{{Short description|Barium titanium cyclosilicate mineral}} {{Distinguish|text=the swelling clay Bentonite}} {{Infobox mineral | name = Benitoite | category = Cyclosilicate | image = Benitoite HD.jpg | imagesize = 260px | caption = Benitoite on natrolite | formula = BaTiSi<sub>3</sub>O<sub>9</sub> | IMAsymbol = Bni<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3 |pages=291–320|doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}</ref> | strunz = 9.CA.05 | system = Hexagonal | class = Ditrigonal dipyramidal ({{overline|6}}m2) <br />H-M symbol: ({{overline|6}} m2) | symmetry = ''P''{{overline|6}}c2 | unit cell = a = 6.641, c = 9.7597(10) [Å]; Z = 2 | color = Blue, colorless | habit = Tabular dipyramidal crystals, granular | twinning = On {0001} by rotation | cleavage = [10{{overline|1}}1] poor | fracture = Conchoidal | mohs = 6–6.5 | luster = Vitreous | refractive = n<sub>ω</sub> = 1.756 – 1.757 n<sub>ε</sub> = 1.802 – 1.804 | opticalprop = Uniaxial (+) | birefringence = δ = 0.046 | pleochroism = O = colorless; E = purple, indigo, greenish blue | dispersion = 0.036–0.046<ref>{{cite book |title=Gems: Their Sources, Descriptions and Identification | author=O'Donoghue, Matthew |edition =6th |year=2006 |page=389 |publisher=Butterworth-Heinemann |place=Oxford |isbn=978-0-75-065856-0}}</ref> | streak = White | gravity = 3.65 | melt = | fusibility = | diagnostic = | solubility = Insoluble: HCl, H<sub>2</sub>SO<sub>4</sub><br> Soluble: HF | diaphaneity = Transparent to translucent | other = Blue fluorescence under SW UV; intense blue cathodoluminescence | references = <ref>[http://webmineral.com/data/Benitoite.shtml WebMineral Listing]</ref><ref>[http://www.mindat.org/min-624.html MinDat Listing]</ref><ref name=HBM/> }}
'''Benitoite''' ({{IPAc-en|b|ə|ˈ|n|iː|t|oʊ|aɪ|t}}) is a rare, blue, barium titanium cyclosilicate mineral, found in hydrothermally altered serpentinite. It forms in low-temperature, high-pressure environments typical of subduction zones at convergent plate boundaries. Benitoite fluoresces under short-wave ultraviolet light, appearing bright blue to bluish white in color. The more rarely seen clear to white benitoite crystals fluoresce red under long-wave UV light.
It was discovered in 1907 by prospector James M. Couch in the San Benito Mountains, located in central California, southeast of San Jose. Due to its similar color, Couch originally believed it to be sapphire, a variety of corundum. In 1909, a sample was sent to the University of California, Berkeley, where mineralogist Dr. George D. Louderback realized it was a previously unknown mineral. Corundum (sapphire) has a defined Mohs hardness of 9, while benitoite is much softer. He named it "benitoite" for its occurrence near the headwaters of the San Benito River in San Benito County, California.<ref>{{cite journal |last=Louderback |first=George Davis |date=July 30, 1907 |title=Benitoite, A New California Gem Mineral. |journal=Bulletin of the Department of Geology |publisher=University of California Publications |volume=5 |issue=9 |pages=149–153 |oclc=6255540}}</ref><ref>{{cite web |url=http://www.mineralsocal.org/scfm/newsletters/2002%20march.htm |title=SCFM News March 02, Featuring Benitoite |author=Wilkins, Al |publisher=Mineralogical Society of Southern California |date=March 23, 2002 |access-date=April 3, 2014 |archive-url=https://web.archive.org/web/20130516071924/http://www.mineralsocal.org/scfm/newsletters/2002%20march.htm |archive-date=May 16, 2013 |url-status=dead }}</ref>
Benitoite occurs in a number of isolated locations globally, but gemstone-quality material has only been found in California at the Benito Gem Mine, where it was first discovered. It has been correctly identified in Montana, Arkansas, Japan, and Australia, although they formed under slightly different conditions and only grow large enough to be considered an accessory mineral. In 1985, benitoite was named as the official state gem of California.<ref>{{cite web |url=http://www.consrv.ca.gov/cgs/geologic_resources/mineral_resource_mapping/ |title=Mineral Resources |publisher=California Department of Conservation - California Geological Survey |access-date=April 3, 2014 |archive-url=https://web.archive.org/web/20080821154254/http://www.consrv.ca.gov/cgs/geologic_resources/mineral_resource_mapping/ |archive-date=August 21, 2008 |url-status=dead }}</ref><ref>{{cite web|title=Benitoite|url=http://www.gemologyonline.com/benitoite.html|work=Gemology Online|access-date=8 November 2012}}</ref>
Benitoite typically crystallizes hexagonally. Nongem crystals of benitoite can have a very rare, six-pointed, twinned form.<ref>{{cite journal |url=https://www.gia.edu/doc/FA97.pdf |title=Benitoite from the New Idria District, San Benito County, California |date=Fall 1997 |journal=Gems & Gemology |volume=33 |page=173 |issn=0016-626X |publisher=Gemological Institute of America |last1=Laurs |first1=Brendan |last2=Rohtert |first2=William |last3=Gray |first3=Michael|issue=3 |doi=10.5741/GEMS.33.3.166 }}</ref>
==Associated minerals and locations==
Benitoite typically occurs with an unusual set of minerals, along with minerals that make up its host rock. Frequently associated minerals include natrolite, neptunite, joaquinite, serpentine, and albite.
Benitoite is a rare mineral found in very few locations including San Benito County, California, Japan, and Arkansas. In the San Benito occurrence, it is found in natrolite veins within glaucophane schist within a serpentinite body. In Japan, the mineral occurs in a magnesio-riebeckite-quartz-phlogopite-albite dike cutting a serpentinite body.<ref name=HBM>[http://rruff.geo.arizona.edu/doclib/hom/benitoite.pdf Handbook of Mineralogy]</ref>
<gallery widths="200px" heights="200px" perrow="2"> File:benitoite09.jpg|Blue benitoite crystals on white natrolite, Dallas Gem Mine, San Benito Co., California, US File:Benitoite, UVL.jpg|Benitoite crystals under UV light </gallery>
==References== {{reflist|25em}}
==External links== * {{Commons category inline|Benitoite}}
Category:Cyclosilicates Category:Gemstones Category:Titanium minerals Category:Hexagonal minerals Category:Minerals in space group 188 Category:Barium minerals Category:Luminescent minerals Category:Geology of San Benito County, California Category:Geology of California Category:Symbols of California