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How many alacranites do exist? A structural study of non-stoichiometric As₈S_9-x crystals

¹ Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via La Pira 4, I-50121 Firenze, Italy
² CNR — Istituto di Geoscienze e Georisorse — sezione di Firenze, via La Pira 4, I-50121 Firenze, Italy

e-mail: pbcry{at}steno.geo.unifi.it

Crystals of arsenic sulphide (reported as alacranite), coming from the burning dump of Kateina 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), 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-As₄S₄ phase. The first one is identical to the As₄S₄ molecule found in the structures of both realgar (-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.

Key-words: alacranite, crystal structure, arsenic sulphides, chemical composition, Kateina Mine..

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