How many alacranites do exist?: A structural study of non-stoichiometric AS8S9-x crystals

Crystals of arsenic sulphide (reported as alacranite), coming from the burning dump of Katerina Mine (Czech Republic), have been investigated by single crystal X-ray diffraction and chemical microanalysis. Both analytical data and unit-cell parameters strongly suggest the existence of a continuous series between the high-temperature polymorph (beta-As4S4) and the mineral alacranite (As8S9). As the S content increases in the series, the unit-cell volume increases accordingly. The structural model has been obtained for two crystals, exhibiting different unit-cell volumes (ALA15: a = 9.940(2), b = 9.398(2), c = 9.033(2), beta =102.12(2), V = 825.0(3), R-obs = 6.12%; ALA2: a = 9.936(2), b = 9.458(2), c = 9.106(2), beta = 101.90(2), V = 837.3(3), R-obs = 6.41%). We found that the non-stoichiometric compounds crystallize as a disordered mixture of two kinds of cage-like molecules, packed together as in the beta-As4S4 phase. The first one is identical to the As4S4 molecule found in the structures of both realgar (alpha-As4S4) and beta-As4S4. The second molecule is chemically and structurally identical to that found in the As4S5 compound. The simultaneous presence of As4S4 (C2/c) and As8S9 (P2/c) microdomains could be a reason for the observed gradual change of the translation symmetry from the beta-phase to alacranite s.s..