Uzonite, As4S5, from the type locality:: Single-crystal X-ray study and effects of exposure to light

The crystal structure of uzonite from the Uzon caldera, Kamchatka, Russian Federation, has been refined in space group P2(1)/m to a final R index of 2.66%. Unit-cell parameters are: a 7.973(2), b 8.096(2), c 7.148(2) Angstrom, beta 101.01(2)degrees, V 452.9(2) Angstrom(3) Z = 2. The structural model previously reported for the synthetic analogue is confirmed, although a higher precision was achieved. The structure consists of discrete covalently bonded As4S5 molecules, which are held together by van der Waals forces. Both the intramolecular As-S (2.237-2.261 Angstrom) and As-As (2.527 Angstrom) bond lengths match closely the values commonly observed in the molecule-based structures of other arsenic sulfides. In order to contribute to the understanding of the effects of light exposure on the structure of the arsenic sulfides, we exposed the crystal to filtered polychromatic light. The unit-cell parameters were measured after each exposure. The results obtained seem to indicate that the As4S4 molecules easily undergo such a transformation, whereas the As4S5 molecules do not.