16.67: Taaffeite

Last updated
Save as PDF

Page ID: 6497

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

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.

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 Edition (1990) - Robert Webster (6th ed.)
Two remarkable taaffeite crystals - K. Schmetzer, L. Kiefert et al., Journal of Gemmology Vol. 29 No. 7/8, July/October 2005
Count Richard Taaffe (1898-1967) Irish gemologist, discoverer of taaffeite (1945)