THE CRYSTAL CHEMISTRY OF DUMORTIERITE AND HOLTITE, ALUMINOBOROSILICATES WITH HEAVY ELEMENTS

Dumortierite [c. (Al,vac)Al₆(BO₃)Si₃O₁₃(O,OH)₂] is second only to tourmaline as the most abundant borosilicate in pegmatites and aluminous metamorphic and metasomatic rocks, whereas the isostructural holtite [c. (Al,Ta,vac)Al₆(BO₃)(Si,Sb)₃O₁₂(O,OH,vac)₃] is known in pegmatites from only three localities worldwide. Dumortierite is generally considered a "clean" aluminoborosilicate in which variable amounts of Ti, Fe and Mg substitute for Al, and P for Si. However, our electron microprobe analyses of 44 samples of dumortierite from 37 localities reveal the presence of significant As, Sb, Bi, Nb and Ta, suggesting some miscibility between dumortierite and holtite. Arsenic is a regular constituent, e.g., dumortierite from the southern Urals, East Antarctica, Ogilby, California, and Saxony contain up to 1.89, 3.01, 3.45, and 3.58 wt.% As₂O₃, respectively; the last corresponds to 0.22 As atoms per formula unit (apfu). The sample from Antarctica also contains up to 1.77 wt.% Ta₂O₅ (0.05 Ta apfu), 0.46 wt.% Nb₂O₅, 1.03 wt.% Sb₂O₃ (0.04 Sb apfu), whereas a crystal from Madagascar shows 1.36 Nb₂O₅ (0.06 Nb apfu). Up to 1.04 wt.% Bi₂O₃ is present in the Urals sample.

Two varieties of holtite were found in the analyzed samples from Greenbushes, Australia (type locality) and from the Kola Peninsula. One appears to be of primary origin and has a low Sb content (4.67-5.22 wt.% Sb₂O₃, 0.22-0.25 Sb apfu), whereas the other is a replacement phase with high-Sb content (14.75-16.41 wt.% Sb₂O₃, 0.71-0.79 Sb apfu), which correspond to holtite I and holtite II of Voloshin et al. (1987), respectively. Holtite I has higher Al, Si, and Ta contents (to 13.81 wt.% Ta₂O₅, 0.43 Ta apfu), but Nb concentrations are uniformly low (to 0.31 wt.% Nb₂O₅, 0.02 Nb apfu). Crystal structure analysis of the holtite II from the Kola Peninsula shows As > Ta at the Al(1) site. Preliminary crystal structure work on a fibrous sample from Poland indicates doubled a and b unit cell dimensions resulting from ordering at the Al(1) sites.

Individual samples of both minerals are compositionally heterogeneous. Calculating their formulae is further complicated by the likely presence of (1) vacancies on the Al(1) sites, (2) pentavalent as well as trivalent As, Sb, and Bi and (3) variations in anion composition, e.g., protons and vacancies.