Percleveite-(Ce), (Ce, La, Nd)2Si2O7, is a new mineral species from the Bastnäs Fe-Cu-REE deposit, Skinnskatteberg, Västmanland, Sweden. It occurs closely associated with mainly cerite-(Ce), bastnäsite-(Ce) and quartz. The colour is greyish and the luster greasy to resinous. The anhedral crystals, up to 0.5 mm in size, are colourless in thin section and optically uniaxial (+). H ∼ 6. Cleavage is imperfect parallel to {001}. Electron-microprobe analyses give: La2O3 14.66, Ce2O3 31.36, Pr2O3 3.41, Nd2O3 12.97, Sm2O3 2.69, Gd2O3 2.26, Dy2O3 0.53, Ho2O3 0.07, Er2O3 0.21, Yb2O3 0.04, Y2O3 2.93, CaO 0.10, FeO 0.01, SiO2 26.55, sum 97.79, yielding the empirical formula (Ce0.87La0.41Nd0.35Y0.12Pr0.09Sm0.07Gd0.06Dy0.01Ca0.01)Σ=1.99Si2.01O7 based on 7 O atoms. The mineral is shown to be isostructural with the synthetic, low-temperature lanthanide disilicates (general formula Ln2Si2O7) with tetragonal symmetry (space group P41). Unit-cell parameters are a = 6.7805(8) and c = 24.689(4) Å (refined from powder data) with Z = 8. The structure has been refined from single crystal data to a weighted Rw value of 0.036. All four symmetry independent Ln positions are slightly differently occupied by the lanthanide ions. Percleveite is formed at conditions with slightly higher SiO2 activities, and lower concentrations of Ca and Mg, than conditions favourable for the crystallization of cerite-(Ce).

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