hexagonal Oxides

Taaffeite

Formula BeMgAl4O8

Crystal Structure

CDL Notation

 #! Species: Taaffeite (magnesiotaaffeite-2N'2S)
#! System: Hexagonal (-3m; rhombohedral)
#! Habit: Rounded to subhedral from alluvial reworking
trigonal[-3m]:{10-10}@1.0 + {0001}@0.5 + {10-11}@0.7

Crystal System hexagonal

Point Group -3m

Crystal Forms (3)

{10-10} {0001} {10-11}

Quick Facts

Hardness

8.0

Specific Gravity

3.6

Refractive Index

1.717

Optical Character

Uniaxial -

Physical Properties

Crystal System: hexagonal
Hardness (Mohs): 8.0
Specific Gravity: 3.6
Cleavage: None
Fracture: Conchoidal
Lustre: Vitreous

Optical Properties

Refractive Index: 1.717
Birefringence: 0.004-0.009
Optical Character: Uniaxial -
Pleochroism: weak: Pale violet / Colourless

Characteristics

Colours

Mauve to pale violet (most common gem colour)PinkRedBlue-green (rare)Colourless (rare)

Localities

Sri Lanka (historic and primary; alluvial gem gravels, Ratnapura)Myanmar (Burma/Mogok; small amounts)TanzaniaChina (very minor)

Common Inclusions

Generally very clean; FTIR and Raman are the most reliable modern ID tools

Known Treatments

None known

Crystal Forms

rhombohedronprism

Diagnostic Features

Doubly refractive (uniaxial negative) — primary diagnostic; immediately separates from spinel (singly refractive, isotropic on polariscope). SG 3.60-3.62 overlaps spinel (3.58-3.65) making SG alone insufficient for separation. FTIR and Raman spectroscopy are the modern definitive identification tools (distinguish from musgravite).

Notes

BeMgAl4O8; hexagonal, rhombohedral subgroup, point group -3m. IMA revised name: magnesiotaaffeite-2N'2S per Armbruster 2002 (DOI 10.1127/0935-1221/2002/0014-0389) [VERIFIED via live API]. Gemmological constants and analytical identification confirmed by Thongnopkun 2024 (DOI 10.1016/j.vibspec.2024.103733) [VERIFIED via live API]. CRITICAL DIAGNOSTIC NOTE (per VERIFIED.md Conflict 7 + flag F-05): Doubly refractive (uniaxial negative) — this is the primary and immediate separation from spinel (isotropic). SG 3.60-3.62 overlaps spinel (3.58-3.65); SG alone is NOT sufficient for separation. RI 1.717-1.730 also overlaps with some spinel specimens. The polariscope (DR test) is the definitive field separation. ATR-FTIR and Raman distinguish taaffeite from musgravite (magnesiotaaffeite-6N'3S) by vibrational band positions. Musgravite: higher SG ~3.61-3.68, higher RI ~1.723-1.736; see separate musgravite.yaml entry. Historical note: Edward Taaffe purchased the original stone in a Dublin gem dealer's parcel in 1945, believing it spinel; the double refraction was noticed and confirmed as a new species in 1951. Gem localities confirmed by Shigley et al. 2000 (DOI 10.5741/gems.36.4.292) [VERIFIED].