Taaffeite

Taaffeite (/ˈtɑːfaɪt/; BeMgAl4O8) is a mineral, named after its discoverer Richard Taaffe (1898–1967) who found the first sample, a cut and polished gem, in October 1945 in a jeweler's shop in Dublin, Ireland. 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 it is still one of the rarest gemstone minerals in the world. 

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

Taaffeite

General

Category Oxide minerals

Formula BeMgAl4O8

IMA symbol Tf

Strunz classification 4.FC.25

Crystal system Hexagonal

Crystal class Dihexagonal pyramidal (6mm)

Trigonal dipyramidal (3m)

(magnesiotaaffeite-6N'3S and ferrotaaffeite-6N'3S)

Identification

Color Colorless, greyish violet, violet red, red, greenish, light green, pink violet, mauve

Crystal habit Prismatic, alluvial grains

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

Discovery

Taaffe bought a number of precious stones from a jeweller in October 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 Gemologist.

Properties

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

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

Features

Magnesiotaaffeite- 2N'2S is an oxide with the chemical formula Mg₃Al₈BeO 16. It crystallizes in the hexagonal system. Its hardness on the Mohs scale is between 8 and 8.5. It was named taaffeite in honor of Count Edward Charles Richard Taaffe, a gemologist from Dublin (Bohemia, Austria-Hungary, 1898 - Dublin, Ireland, 1967), who discovered the mineral in 1945. It was the first case of a new mineral discovered as a faceted gemstone. The suffix was added by Armbruster et al. (2002) to indicate polysome.

Classification

According to the Nickel-Strunz classification, magnesiotaaffeite-2N'2S belongs to «04.FC: Hydroxides (without V or U), with OH, without H₂O; angle-sharing octahedra" together with the following minerals: bernalite, dzhalindite, söhngeïte, burtite, mushistonite, natanite, schoenfliesite, vismirnovite, wickmanite, jeanbandyite, mopungite, stottite, tetrawickmanite, ferronigerite-2N1S, magnesionigerite-6N6S, magnesionigerite-2N1S, ferronigerite-6N6S, zinconigerite-2N1S, zinconigerite-6N6S and magnesioaffeite-6N'3S.

Usage

Because of its rarity, taaffeite is used only as a gemstone.

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 and southern Tanzania, as well as lower grade taaffeite in limestone sediments in China.

Sourced from Wikipedia