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 AlAl₆BSi₃O_18, and magnesiodumortierite, end-member composition MgAl₆BSi₃O₁₇(OH), plus three end-members, “hydroxydumortierite”, oAl₆BSi₃O₁₅(OH)₃, two Mg-Ti analogues of dumortierite, (Mg_0.5Ti_0.5)Al₆BSi₃O₁₈ and (Mg_0.5Ti_0.5)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 (Ta_0.6o_0.4)Al₆BSi₃O₁₈, nioboholite (2012-68), end-member composition (Nb_0.6o_0.4)Al₆BSi₃O_18, and titanoholtite (2012-69), end-member composition (Ti_0.75o_0.25)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.