Large Faceted Hibonite
The Carlsbad laboratory recently examined an unusually large 134.43 ct opaque, very dark brown faceted hibonite (see above image). Standard gemological testing gave spot RI values of 1.79 to 1.81 and a hydrostatic SG of 3.81. Hibonite, which crystallizes in the hexagonal system, has published RI values of 1.790 to 1.807, but we were unable to observe birefringence, probably due to its opacity and these values’ proximity to the limits of the refractometer. The stone showed a high luster and was inert to long-wave and short-wave UV. Testing with a Geiger counter showed it was slightly radioactive. All of these features are consistent with hibonite, yet more advanced testing was needed for a positive identification of this unusual material.
Raman spectroscopy gave a spectrum consistent with hibonite. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis revealed large amounts of Mg, Ca, Al, Ti, and Fe; small amounts of Na, Si, Sr, La, and Ce; and traces of numerous other elements, including the radioactive elements thorium (62.62–122.60 ppma) and uranium (0.12–0.56 ppma). The presence of these elements caused the mineral to react to the Geiger counter. The thorium and REE content is consistent with samples from Madagascar (M.A.F. Rakotondrazafy et al., “Mode of formation of hibonite (CaAl12O19) within the U-Th skarns from the granulites of S-E Madagascar,” Contributions to Mineralogy and Petrology, Vol. 123, No. 2, 1996, pp. 190–201).
Microscopic examination revealed one deep green mineral inclusion breaking the surface of the table. Its Raman spectrum was consistent with spinel, a known associated mineral of hibonite. LA-ICP-MS analysis showed it was an iron-rich spinel within the hercynite(FeAl2O4)-spinel (MgAl2O4) series, with almost one-third of the magnesium substituted by iron. Although difficult to see internally, other whitish mineral inclusions were visible, as well as an extensive fracture and cleavage network.
Orangy brown, transparent, and well-formed hexagonal crystals of gem-quality hibonite have been reported from Myanmar (T. Hainschwang et al., “Hibonite: A new gem mineral,” Summer 2010 G&G, pp. 135–138; Summer 2012 Lab Notes, p. 136). Although not gem quality, the stone’s size makes it a rare collector’s mineral as well as the largest hibonite GIA has identified to date.