Benitoite

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Benitoite
Bénitoïte, neptunite, joaquinite- (Ce), natrolite 2 (USA) .jpg
Benitoite (blue), joaquinite (brown) and neptunite (black) on natrolite (white)
General and classification
chemical formula BaTi [Si 3 O 9 ]
Mineral class
(and possibly department) 		Silicates (and Germanates)
System no. to Strunz
and to Dana 		9.CA.05 ( 8th edition : VIII / E.01)
59.01.01.02
Crystallographic Data
Crystal system hexagonal
Crystal class ; symbol ditrigonal-dipyramidal; 6 m 2
Space group P 6 c 2 (No. 188)Template: room group / 188
Lattice parameters a  = 6.641  Å ; c  = 9.759 Å
Formula units Z  = 2
Frequent crystal faces {0001}
Twinning Rotation twins according to [0001]
Physical Properties
Mohs hardness 6 to 6.5
Density (g / cm 3 ) 3.65
Cleavage indistinct after {10 1 1}
Break ; Tenacity shell-like
colour colorless, white, blue, pink to purple
Line color White
transparency transparent to translucent
shine Glass gloss to diamond gloss
Crystal optics
Refractive indices n ω  = 1.756 to 1.757
n ε  = 1.802 to 1.804
Birefringence δ = 0.046
Optical character uniaxial positive
Pleochroism ω = colorless; ε = purple, indigo blue, greenish blue
Other properties
Special features Fluorescence: short-wave UV light: light blue, long-wave UV light: rarely reddish

The rare mineral benitoite is a ring silicate with the chemical composition BaTi [Si 3 O 9 ]. It crystallizes in the hexagonal crystal system and develops predominantly trigonal and / or hexagonal, prismatic to tabular pyramidal crystals in mostly sapphire blue color.

Benitoite are exclusively processed into gemstones .

Etymology and history

The first find of benitoite in February 1907 was made by prospector James M. Couch. He was looking for cinnabar or copper ore in the Diablo Mountains on behalf of Roderick W. Dallas . On a slope, Couch stepped into a man-high cavity between two crevices in the rock and saw countless blue benitoite crystals on the ground, weathered out of white natrolite . Some pieces were sent to a San Francisco jeweler for faceting . Eventually, Georg Davis Louderback , professor of geology at the University of California at Berkeley , was tasked with identifying the as yet unknown crystals. He recognized the new mineral, described it in detail that same year and named it benitoite after its type locality, San Benito County .

The "Dallas Mining Company" was founded and a camp was built for the mining. In the following, only about 1% of the benitoites were preserved during mining using dynamite to make work easier. In 1912 the mine closed because Dallas believed it was exhausted. After a brief reopening in 1933, the mine was leased to various entrepreneurs. In 1967 B. Gray and B. Forrest leased the mine. The two bought the mine in 1987 and expanded it, the name was changed to "Benitoite Gem Mine". In 1997 the mine was leased to the AZCO Company, which despite major investments did not find any significant amounts of benitoite. Collector's Edge Minerals, Inc. purchased the mine in 2000. In 2005, the Company sold the mine to Dave Schreiner in 2005. Dave is under a contract not to use heavy equipment until 2015. It allows visitors to dig on the mine site for a fee.

In 1985, benitoite was selected as the official State Gemstone of California.

classification

In the old (8th edition) and new systematics of minerals (9th edition) according to Strunz , benitoite belongs to the division of " ring silicates (cyclosilicates)". However, since the new Strunz'schen mineral systematics, this department has been further subdivided and, according to its crystal structure, the mineral now belongs to the subdivision of "ring silicates with [Si 3 O 9 ] 6− triple single rings without island-like, complex anions".

The classification of minerals by Dana assigns the Benitoite in the department "Cyclosilicates cyclosilicates Three-membered ring anhydrous, no other anion" (Translated: Anhydrous ring silicates with three rings without further anions ), where he together with Bazirit , Pabstit and Wadeit after benitoite group named for him with the number 59.01.01.

Crystal structure

3 single ring of benitoit

Benitoite crystallizes in the hexagonal crystal system in the space group P 6 c 2 (space group no. 188) with the lattice parameters a  = 6.641  Å and c  = 9.759 Å as well as two formula units per unit cell . Template: room group / 188

Benitoite is a ring silicate with three single rings as a silicate anion complex.

properties

The sapphire blue (with a hint of purple) in benitoite (sometimes called the “blue diamond ”) may be caused by traces of iron . Clear, white, green-gray, pink to purple-colored, orange, red or maroon crystals (with inclusions of other minerals such as neptunite ) are also very rarely found . Because of its strong dichroism , benitoite shows a light cornflower blue to deep, slightly violet blue perpendicular to the C-axis and appears colorless parallel to the C-axis. Benitoite fluoresces light blue under short-wave UV light , while some crystals show a reddish color under long-wave UV light. The dispersion (different refraction of light depending on the wavelength of the incident light) of benitoite can cause a sparkle of red and green flashes .

