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 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-As 4 S 4 ) and the mineral alacranite (As 8 S 9 ). 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), R obs = 6.12%; ALA2: a = 9.936(2), b = 9.458(2), c = 9.106(2), β = 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 β-As 4 S 4 phase. The first one is identical to the As 4 S 4 molecule found in the structures of both realgar (α-As 4 S 4 ) and β-As 4 S 4 . The second molecule is chemically and structurally identical to that found in the As 4 S 5 compound. The simultaneous presence of As 4 S 4 ( C 2/c) and As 8 S 9 ( P 2/c) microdomains could be a reason for the observed gradual change of the translation symmetry from the β-phase to alacranite s.s.