Petersite-(Y) from the Mednorudyanskoe Deposit (Middle Urals, Russia) and Its Crystal Structure

Petersite-(Y) was found in the oxidation zone of the Mednorudyanskoe deposit (Middle Urals). The mineral occurs as light blue acicular hexagonal crystals up to 0.5 mm long and up to 7 μm thick, usually combined in bunches. Associated minerals are malachite, chrysocolla, opal, and rancieite. This is the first find of a mixite-group phosphate mineral in Russia. The chemical composition of the studied petersite-(Y) (electron-microprobe data, H₂O content is calculated by stoichiometry, wt %) is: 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 100.7. 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 mixite-type phosphate with species-defining REE was first studied on the single crystal extracted from this specimen, R₁ = 2.5%. The mineral is hexagonal, space group P6₃/m, a = 13.2348(4), c = 5.8574(2) Å, V = 888.53(6) ų, and Z = 2. In general, petersite-(Y) is isotypic to mixite-group arsenates, however, it has a crystal chemical feature earlier unknown for representatives of the mixite structure type: the presence of the octahedral [Ca(H₂O)₆]²⁺ clusters in wide channels. The powder X-ray diffraction pattern reliably indexed based on the crystal-structure data is first reported for a mixite-type phosphate.