Benitoite
| 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:(6m2) |
| 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 scalehardness | 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 bluebariumtitaniumcyclosilicate mineral,found inhydrothermallyalteredserpentinite.It forms in low temperature, high pressure environments typical ofsubductionzones atconvergent plate boundaries.Benitoitefluorescesunder short waveultraviolet 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 theSan Benito Mountainsroughly halfway betweenSan FranciscoandLos Angeles.Due to its similar color, Couch originally believed it to besapphire,a variety ofcorundum.In 1909, a sample was sent to theUniversity of California, Berkeley,where mineralogistDr. George D. Louderbackrealized it was a previously unknown mineral. Corundum (sapphire) has a defined Mohs hardness of 9, while benitoite is much softer. He named itbenitoitefor its occurrence near the headwaters of theSan Benito RiverinSan 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 inMontana,Arkansas,Japan,andAustraliaalthough they formed under slightly different conditions and only grow large enough to be considered an accessory mineral. In 1985 benitoite was named as theofficial state gemofCalifornia.[8][9]
Benitoite typically crystallises hexagonally. Non-gem crystals of benitoite can have a very rare, six-pointed twinned form.[10]
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,serpentineandalbite.
Benitoite is a rare mineral found in very few locations including San Benito County, California,JapanandArkansas.In the San Benito occurrence, it is found innatroliteveinswithinglaucophaneschistwithin aserpentinitebody. In Japan, the mineral occurs in a magnesio-riebeckite-quartz-phlogopite-albitedikecutting a serpentinite body.[5]
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Blue benitoite crystals on white natrolite, Dallas Gem Mine, San Benito Co., California, US -
Benitoite crystals under UV light
References
[edit]- ^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.S2CID235729616.
- ^O'Donoghue, Matthew (2006).Gems: Their Sources, Descriptions and Identification(6th ed.). Oxford:Butterworth-Heinemann.p. 389.ISBN978-0-75-065856-0.
- ^WebMineral Listing
- ^MinDat Listing
- ^abHandbook of Mineralogy
- ^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.OCLC6255540.
- ^Wilkins, Al (March 23, 2002)."SCFM News March 02, Featuring Benitoite".Mineralogical Society of Southern California. Archived fromthe originalon May 16, 2013.RetrievedApril 3,2014.
- ^"Mineral Resources".California Department of Conservation-California Geological Survey.Archived fromthe originalon August 21, 2008.RetrievedApril 3,2014.
- ^"Benitoite".Gemology Online.Retrieved8 November2012.
- ^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.ISSN0016-626X.