foitite
{{Short description|Tourmaline mineral}}
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{{Infobox mineral
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Foitite is a mineral in the tourmaline group,{{Cite web|title=Mineral Database - Mineralogy of Wales|url=https://museum.wales/mineralogy-of-wales/database/?mineral=334&name=Foitite|access-date=2021-10-12|website=National Museum Wales|language=en}} it is a vacancy-dominant member of the group.{{Cite web|title=Foitite gemstone information|url=https://www.gemdat.org/gem-1579.html|access-date=2021-10-28|website=www.gemdat.org}} Foitite is in the 'vacancy' group, due to the absence of atoms in the X site.
According to the Czech Geological Society, foitite is rare.{{Cite book|url=https://books.google.com/books?id=2ZRPAQAAIAAJ&q=Foitite+gem|title=Journal of Geosciences|date=2010|publisher=Czech Geological Society|pages=17|language=en}} However foitite is quite common among low-temperature tourmalines{{Cite web|title=Foitite|url=https://www.mindat.org/min-1579.html|access-date=2021-10-12|website=www.mindat.org}} and it might be more common than previously thought.{{cite web|title=Foitite|url=https://museum.mtu.edu/pdfs/FOITITE.pdf|access-date=29 October 2021|publisher=The A. E. Seaman Mineral Museum}}
The color of the mineral is dark indigo with purple tints to bluish-black.
Name
Chemistry
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It lacks alkalis like sodium.{{Cite web|title=Wisconsin Geological & Natural History Survey » Foitite|url=https://wgnhs.wisc.edu/minerals/foitite/|access-date=2021-10-14|website=wgnhs.wisc.edu}}
Foitite is a member of the tourmaline mineral group, which is a complex aluminum borosilicate. The chemical formula of foitite is (NaFe3+Al6)(Al6Si6O18)(BO3)3(OH)4.
In this formula, sodium{{Cite web |last=UNIVERSITY of WISCONSIN–MADISON |title=Foitite |url=https://home.wgnhs.wisc.edu/minerals/foitite/}} (Na) acts as the dominant cation, iron (Fe3+) replaces some of the aluminum (Al) in the crystal structure, and boron (B) replaces some of the silicon (Si) atoms. The BO3 groups are trigonal planar borate ions, and the OH groups represent hydroxyl ions.
The crystal structure of foitite consists of continuous chains of silicon-oxygen tetrahedra, with aluminum and boron ions substituting within these chains. The aluminum and boron substitutions create charge imbalances, which are compensated by the presence of sodium and iron ions.
Foitite typically forms in granitic rocks enriched in boron, such as pegmatites. It is commonly associated with other minerals such as quartz, feldspar, and muscovite. The presence of iron in foitite gives it a characteristic black color.
Occurrence
It occurs in quartz veins{{Cite CiteSeerX|citeseerx = 10.1.1.615.277|title = Tourmaline-Bearing Quartz Veins in the Baraboo Quartzite, Wisconsin: Occurrence and Significance of Foitite and "Oxy-Foitite"}} and possibly occurs in granite pegmatites.{{cite web |title=Foitite |url=http://www.handbookofmineralogy.org/pdfs/foitite.pdf |publisher=Mineral Data Publishing |access-date=29 October 2021 |date=2001}}
Its type locality is California. Foitite has also been reported in Marquette County, Michigan. It has been reported in Wales and it is predicted that a more widespread distribution of the mineral in Wales will be shown.
References
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