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Geosciences LibreTexts

16.23: Benitoite

  • Page ID
    4359
  • Benitoite
    Chemical composition BaTiSi3O9
    Crystal system Trigonal
    Habit Bipyramids
    Cleavage None
    Fracture Conchoidal
    Hardness 6.5
    Optic nature Uniaxial +
    Refractive index 1.75-1.80
    Birefringence 0.047
    Dispersion High, 0.046
    Specific gravity 3.65
    Lustre Bright vitreous
    Pleochroism Strong

    Benitoite is a very rare mineral that was discovered in 1907 in San Benito County in California. It has never been found elsewhere. It was originally mistaken for sapphire. Benitoite is very highly dispersive (the ability to take white light and disperse it into various components of the spectrum) with an adamantine luster when polished.
    Benitoite is associated with other rare minerals such as black-red neptunite, snow white natrolite, and brown-yellow joaquinite. Since the only source of this rare combination occurs at San Benito County, California, it has been designated the California state gemstone. Crystals, and cut stones tend to be small and quite expensive!

    File:Benitoite1.jpg

    Figure \(\PageIndex{1}\): Photo Courtesy of Rob Lavinsky

    Figure \(\PageIndex{2}\): Photo Courtesy of Barbra Voltaire

    Spectrum

    File:Benitoite3.jpg

    Figure \(\PageIndex{3}\): 4mm faceted benitoites, Colorless to Dark Blue
    Photo Courtesy of Barbra Voltaire


    Figure \(\PageIndex{4}\): Transmission Spectrum
    The more saturated the blue, the higher the peak in the 400-500 nm range

    Cause of Color

    The origin of color in benitoite has not been definitively established.
    Proposed ideas include:
    Fe2+ - Ti4+ or the Fe2+ - Fe3+ intervalance charge transfer.
    While most benitoite is colorless when viewed down the c-axis, there is a very small number (3) of exceedingly rare stones (now cut into gemstones) which are pink in this direction.

    The Benitoite Gem Mine

    The Early Days: An Interview with Ed Swoboda and Peter Bancroft