Benitoite: California’s state gemstone.This blue lovely is only found in sizes larger than

Benitoite: California’s state gemstone.This blue lovely is only found in sizes larger than sand grains in one location worldwide, the mines in San Benito county of California located at the headwaters of the river bearing the same name. Discovered in 1907, it is a very popular collector’s piece with American mineralogists. Many specimens have a greyish colour component added to their usual blue-violet, and the shade of this specimen is exceptionally fine. Chemically it is a barium titanium silicate. Benitoite fluoresces a beautiful sky blue shade in UV light (see our past post on glowing diamonds for an explanation of fluorescence at http://tinyurl.com/kebod3b). It occurs in association with several other rare minerals, including neptunite and natrolite, and results from an uncommon association of geological forces. Hot fluids percolate through serpentine (altered olivine, see our past post at http://tinyurl.com/kjxc3es). The rarity comes from the particular chemistry of these hydrothermal solutions, combining elements in unusual combinations. It is suggested that fluids that had leached elements from two different geological formations mixed within the serpentine. How these formed is still poorly understood though they were metamorphic, The veins of serpentine (possibly old altered dykes) occur within brecciated grey-green glaucophane schists. Such schists are usually formed when a piece of crust is partly subducted and then springs back up due to its greater buoyancy than the mantle, being metamorphosed on its plunge down and bouncing up so fast the new minerals don’t have time to return to their original form. Such minerals are known as metastable, ie existing outside their formation environment.Its colour is probably caused by the same combination of iron and titanium passing electrons back and forth as blue sapphire. It is strongly dichroic, exhibiting different colours when viewed from different angles.With a Mohs hardness of 6.5 faceted gems are used in jewellery, mainly expensive and unique designer pieces from California. Supply cannot keep up with demand, so prices are high, and since the mine is getting depleted, this situation will continue into the future, unless a new deposit is found.LozImage credit: marinminerals.comhttp://www.benitoite.com/benitoite/benitoit.shtmlhttp://www.statesymbolsusa.org/California/gemstone_benitoite.htmlhttp://www.galleries.com/BenitoiteA visit to the mine: http://luminousminerals.com/benitoite.shtmlhttp://webmineral.com/data/Benitoite.shtmlhttp://www.mindat.org/min-624.html -- source link

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