Adam Pieczka 
 Department of Mineralogy, Petrography and Geochemistry, AGH-University of Science and Technology, Kraków, Poland

R. James Evans, Lee A. Groat
Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada

Edward S. Grew
Department of Earth Sciences, University of Maine, Orono, Maine, U.S.A.

 Chi Ma and George R. Rossman
'Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, U.S.A.

 

Abstract

As presently constituted, the dumortierite group, space group Pnma (no. 62),  comprises three minerals: dumortierite, (Al,o)Al6BSi3O16(O,OH)2, magnesiodumortierite, (Mg,o)Al6BSi3O16(O,OH)2, and holtite, (Al,Ta,o)Al6B(Si,Sb,As)3O15(O,OH,o)3, where o denotes cation or anion vacancy. Although the distinction between magnesiodumortieite and dumortierite, i.e., Mg vs. Al dominance at the partially vacant octahedral Al1 site, meets current criteria of the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) for distinguishing mineral species, the distinction between holtite and dumortierite does 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. In addition, a mineral from the Szklary pegmatite (Lower Silesia, Poland) giving EBSD patterns consistent with the dumortierite structure is the first example of a dumortierite-group mineral with trivalent cations dominant at the Si-(As,Sb) sites, which has been approved as the new mineral szklaryite (2012-70) (Pieczka et al., submitted). Other minerals from the Szklary pegmatite showing EBSD patterns consistent with the dumortierite structure have Nb or Ti dominant at Al1 if allowance is made for charge balance, which requires that Al3+ be replaced by 0.6(Ta5+, Nb5+) + 0.4o and by 0.75Ti4+ + 0.25o, respectively. The Nb- and Ti-dominant compositions have been approved as the new minerals nioboholtite (2012-68) and titanoholtite (2012-69)(Pieczka et al., submitted).

On the basis of these findings we propose more specific end-member compositions for dumortierite and holtite together with a classification of a dumortierite supergroup based on occupancy of the Al1 site. The supergroup comprises two groups and one representative of a potential third group:

(1) Dumortierite group, with Al1 = Al3+, Mg2+ and o. Charge balance is provided by OH substitution for O at the O2, O7 and O10 sites. This includes the minerals dumortierite, end-member composition AlAl6BSi3O18, and magnesiodumortierite, end-member composition MgAl6BSi3O17(OH), plus three end-members, “hydroxydumortierite”, oAl6BSi3O15(OH)3, two Mg-Ti analogues of dumortierite, (Mg0.5Ti0.5)Al6BSi3O18 and (Mg0.5Ti0.5)Mg2Al4BSi3O16(OH)2, which do not correspond to mineral species. Three more hypothetical end-members are derived by homovalent substitutions of Fe3+ for Al and Fe2+ for Mg.

(2) Holtite group, with Al1 = Ta5+, Nb5+, Ti4+ and o. 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, end-member composition (Ta0.6o0.4)Al6BSi3O18, nioboholite (2012-68), end-member composition (Nb0.6o0.4)Al6BSi3O18, and titanoholtite (2012-69), end-member composition (Ti0.75o0.25)Al6BSi3O18.

(3) Szklaryite (2012-70) with Al1 = o and an end-member formula oAl6BAs3+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. A Sb3+ analogue to szklaryite is possible.