| Summary: | Spryite (Ag<sub>7.98</sub>Cu<sub>0.05</sub>)<sub>Σ=8.03</sub>(As<sup>5+</sup><sub>0.31</sub>Ge<sub>0.36</sub>As<sup>3+</sup><sub>0.31</sub>Fe<sup>3+</sup><sub>0.02</sub>)<sub>Σ=1.00</sub>S<sub>5.97</sub>, and ideally Ag<sub>8</sub>(As<sup>3+</sup><sub>0.5</sub>As<sup>5+</sup><sub>0.5</sub>)S<sub>6</sub>, is a new mineral recently described from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. Its room temperature structure exhibits an orthorhombic symmetry, space group <i>Pna</i>2<sub>1</sub>, with lattice parameters <i>a</i> = 14.984(4), <i>b</i> = 7.474(1), <i>c</i> = 10.571(2) Å, <i>V</i> = 1083.9(4) Å<sup>3</sup>, <i>Z</i> = 4, and shows the coexistence of As<sup>3+</sup> and As<sup>5+</sup> distributed in a disordered fashion in a unique mixed position. To analyze the crystal-chemical behaviour of the arsenic distribution at ultra-low temperatures, a structural study was carried out at 30 K by means of in situ single-crystal X-ray diffraction data (helium-cryostat) on the same sample previously characterized from a chemical and structural point of view. At 30 K, spryite still crystallizes with orthorhombic symmetry, space group <i>Pna</i>2<sub>1</sub>, but gives rise to a <b>a</b> × 3<b>b</b> × <b>c</b> superstructure, with <i>a</i> = 14.866(2), <i>b</i> = 22.240(4), <i>c</i> = 10.394(1) Å, <i>V</i> = 3436.5(8) Å<sup>3</sup> and <i>Z</i> = 4 (Ag<sub>24</sub>As<sup>3+</sup>As<sup>5+</sup>Ge<sup>4+</sup>S<sub>18</sub> stoichiometry). The twin laws making the twin lattice simulating a perfect hexagonal symmetry have been taken into account and the crystal structure has been solved and refined. The refinement of the structure leads to a residual factor <i>R</i> = 0.0329 for 4070 independent observed reflections [with <i>F</i><sub>o</sub> > 4σ(<i>F</i><sub>o</sub>)] and 408 variables. The threefold superstructure arises from the ordering of As<sup>3+</sup> and (As<sup>5+</sup>, Ge<sup>4+</sup>) in different crystal-chemical environments.
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