The crystal chemistry of holtite

Abstract

Sb 3+ ,As 3+ ) S3 O 12 (O,OH,&) S3 , is a member of the dumortierite group that has been found in pegmatite, or alluvial deposits derived from pegmatite, at three localities: Greenbushes, Western Australia; Voron'i Tundry, Kola Peninsula, Russia; and Szklary, Lower Silesia, Poland. Holtite can contain >30 wt.% Sb 2 O 3 , As 2 O 3 , Ta 2 O 5 , Nb 2 O 5 , and TiO 2 (taken together), but none of these constituents is dominant at a crystallographic site, which raises the question whether this mineral is distinct from dumortierite. The crystal structures of four samples from the three localities have been refined to R 1 = 0.02À0.05. The results show dominantly: Al, Ta, and vacancies at the Al(1) position; Al and vacancies at the Al(2), (3) and (4) sites; Si and vacancies at the Si positions; and Sb, As and vacancies at the Sb sites for both Sb-poor (holtite I) and Sb-rich (holtite II) specimens. Although charge-balance calculations based on our single-crystal structure refinements suggest that essentially no water is present, Fourier transform infrared spectra confirm that some OH is present in the three samples that could be measured. By analogy with dumortierite, the largest peak at 3505À3490 cm À1 is identified with OH at the O(2) and O(7) positions. The single-crystal X-ray refinements and FTIR results suggest the following general formula for holtite: Al 7À[5x+y+z]/3 (Ta,Nb) x & [2x+y+z]/3 BSi 3Ày (Sb,As) y O 18ÀyÀz (OH) z , where x is the total number of pentavalent cations, y is the total amount of Sb + As, and z 4 y is the total amount of OH. Comparison with the electron microprobe compositions suggests the following approximate general formulae Al 5.83 (Ta,Nb) 0.50 & 0.67 BSi 2.50 (Sb,As) 0.50 O 17.00 (OH) 0.50 and Al 5.92 (Ta,Nb) 0.25 & 0.83 BSi 2.00 (Sb,As) 1.00 O 16.00 (OH) 1.00 for holtite I and holtite II respectively. However, the crystal structure refinements do not indicate a fundamental difference in cation ordering that might serve as a criterion for recognizing the two holtites as distinct species, and anion compositions are also not sufficiently different. Moreover, available analyses suggest the possibility of a continuum in the Si/(Sb + As) ratio between holtite I and dumortierite, and at least a partial continuum between holtite I and holtite II. We recommend that use of the terms holtite I and holtite II be discontinued.

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

“…We recommend a modification of the approach adopted by Pieczka et al (2011) based on a simplified version of the formula derived by Groat et al (2009). Assuming 1.000 B p.f.u.…”

“…An Sb 3+ analogue to the new mineral has not yet been discovered. However, many minerals called holtite contain more Sb 3+ than As 3+ (Pryce, 1971;Voloshin et al, 1987;Groat et al, 2009;Pieczka et al, 2011), and thus the component &Al 6 BSb 3+ 3 O 15 plays a major role in the dumortierite supergroup, and has been listed in Table 1 and included in Fig. 3.…”

“…The crystal structure of dumortierite viewed down the a axis. Pnma setting (from Groat et al, 2009Groat et al, , 2012.…”

“…They reported that As 3+ is incorporated into the crystal structure in (Sb,As)O 3 pyramids substituting for SiO 4 tetrahedra just as SbO 3 pyramids substitute for SiO 4 tetrahedra in holtite from Greenbushes, and there was no preference for one of the two Si positions. In their refinement of four holtite crystals, Groat et al (2009) (OH) z , where x is the total number of pentavalent cations, y is the total amount of Sb + As and z 4 y is the total amount of OH. The splitting of (Sb, As) and Al1 sites observed in dumortierite has not been found in holtite .…”

“…Voloshin et al (1977Voloshin et al ( , 1987 introduced the terms ''holtite I'' and ''holtite II'' for clusters of holtite compositions differing in (As+Sb) content, but compositions from other localities, particularly the Szklary pegmatite, filled the apparent gap between the two compositions (Pieczka et al, 2011). Groat et al (2009) suggested the clustering could be an artifact resulting from there being two generations of holtite formed in different chemical environments at Voron'i Tundry, rather than from a crystallographic control. Moreover, the compositional range in holtite from the Szklary pegmatite extended to higher (Sb+As) contents than the high (Sb+As) composition of ''holtite II'' (Pieczka et al, 2011).…”