic" refers to materials that contain feldspar. The alternate spelling,
felspar, has now largely fallen out of use.
Compositions
This group of minerals consists of framework
tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three
endmembers:
Potassium-Feldspar (K-spar) endmember
KAlSi3O8 — 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. 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 colours 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.
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.
Potassic feldspars are used frequently in K-Ar dating and argon-argon dating, with applications to both geochronology and thermochronology.
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
List of minerals
List of countries by feldspar production
References
Bonewitz, Ronald Louis. (2005). Rock and Gem, New York: DK Publishing. ISBN 978-0-7566-3342-4
Category:Tectosilicates
Category:German loanwords
Category:Triclinic minerals
Category:Monoclinic minerals
![Uluru is dominantly composed of coarse-grained arkose, a type of sandstone characterized by an abundance of feldspar, and some conglomerate.[5][7] Average composition is 50% feldspar, 25–35% quartz and up to 25% rock fragments; most feldspar is K-feldspar with only minor plagioclase as subrounded grains and highly altered inclusions within K-feldspar.](http://web.archive.org./web/20110210120113im_/http://cdn.wn.com/pd/73/7e/cd55d6403c077a47e927aa0b6f0f_grande.jpg "Uluru is dominantly composed of coarse-grained arkose, a type of sandstone characterized by an abundance of feldspar, and some conglomerate.[5][7] Average composition is 50% feldspar, 25–35% quartz and up to 25% rock fragments; most feldspar is K-feldspar with only minor plagioclase as subrounded grains and highly altered inclusions within K-feldspar.")
![Uluru is dominantly composed of coarse-grained arkose, a type of sandstone characterized by an abundance of feldspar, and some conglomerate.[5][7] Average composition is 50% feldspar, 25–35% quartz and up to 25% rock fragments; most feldspar is K-feldspar with only minor plagioclase as subrounded grains and highly altered inclusions within K-feldspar.](http://web.archive.org./web/20110210120113im_/http://cdn.wn.com/pd/73/7e/cd55d6403c077a47e927aa0b6f0f_thumb.jpg "Uluru is dominantly composed of coarse-grained arkose, a type of sandstone characterized by an abundance of feldspar, and some conglomerate.[5][7] Average composition is 50% feldspar, 25–35% quartz and up to 25% rock fragments; most feldspar is K-feldspar with only minor plagioclase as subrounded grains and highly altered inclusions within K-feldspar.")
