Benitoite

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Benitoite
Benitoite HD.jpg
Benitoite on natrolite
General
Category Cyclosilicate
Formula
(repeating unit)
BaTiSi3O9
Strunz classification 09.CA.05
Crystal symmetry Hexagonal 6 m2 ditrigonal dipyramidal
Unit cell a = 6.641 Å, c = 9.7597(10) Å; Z = 2
Identification
Color Blue, colorless
Crystal habit Tabular dipyramidal crystals, granular
Crystal system Hexagonal
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 3.6
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[1]
Solubility Insoluble: HCl, H2SO4
Soluble: HF
Other characteristics Blue fluorescence under SW UV; intense blue cathodoluminescence
References [2][3][4]

Benitoite (pronounced /bɛn.ˈi.t.t/) is a rare blue barium titanium silicate mineral, found in hydrothermally altered serpentinite. 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 first described in 1907 by George D. Louderback, who named it benitoite for its occurrence near the headwaters of the San Benito River in San Benito County, California.[5][6]

Benitoite occurs in a number of sites, but gemstone quality material has only been found in California. In 1985 benitoite was named as the official state gem of California.[7][8]

Benitiote has a rare 5 pointed crystal form, and an even rarer 6 pointed form, "star of David", with about 24 samples known.[9]

Associated minerals and locations[edit]

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.[4]

References[edit]

  1. ^ O'Donoghue, Matthew (2006). Gems: Their Sources, Descriptions and Identification (6th ed.). Oxford: Butterworth-Heinemann. p. 389. ISBN 978-0-75-065856-0. 
  2. ^ WebMineral Listing
  3. ^ MinDat Listing
  4. ^ a b Handbook of Mineralogy
  5. ^ Louderback, George Davis (July 30, 1907). "Benitoite, A New California Gem Mineral.". Bulletin of The Department of Geology (University of California Publications) 5 (9): 149–153. OCLC 6255540. 
  6. ^ Wilkins, Al (March 23, 2002). "SCFM News March 02, Featuring Benitoite". Mineralogical Society of Southern California. Retrieved April 3, 2014. 
  7. ^ "Mineral Resources". California Department of Conservation - California Geological Survey. Retrieved April 3, 2014. 
  8. ^ "Benitoite". Gemology Online. Retrieved 8 November 2012. 
  9. ^ Pool, Bob (August 17, 2013). "California visitor's rare find: A star-shaped piece of benitoite". Retrieved April 3, 2014.