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) Å, ␤ 101.01(2)°, V 452.9(2) Å 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 As 4 S 5 molecules, which are held together by van der Waals forces. Both the intramolecular As-S (2.237-2.261 Å) and As-As (2.527 Å) 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 As 4 S 4 molecules easily undergo such a transformation, whereas the As 4 S 5 molecules do not. SOMMAIRE Nous avons affiné la structure cristalline de l'uzonite provenant de la localité type, la caldeira de Uzon, au Kamchatka, en Russie, dans le groupe spatial P2 1 /m jusqu'à un résidu R de 2.66%. Les paramètres réticulaires sont: a 7.973(2), b 8.096(2), c 7.148(2) Å, ␤ 101.01(2)°, V 452.9(2) Å 3 , Z = 2. Le modèle structural proposé antérieurement pour l'analogue synthétique a été confirmé, tout en atteignant une précision plus élevée. La structure contient des molécules distinctes de As 4 S 5 à liaisons covalentes, retenues par liaisons van der Waals. Les liaisons intramoléculaires As-S (2.237-2.261 Å) et As-As (2.527 Å) correspondent étroitement aux valeurs observées dans la structure d'autres sulfures d'arsenic à base de molécules. Afin de contribuer à une explication des effets sur la structure des sulfures d'arsenic d'une exposition à la lumière, nous avons exposé le cristal à une lumière polychromatique filtrée, et nous avons mesuré les paramètres réticulaires après chaque exposition. Les résultats semblent indiquer que seules les molécules de As 4 S 4 sont facilement transformées, tandis que les molécules de As 4 S 5 ne montrent aucune réaction. (Traduit par la Rédaction)Mots-clés: uzonite, affinement de la structure cristalline, effets d'une exposition à la lumière, sulfure d'arsenic, caldeira de Uzon, Russie. §

Phase relations in two binary systems MgAl 2 O 4 -ZnAl 2 O 4 and MgAl 2 O 4 -Mg 2 TiO 4 have been studied and phase diagrams for them have been constructed. Based on the data of x-ray phase and crystal-optical analyses, the formation of a continuous series of solid solutions with spinel structure between the terminal members of the systems studied has been established. In the MgAl 2 O 4 -ZnAl 2 O 4 system the solid solution is stable in the range from room temperature to melting temperature. In the MgAl 2 O 4 -Mg 2 TiO 4 system the solid solution decomposes below 1380 ± C, yielding the formation of limited regions of homogeneity on the basis of MgAl 2 O 4 and Mg 212d Ti 12d O 4 . Decomposition of the solid solution is accompanied by crystallization of MgTiO 3 .

Alacranite from the type locality (Uzon caldera, Kamchatka, Russian Federation) was submitted for structural analysis. A single crystal was selected and the following lattice parameters were determined: a = 9.942(4), b = 9.601(2), c = 9.178(3) Å, b = 101.94(3)∞, V = 857.1 (5) Å 3 . The crystal structure was solved in the P2/c space group using direct methods and refined to R = 6.79% for 472 observed reflections. The structure of alacranite consists of an ordered sequence of As 4 S 4 and As 4 S 5 cage-like molecules, with a molecular packing closely resembling that found in the b-As 4 S 4 phase. Both As-As and As-S intramolecular distances are in the range usually observed for covalent bonds. The structural model confirms the chemical formula As 8 S 9 for alacranite, and accounts for differences in the unit-cell parameters of alacranite compared to those of the natural analogue of b-As 4 S 4 . This latter mineral, therefore, should receive a new name.

No abstract

No abstract