Crystals of arsenic sulphide (reported as alacranite), coming from the burning dump of Kateřina 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 (β-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), β = 102.12(2), V = 825.0(3), Robs = 6.12%; ALA2: a = 9.936(2), b = 9.458(2), c = 9.106(2), β = 101.90(2), V = 837.3(3), Robs = 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 β-As4S4 phase. The first one is identical to the As4S4 molecule found in the structures of both realgar (α-As4S4) and β-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 β-phase to alacranite s.s.