Exceptionally Rare Titanoholtite

Britni LeCroy and Aaron Palke

A first-time submission of a rare dumortierite supergroup member was recently examined at the Carlsbad laboratory. A 0.85 ct titanoholtite (figure 1) was identified after extensive testing. The stone was submitted as dumortierite, but its physical properties and chemistry did not support this. The dark orangy brown stone had a hydrostatic specific gravity of 3.65 and a refractive index of 1.720–1.740 (birefringence of 0.020). These properties stand in contrast to dumortierite, which usually forms as an inclusion rather than a single crystal and typically has a blue to violet color, a specific gravity of 3.21 to 3.41, and a refractive index of 1.659 to 1.692 with a birefringence of 0.027.

A look into the stone offered a vibrant scene composed of a reflective surface-reaching fracture breaking the crown (figure 2, left) and an abundance of transparent graining throughout with small flecks of reflective thin films and unidentified particles (figure 2, right). Microscopic observation revealed a strong resemblance to garnet, but the material’s doubly refractive nature was inconsistent with garnet. Raman spectroscopy results did not match any existing minerals within GIA’s reference databases, but they did show some general similarities with dumortierite. Finally, laser ablation–inductively coupled plasma–mass spectrometry was performed on the stone for a full quantitative chemical analysis. Given the general similarities with dumortierite, the chemical data was fit to a dumortierite supergroup formula of (Al_0.35Ti_0.41□0.24Fe_0.02)Al₆B(Si_2.60Sb_0.32)O_17.67. The dominant component in this formula is titanoholtite, (Ti_0.75□0.25)Al₆BSi₃O₁₈, with minor dumortierite, Al₇BSi₃O₁₈, and incorporation of significant antimony.

Titanoholtite is a recently discovered mineral within the holtite group and a member of the dumortierite supergroup. It was first published in 2013, when microscopic crystals were found in a Polish pegmatite (A. Pieczka et al., “The dumortierite supergroup. II. Three new minerals from the Szklary pegmatite, SW Poland: Nioboholtite, (Nb_0.6□0.4)Al₆BSi₃O₁₈, titanoholtite, (Ti_0.75□_0.25)Al₆BSi₃O₁₈, and szklaryite, □Al₆BAs³⁺₃O₁₅,” Mineralogical Magazine, Vol. 77, No. 6, 2013, pp. 2841–2856). This stone submitted to GIA appears to be the first macroscopic titanoholtite specimen ever reported in the literature.