Dumortierite samples from two pegmatite localities in Antarctica and one each in Germany and Russia show a range of As and Sb compositions (As + Sb + minor Bi = 0.001–0.212 apfu) and low Ta + Nb + Ti (0.001–0.079 apfu). Single-crystal diffraction data obtained from crystals from each sample refined to R1 = 0.0161–0.0285, the latter value for a twinned crystal. Initial refinements of three of the four crystals showed considerable electron density at the Sb1 and Sb2 sites; however, the atoms at these sites are also highly anisotropic, and consequently the sites were split into distinct As1, Sb1, As2, and Sb2 positions. Such distinct As and Sb sites are not seen in the isostructural mineral holtite, which contains considerably more As and Sb (and Ta, Nb, and Ti). Initial refinements also showed that in all four crystals, the atom at the All site, with occupancies of 0.81–0.88, is highly anisotropic with most of the positional displacement in the a direction. The Al1 site was then split into Al1a, Al1, and Al1b positions, whose occupancies refined to All > Al1a > Al1b. The unequal occupancy of Al1a, Al1, and Al1b suggests that the hexagonal channel contains a disordered mix of face-sharing octahedron dimers, trimers and longer units separated by vacancies. A plot of Si + P apfu versus As + Sb + Bi apfu for 340 dumortierite and 627 holtite compositions shows no gap between the two minerals. Although there is a pronounced gap in terms of As and Sb occupancy, it separates dumortierite and Sb-poor holtite from Sb-bearing holtite. The continuum of compositions between dumortierite and holtite, and the discovery of very (As, Sb)-rich, (Ta, Nb)-poor compositions, suggest that the distinction between what has been called dumortierite and what has been called holtite should be reconsidered.

You do not currently have access to this article.