Difference between revisions of "Taaffeite"
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The chromium colored red stones are the most rare of this rare gemstone species. | The chromium colored red stones are the most rare of this rare gemstone species. | ||
− | + | Colorless, greyish violet, violet red, red, greenish, light green, pink violet, mauve. | |
===Spectroscope=== | ===Spectroscope=== |
Revision as of 06:36, 18 August 2007
Taaffeite | |
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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 |
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 facetted gemstone instead of a rough crystal.
Taaffeite is named after its discoverer, Count (suspended title) Edward Charles Richard Taaffe (1898-1967) whom 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 indentified 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.
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, greyish 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.
Occurence
Taaffeite is predominantly found in Sri Lanka. Rare sources are Myanmar and Tanzania.
Sources
- Taaffeite and Musgravite article from the GAAJ
- Anderson B W, Payne C J, Claringbull G F, Hey M H (1951) Taaffeite, a new beryllium mineral, found as a cut gemstone, Mineralogical Magazine 29, 765-772 (from RRUFF)
- Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
- Two remarkable taaffeite crystals - K. Schmetzer, L. Kiefert et al., Journal of Gemmology Vol. 29 No. 7/8, July/October 2005