The dumortierite supergroup. I. A new nomenclature for the dumortierite and holtite groups

Abstract

AbstractAlthough the distinction between magnesiodumortieite and dumortierite, i.e. Mg vs. Al dominance at the partially vacant octahedral Al1 site, had met current criteria of the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) for distinguishing mineral species, the distinction between holtite and dumortierite had not, since Al and Si are dominant over Ta and (Sb, As) at the Al1 and two Si sites, respectively, in both minerals. Recent studies have revealed extensive solid solution between Al, Ti, Ta and Nb at Al1 and between Si, As and Sb at the two Si sites or nearly coincident (As, Sb) sites in dumortierite and holtite, further blurring the distinction between the two minerals.This situation necessitated revision in the nomenclature of the dumortierite group. The newly constituted dumortierite supergroup, space group Pnma (no. 62), comprises two groups and six minerals, one of which is the first member of a potential third group, all isostructural with dumortierite. The supergroup, which has been approved by the CNMNC, is based on more specific end-member compositions for dumortierite and holtite, in which occupancy of the Al1 site is critical.(1) Dumortierite group, with Al1 = Al3+, Mg2+ and ☐, where ☐ denotes cation vacancy. Charge balance is provided by OH substitution for O at the O2, O7 and O10 sites. In addition to dumortierite, endmember composition AlAl6BSi3O18, and magnesiodumortierite, endmember composition MgAl6BSi3O17(OH), plus three endmembers, “hydroxydumortierite”, ☐Al6BSi3O15(OH)3 and two Mg-Ti analogues of dumortierite, (Mg0.5Ti0.5)Al6BSi3O18 and (Mg0.5Ti0.5)Mg2Al4BSi3O16(OH)2, none of which correspond to mineral species. Three more hypothetical endmembers are derived by homovalent substitutions of Fe3+ for Al and Fe2+ for Mg.(2) Holtite group, with Al1 = Ta5+, Nb5+, Ti4+ and ☐. In contrast to the dumortierite group, vacancies serve not only to balance the extra charge introduced by the incorporation of pentavalent and quadrivalent cations for trivalent cations at Al1, but also to reduce repulsion between the highly charged cations. This group includes holtite, endmember composition (Ta0.6☐0.4)Al6BSi3O18, nioboholite (2012-68), endmember composition (Nb0.6☐0.4)Al6BSi3O18, and titanoholtite (2012-69), endmember composition (Ti0.75☐0.25)Al6BSi3O18.(3) Szklaryite (2012-70) with Al1 = ☐ and an endmember formula ☐Al6BAs3+3O15. Vacancies at Al1 are caused by loss of O at O2 and O7, which coordinate the Al1 with the Si sites, due to replacement of Si4+ by As3+ and Sb3+, and thus this mineral does not belong in either the dumortierite or the holtite group. Although szklaryite is distinguished by the mechanism introducing vacancies at the Al1 site, the primary criterion for identifying it is based on occupancy of the Si/As, Sb sites: (As3+ + Sb3+) > Si4+ consistent with the dominant-valency rule. A Sb3+ analogue to szklaryite is possible.