Bar

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Bar

The Mineral & Gemstone Benitoite


Bar
 

Benitoite mineral specimenBenitoite mineral specimen
PHYSICAL PROPERTIES:
Chemistry:
BaTiSi3O9
Composition: Barium titanium silicate
Class: Silicates
Subclass: Cyclosilicates
Group: Benitoite
Crystal system: hexagonal; bar 6 m 2
Fracture: irregular
Hardness: 6 - 6.5
Specific Gravity: 3.6
Refractive Index: 1.75-1.8
Luster: vitreous
Streak:
white
Color: usually blue, but can be yellowish or colorless
Cleavage: absent
Transparency: Crystals are transparent to translucent
Associated Minerals: albite, fresnoite, joaquinite, neptunite, natrolite, sanbornite, serpentine, and taramellite

Composition:
Barium titanium silicate (36.3% BaO, 20.2% Ti02, 43.5% Si02)

DISTINGUISHING CHARACTERISTICS:
Benitoite (pronounced "ben-ee-toe-ite") is named after San Benito County, California where it was discovered in 1907. It has a sapphire blue color and was first thought to be a variety of sapphire. But it is one of only a few minerals to crystallize in the bar 6 m 2 class called the ditrigonal-dipyramidal symmetry class. While this class is techically hexagonal, it produces trigonal (triangular) looking crystals. Additionally benitoite is also a fluorescent mineral.

 
ENVIRONMENT:
Benitoite from San Benito, County is associated with the minerals neptunite, natrolite and joaquinite. This combination of minerals was formed from hydrothermal solutions in a natrolite dike in the green schist of the serpentine parent rock. These highly saturated solutions contained a number of unusual elements such as barium, cesium, fluorine, iron, lithium, manganese, niobium and titanium.

CRYSTAL DESCRIPTION:
Benitoite belongs to the hexagonal class of minerals. Early in the theoretical development of crystallography it was hypothesized that there was a class of the hexagonal system that would produce trigonally shaped crystals. Until this mineral was found in 1907, there was no known naturally occurring representative of this crystal class. Benitoite is the first species known to crystallize in the ditrigonal dipyramidal class of the hexagonal crystal system.

TESTS:
Crystals are so distinctive that no tests are necessary.

LOCALITIES:

Only the mines of San Benito County, California have good crystals some of which are good enough to cut stones from. SW Texas produces tiny grains in Eocene sands. There are also some other California localities that produce small crystals.

USES:
As a gemstone and as a mineral specimen

FACTS & HISTORY:
When first discovered Benitoite was initially thought to be sapphire. Samples were sent to the University of California, Berkeley for identification. Dr. George Louderback found it to be a new mineral. Two other minerals were also found. He thought they too, were new to science. He formally named Benitoite and called the other two new minerals carlosite and joaquinite.

After further analysis carlosite was found to be neptunite which had been previously described from specimens found in Greenland and the Ural Mountains in Russia. Even so, the specimens of neptunite from the San Benito County were far superior to any of the other known localities of this species. Joaquinite was also a new mineral species but remained incompletely described until the 1970s.