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16.67: Taaffeite

  • Page ID
    6497
  •  

    Taaffeite
    Chemical composition Magnesium-beryllium-aluminium oxide

    Mg3BeAl8O16

    Crystal system Hexagonal
    Hardness 8
    Luster Vitreous
    Optic nature Uniaxial -
    Refractive index 1.717 - 1.724
    Birefringence 0.004 - 0.005
    Dispersion 0.019
    Specific gravity 3.60 - 3.62
    Pleochroism Faint
    Fluorescence

    LW: inert to lightgreen
    SW: inert

    File:TaaffeiteGroup.gif

    Figure \(\PageIndex{1}\): Photo courtesy of The Gem Trader


    In mineralogy, taaffeite is the name of a polytype group to which also musgravite belongs. The current (2002) IMA approved name for taaffeite as a mineral is magnesiotaaffeite-2N'2S. Prior to 2002, it was known as just "taaffeite". When gemologists refer to "taaffeite", "magnesiotaaffeite-2N'2S" is usually implied.
    Taaffeite is, to date (2007), the only mineral that was discovered as a faceted gemstone instead of a rough crystal.

    Taaffeite is named after its discoverer, Count Edward Charles Richard Taaffe (1898-1967) who discovered the stone by chance in October 1945 from a lot of gemstones bought from Dublin, Ireland based jeweler and watchmaker Robert Dobbie.
    After examination of the lot with a microscope, hydrostatic balance, polariscope and immersion, he found a stone having properties close to spinel but showing double refraction. As he was puzzled by this cut gemstone (wondering if anomalous birefringence could be that strong), he sent the stone off to Basil Anderson at the Laboratory of the London Chamber of Commerce. There it was identified as a new mineral species.

    Alternative name: Taprobanite (rarely used).

     

    Diagnostics

    Taaffeite can easily be mistaken for spinel judging from its refractive index and specific gravity range, but taaffeite is most definitely birefringent while spinel is not, although spinel can exhibit anomalous double refraction(ADR) due to internal stain. Therefore do not rely on polariscope reaction for identification, unless you are fortunate enough to detect an optic figure with the coniscope.
    Distinguishing taaffeite from musgravite can be a challenge and usually requires laboratory techniques as Raman spectroscopy and energy dispersive X-ray fluorescency (EDXRF).

     

    Color

    The chromium colored red stones are the most rare of this rare gemstone species.

    Colorless, grayish violet, violet red, red, greenish, light green, pink-violet, mauve.

     

    Spectroscope

    Iron colored taaffeite may show the same spectrum as iron-bearing spinel in the visible range.
    Chromium rich taaffeite gemstones may have an absorption band around 550nm.

     

    Occurrence

    Taaffeite is predominantly found in Sri Lanka. Rare sources are Myanmar and Tanzania.