The mineral is hard enough ( Mohs hardness 6.5) to be processed as a gem stone , but it is still fairly unknown. Faceted benitoites typically weigh less than 1 ct. A faceted, clear benitoite of good color that weighs more than 0.5 ct is rare and correspondingly expensive.

Education and Locations

Benitoite in trigonal habit together with joaquinite (brown) and neptunite (black)

Benitoite crystals are formed in natrolite veins in hydrothermal solutions , which contain relatively high proportions of elements that are unusual for hydrothermal solutions, such as barium , titanium , fluorine , iron and others. Benitoite usually occurs together with albite , joaquinite , neptunite and serpentine . Natrolite, the last mineral formed in the benitoite deposits, normally covers the benitoite crystals, which is why it is used e.g. B. has to be removed with hydrochloric acid to make the benitoite crystals visible.

Benitoites come almost exclusively from the richly mineralized (more than 150 semi-precious stones and precious stones) serpentine body of the New Idria mine area near the San Benito River in San Benito County in California (USA). There you can find benitoite at the southern end of the " Diablo Range ", a mountain belt that contains basalt , chert , greywacke , slate , serpentinite and Cretaceous and Tertiary sandstones .

The benitoite mine best known for its precious crystals is the Benitoite Gem Mine (also known as the Benitoite Mine and Dallas Gem Mine) owned by Dave Schreiner (of Coalinga , California ). Dave allows visitors to dig in the mine area for a fee.

There are four other benitoite mines in Clear Creek within a radius of 10 km from the “Benitoit Edelstein Mine” : The “Junnila Mine”, about 8 km northwest of the “Benitoit Edelstein Mine”, the “Numero Uno Mine”, the "Victor Mine" and the "Santa Rita Peak Property". In addition to the discovery area in San Benito County, less valuable benitoite can be found in Arkansas ("Diamond Jo Quarry" near Magnet Cove in Hot Spring County), "Big Creek" (in Fresno County in Sierra Nevada in California), Japan (Ohmi near Itoigawa in Niigata Prefecture , Chubu area on Honshū ) and finally in Broken Hill ( New South Wales , Australia ).

Use as a gem stone

Large, polished benitoites (over 1 carat ) are very rare. Usually they show a deep, intense blue. Clear crystals of good quality are ground into gemstones and can definitely withstand comparison with the sapphire . There is a risk of confusion with other gemstones such as cordierite , spinel , tanzanite , the blue varieties of the tourmaline group and the colorless zircon .

The strong dichroism and the resulting alternation of lighter and more intense shades of blue, the easily identifiable doubling of the lower facets with magnifying glasses of the stone (colorless and blue diamonds would be single-refractive) make benitoite a relatively easily identifiable gemstone. Only the extremely rare colorless stones can be confused with colorless zircon.

Colorless benitoites can be changed into an orange to pinkish-orange color by heat treatment. The exact procedure is secret. Original blue benitoite does not change color when fired.

Similar to sapphire, ruby or other well-known gemstones, large, pure benitoites with a good blue are very expensive. Stones with a very light or very dark blue are less valuable. The cut quality is generally above average.

See also

literature

  • George Davis Louderback, Walter C. Blasdale: Benitoite, a new California gem mineral . In: University of California Publications. Bulletin of the Department of Geology . tape 5 , no. 9 , 1907, pp. 149–153 (English, rruff.info [PDF; 459 kB ; accessed on August 28, 2019]).
  • Benitoite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 64 kB ; accessed on August 28, 2019]).
  • Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 200 .

Web links

Commons : Benitoite  - collection of images, videos and audio files

Individual evidence

  1. ^ A b c d David Barthelmy: Benitoite Mineral Data. In: webmineral.com. Accessed August 28, 2019 .
  2. ↑ a b c American-Mineralogist-Crystal-Structure-Database - Benitoite. In: rruff.geo.arizona.edu. Accessed August 28, 2019 .
  3. ↑ a b c d Benitoite. In: mindat.org. Hudson Institute of Mineralogy, accessed August 28, 2019 .
  4. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
  5. ↑ George Davis Louderback, Walter C. Blasdale: Benitoite, a new California gem mineral . In: University of California Publications. Bulletin of the Department of Geology . tape 5 , no. 9 , 1907, pp. 149–153 (English, rruff.info [PDF; 459 kB ; accessed on August 28, 2019]).
  6. ↑ Benitoite. In: geology.com. Retrieved August 28, 2019 .
  7. ↑ Orange Benitoite. In: thegemtrader.com. The Gem Trader, accessed August 28, 2019 .