Taaffeite

Taaffeite

Magnesiotaaffeite-2N’2S (Mg3Al8BeO16)
General
Category Oxide minerals
Chemical formula BeMgAl4O8
Strunz classification 4.FC.25
Identification
Color Colorless, greyish violet, violet red, red, greenish, light green, pink violet, mauve
Crystal habit Prismatic, alluvial grains
Crystal system Hexagonal, 6/mmm (6/m 2/m 2/m)
Twinning By reflection on (0001)?
Cleavage Imperfect/fair/absent
Fracture Conchoidal
Mohs scale hardness 8-8.5
Luster Vitreous
Streak White
Diaphaneity Transparent to translucent
Specific gravity 3.60–3.61
Optical properties Uniaxial
Refractive index nω = 1.722, nε = 1.777
Birefringence δ = 0.055
Pleochroism Weak
References [1][2]

Taaffeite (pronounced /ˈtɑːfaɪt/; BeMgAl4O8) is a mineral named after its discoverer, Count Edward Charles Richard Taaffe (1898–1967) who found the first cut and polished gem in November 1945.[3] As such, it is the only gemstone to have been initially identified from a faceted stone. Most pieces of the gem, prior to Taaffe, had been misidentified as spinel. For many years afterwards, it was known only in a few samples, and is still one of the rarest gemstone minerals in the world.[4]

Since 2002, the International Mineralogical Association-approved name for taaffeite as a mineral is magnesiotaaffeite-2N'2S.

Contents

Discovery

Taaffe bought a number of precious stones from a jeweller in November 1945. Upon noticing inconsistencies between the taaffeite and spinels, Taaffe sent some examples to B. W. Anderson of the Laboratory of the London Chamber of Commerce for identification on 1 November 1945. When Anderson replied on 5 November 1945, he told Taaffe that they were unsure of whether it was a spinel or something new; he also offered to write it up in Gemmologist.[5]

Properties

In 1951, chemical and X-ray analysis confirmed the principal constituents of taaffeite as beryllium, magnesium and aluminium,[6] making taaffeite the first mineral to contain both beryllium and magnesium as essential components.[1]

The confusion between spinel and taaffeite is understandable as certain structural features are identical in both. Anderson et al.,[5] classified taaffeite as an intermediate mineral between spinel and chrysoberyl.[7] Unlike spinel, taaffeite displays the property of double refraction that allows distinguishing these two minerals.

Usage

Because of its rarity, taaffeite is only used as gemstone.[8]

Formation and occurrence

Taaffeite occurs in carbonate rocks alongside fluorite, mica, spinel and tourmaline. This extremely rare mineral is increasingly found in alluvial deposits in Sri Lanka[9] and southern Tanzania,[1] as well as lower grade taffelite in limestone sediments in China.[7]

References

  1. ^ a b c Thomas, Arthur (2008) Gemstones: properties, identification and use. New Holland Publishers. p. 74. ISBN 1845376021
  2. ^ Magnesiotaaffeite, Mindat
  3. ^ Papers and proceedings of the International Mineralogical Association. General meeting, Mineralogical Society of America, Volume 9, p. 502
  4. ^ Collings, Michael R (2009). Gemlore: An Introduction to Precious and Semi-Precious Stones (2nd Ed). p. 152. Wildside Press LLC. ISBN 1434457028
  5. ^ a b Anderson, B.W., Payne, C.J., and Claringbull, G.F., (1951) Taaffeite, a new beryllium mineral, found as a cut gemstone. Mineralogical Magazine 29, pp. 765–772
  6. ^ Read, Peter G. (2005). Gemmology. Butterworth-Heinemann. p. 5. ISBN 0750664495.
  7. ^ a b Institut mineralogii, geokhimii, i kristallokhimii redkikh ėlementov (1966). Geochemistry and mineralogy of rare elements and genetic types of their deposits, Volume 2. Institut mineralogii, geokhimii i kristallokhimii redkikh elementov. (English Version Publisher: Israel Program for Scientific Translations). pp. 77–79.
  8. ^ Amethyst Galleries Inc.
  9. ^ Geological abstracts, Issues 1–7259 (1992). Elsevier/Geo Abstracts, p. 565