Chabournéite from Monte Arsiccio mine (Apuan Alps, Tuscany, Italy): occurrence and crystal structure

Chabournéite is a very rare Tl-Pb-Sb sulfosalt found only in the hydrothermal deposit of Jas Roux (Hautes-Alpes, France) 1, at the Toya-Takarada mine (Okkaido, Japan) 2, and as an unnamed mineral at Hemlo gold mine 3. The crystal structure of a Pb-rich chabournéite was determined by 4. The As-rich isotype of chabournéite, dalnegroite, was discovered recently 5. The new finding of chabournéite at the baryte-pyrite-iron oxides deposit of Monte Arsiccio allowed a new crystallographic study. At this locality, chabournéite is associated with baryte, pyrite, realgar, sphalerite, stibnite, and zinkenite. Electron microprobe analyses collected on two different grains give (wt.): Tl 16.81(27), Pb 10.65(17), Sb 41.75(17), As 6.59(4), S 23.43(11), total 99.24(36) and Tl 15.05(26), Pb 13.04(32), Sb 45.49(29), As 3.07(4), S 22.77(14), total 99.42(39). On the basis of (SUM)Me 26 apfu, the formula of the chabournéite grains are Tl3.79Pb2.37Sb15.79As4.05S33.65 and Tl3.47Pb2.97Sb17.63As1.93S33.50, in agreement with the structural formula Tl5-xPb2x(Sb,As)21-xS34. Crystal structure determination shows that this chabournéite is triclinic, space group P1, with unit cell parameters a 8.150(2), b 8.716(2), c 21.579(4) Å, alpha 85.18(1)°, beta 96.94(1)°, gamma 88.60(1)°, V 1515(1) Å3. Due to the very small crystal size, intensity data were collected at the XRD beamline of the Elettra synchrotron facility (Basovizza, Trieste, Italy). The crystal structure was solved by direct methods and refined to R 0.048 for all 3392 reflections with Fo > 4sigma(Fo). The new crystal structure solution confirms the general features given by 4, but on the basis of a simpler, more symmetric unit cell, without any visible superstructure. While in 4 all cation positions correspond to pure sites (Tl, Pb, Sb or As), the new structure presents mixed (Tl,Pb), (Pb,Sb) and (Sb,As) sites, together with pure Tl, Pb and Sb sites. According to 6, it is built up on the basis of two kinds of layer. The first one, derived from the SnS archetype, is closely related to sartorite 7 and parapierrotite 8, and is here enriched with As, while the second one, of the PbS archetype, is As-poor. Weak bonding in the middle plane of this last layer may act as a cleavage plane. The high Sb/As ratio of chabournéite from Monte Arsiccio appears as the main factor precluding the 2a superstructure visible in As-rich chabournéite from Jas Roux as well as in dalnegroite. 1 Johan, Z. et al. (1981) Bull. Minéral., 104, 10-15. 2 Shimizu, M. et al. (1999) Res. Geol., 20, 31-37. 3 Harris, D. (1989) Geol. Surv. Canada, Econ. Geol. Rep., 38 p. 4 Nagl, A. (1979) Z. Kristall., 150, 85-106. 5 Nestola, F. et al. (2009) Mineral. Mag., 73, 1027-1032. 6 Makovicky, E. (1985) Fortschr. Miner. 63, 45-89. 7 Berlepsch et al. (2003) Am. Miner. 88, 450-461. 8 Engel, P. (1980) Z. Kristall., 151, 203-216