hexagonal Oxides

Musgravite

Formula BeMg2Al6O12

Crystal Structure

CDL Notation

 #! Species: Musgravite (magnesiotaaffeite-6N'3S)
#! System: Hexagonal (-3m; rhombohedral)
#! Habit: Alluvial rounded to subhedral grains
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

3.61

Refractive Index

1.719

Optical Character

Uniaxial -

Physical Properties

Crystal System: hexagonal
Hardness (Mohs): 8.0
Specific Gravity: 3.61
Cleavage: None
Fracture: Conchoidal
Lustre: Vitreous

Optical Properties

Refractive Index: 1.719
Birefringence: 0.004-0.012
Optical Character: Uniaxial -
Pleochroism: weak: Grey-green / Colourless to pale olive

Characteristics

Colours

Grey-greenOlive greenVioletColourless (extremely rare)

Localities

Australia (Musgrave Ranges, South Australia; type locality; 1967 discovery)Sri Lanka (several reported stones)MadagascarTanzaniaGreenland (minor)

Common Inclusions

Generally very clean

Known Treatments

None known

Crystal Forms

rhombohedron

Diagnostic Features

Doubly refractive (uniaxial negative) — separates from spinel (isotropic); slightly higher RI and SG than taaffeite (SG overlap exists); ATR-FTIR and Raman spectroscopy give definitive separation from taaffeite by vibrational band positions. One of the rarest gem minerals.

Notes

BeMg2Al6O12; hexagonal, rhombohedral, point group -3m. IMA revised name: magnesiotaaffeite-6N'3S per Armbruster 2002 (DOI 10.1127/0935-1221/2002/0014-0389) [VERIFIED via live API]. ATR-FTIR and Raman distinction from taaffeite confirmed by Thongnopkun 2024 (DOI 10.1016/j.vibspec.2024.103733) [VERIFIED via live API]: "ATR-FTIR and Raman spectroscopy efficiently distinguish musgravite from taaffeite using diagnostic vibrational band positions." Higher RI (1.719-1.736) and SG (3.61-3.68) than taaffeite (RI 1.717-1.730, SG 3.60-3.62) but ranges overlap; FTIR/Raman is definitive. Colour cause: Cr, Fe or V substitution (not firmly established for all colours; per VERIFIED.md REJ-07, do not present as confirmed fact). Type locality: Musgrave Ranges, South Australia; fewer than 10 faceted stones known before 2000. Note: musgravite is a separate IMA species from taaffeite, not merely a variety.