Because of its similarity to taaffeite it is probable that some stones originally thought by their owners to be taaffeites, are actually musgravites. Indeed, our own investigations of taaffeites from Tunduru, Tanzania, have confirmed a 10 – 15% incidence of stones originally indentified as taaffeites to actually be musgravite. The only known facet quality specimens of musgravite are from Sri Lanka and Tanzania. The largest known musgravite is owned by Multicolour Gems and weighs a 5.74cts. Please see image below.
Musgravite is a member of the taaffeite mineral group composed of spinel and modified nolanite modules with beryllium (Be), magnesium (Mg) and aluminium (Al) as primary constituents. More specifically the formula for taaffeite is, Mg3BeAl8O16 (Anderson et al., 1951) while musgravite is, Mg2BeAl6O12 (Hudson et al., 1967).
Because of such close structural and chemical similarities between musgravite and taaffeite a new systematic nomenclature was recently proposed and accepted by the IMA (International Mineral Association). The new name for musgravite will be magnesiotaaffeite -6N3S, where N and S represent nolanite and spinel nodules. Magnesiotaafeite-2N2S will be the new name for the mineral formally known as taaffeite. The name musgravite will apparently be relegated to trade name status.
Overlap in taaffeite and musgravite properties make these species impossible to distinguish with standard gemological tests like refractive index and specific gravity. The most convenient way to differentiate the species is with Raman spectroscopy or X-ray diffraction techniques. According to Dr. Robert Downs (University of Arizona) “the X-ray method looks foolproof inasmuch as the cell parameters are so different from each other. Both minerals are hexagonal with the a-cell edges indistinguishable, but magnesiotaafeite-2N2S (formerly known as taaffeite) has a c-cell edge of 18.3 angstroms while magnesiotaafeite-6N3S (formerly known as musgravite) has a c-cell edge of 41.1 angstroms. These are so different from each other that it is next-to-impossible to screw up the identification based on the X-ray patterns.”
Most gemological labs employ the Raman spectroscope for musgravite/taafeite differentiation because it is fast and non-destructive. Still the effects of crystal orientation and similarities in the spectral data make this method difficult or inconclusive in some cases. Please see the SSEF spectral data below.
