бонацциит, алакранит, ральгар, сульфиды мышьяка, псевдофумаролы, угольный пожар, возгоны, Таджикистан, Фан-Ягнобское месторождение, Кухи-Малик
Two rare minerals were found in the sublimates of a natural coal fire at the Fan-Yagnob coal deposit (Tajikistan) – bonazzite As4S4 and alacranite As8S9. Bonazzite occurs as orange, orange-red rhombic tapered crystals, both skeletal, block, and full-faceted, from 0.1 to 1.5 mm in size. Bonazziite grows on realgar crystals or on a matrix, composed of amorphous As2S3. The association contains sal-ammoniac and ermakovite (NH4)(As2O3)2Br; crystals of the latter also grows on bonazziite. In transmitted light in thin sections, bonazzite is yellowish-brown, biaxial (+). Dispersion is strong, v > r. 2Vmeas. = + 60(5)°. Refractive indices are significantly higher than n = 2.05. In reflected light, it is gray with intense internal reflections ranging from yellow-green to red. Digests in 5M KOH, producing brown colored spots. The unit cell parameters from X-ray powder data are as follows: a9.972(5), b 9.435(3), c 8.870(6) Å, β 102.25(4) º, V 815(1) Å3. The X-ray ratio b : c= 1.063, morphological ratio (if to assign the symbol {111} to the most developed face) from the results of goniometric measurements from 0.817 to 0.870. Indicing of this facet, applying X-ray cell parameters leads to irrational symbols.Bonazziite crystals are assumed to be paramorphoses on a related phase with some different cell parameters. Measured density (g/cm3) 3.57(1), calculated 3.52. Chemical analysis by electron microprobe using EDS (12 points), As 69.01, S 29.86, Se 0.47, total 99.34 wt.%. The empirical formula is As4.01S3.96Se0.02. Adjacent realgar does not contain Se at the sensitivity level of the microprobe analysis. Powdery yellow As-sulfide sometimes develops on realgar, it also grows as rhombic brittle crystals, similar to those of bonazzite. X-ray powder data shows, it is close to alacranite with cell parameters a 9.90(5), b 9.58(5), c 9.11(5) Å, β 101.6(6) º, V 847(5) Å3.The growth of bonazzite could have been originated by a local increase in temperature (but no more than up to 320 °C – the upper temperature limit of stability of the realgar) and/or Se occurence in the gas precipitates, which played the role of a stabilizing impurity that caused the growth of bonazzite on the realgar. A local change in the thermal regime probably led to the appearance of related alacranite.
