Benitoite

Not to be confused with the swelling clay Bentonite.

Benitoite
Benitoite on natrolite
General
Category	Cyclosilicate
Formula (repeating unit)	BaTiSi3O9
IMA symbol	Bni[1]
Strunz classification	9.CA.05
Crystal system	Hexagonal
Crystal class	Ditrigonal dipyramidal (6m2) H-M symbol: (6 m2)
Space group	P6c2
Unit cell	a = 6.641, c = 9.7597(10) [Å]; Z = 2
Identification
Color	Blue, colorless
Crystal habit	Tabular dipyramidal crystals, granular
Twinning	On {0001} by rotation
Cleavage	[1011] poor
Fracture	Conchoidal
Mohs scale hardness	6–6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.65
Optical properties	Uniaxial (+)
Refractive index	nω = 1.756 – 1.757 nε = 1.802 – 1.804
Birefringence	δ = 0.046
Pleochroism	O = colorless; E = purple, indigo, greenish blue
Dispersion	0.036–0.046[2]
Solubility	Insoluble: HCl, H2SO4 Soluble: HF
Other characteristics	Blue fluorescence under SW UV; intense blue cathodoluminescence
References	[3][4][5]

Benitoite (/bəˈniːtoʊaɪt/) 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 roughly halfway between San Francisco and Los Angeles. 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.^[6][7]

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.^[8][9]

Benitoite typically crystallizes hexagonally. Non-gem crystals of benitoite can have a very rare, six-pointed twinned form.^[10]

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.^[5]

• 
  Blue benitoite crystals on white natrolite, Dallas Gem Mine, San Benito Co., California, US
• 
  Benitoite crystals under UV light

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. O'Donoghue, Matthew (2006). Gems: Their Sources, Descriptions and Identification (6th ed.). Oxford: Butterworth-Heinemann. p. 389. ISBN 978-0-75-065856-0.
3. WebMineral Listing
4. MinDat Listing
5. Handbook of Mineralogy
6. Louderback, George Davis (July 30, 1907). "Benitoite, A New California Gem Mineral". Bulletin of the Department of Geology. 5 (9). University of California Publications: 149–153. OCLC 6255540.
7. Wilkins, Al (March 23, 2002). "SCFM News March 02, Featuring Benitoite". Mineralogical Society of Southern California. Archived from the original on May 16, 2013. Retrieved April 3, 2014.
8. "Mineral Resources". California Department of Conservation - California Geological Survey. Archived from the original on August 21, 2008. Retrieved April 3, 2014.
9. "Benitoite". Gemology Online. Retrieved 8 November 2012.
10. Laurs, Brendan; Rohtert, William; Gray, Michael (Fall 1997). "Benitoite from the New Idria District, San Benito County, California" (PDF). Gems & Gemology. 33 (3). Gemological Institute of America: 173. doi:10.5741/GEMS.33.3.166. ISSN 0016-626X.