Petersite-(Y) from the Mednorudyanskoe Deposit (Central Urals) and Its Crystal Structure

Petersite-(Y) is identified in cavities of oxidation zone of the Mednorudyanskoe deposit (Central Urals). It forms blue acicular hexagonal crystals up to 0.5 × 0.007 mm in size, which are typically assembled in radial aggregates, and is associated with malachite, chrysocolla, opal, and ranciéite. This is the first finding of a phosphate member of the mixite group in Russia. The chemical composition of the Mednorudyansky petersite-(Y) is as follows (microprobe data; H₂O content is calculated by stoichiometry, wt %): CaO 1.48, CuO 52.65, Y₂O₃ 7.34, La₂O₃ 2.51, Nd₂O₃ 1.01, Dy₂O₃ 0.19, P₂O₅ 23.05, As₂O₅ 0.62, H₂O 11.85, total 100.70. The empirical formula is (Y_0.59Ca_0.24La_0.14Nd_0.05Dy_0.01)_Σ1.03Cu_6.04(P_2.96As_0.05)_Σ3.01O₁₂(OH)₆·3H₂O. The crystal structure of a rare earth element (REE) phosphate with a mixite structure is solved for the first time on a single crystal of the Mednorudyansky petersite-(Y), R₁ = 2.5%. The mineral is hexagonal, spatial group P6₃/m, a = 13.2348(4), c = 5.8574(2) Å, V = 888.53(6) ų, Z = 2. In general, petersite-(Y) is isostructural with arsenates of the mixite group, however, it has the previously unknown crystal chemical feature of the representatives of this structural type: the wide channels (tunnels) host octahedral clusters [Ca(H₂O)₆]²⁺. The X-ray powder pattern of the phosphate member of the mixite group is published for the first time, which is unambiguously resolved on the basis of structural data.