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Percleveite-(Ce) — a new lanthanide disilicate mineral from Bastnäs, Skinnskatteberg, Sweden

¹ Research Division, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
² Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden

¹ E-mail address: dan.holtstam{at}nrm.se

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 (Ce_0.87La_0.41Nd_0.35Y_0.12Pr_0.09Sm_0.07Gd_0.06Dy_0.01Ca_0.01)_=1.99Si_2.01O₇ 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).

Key-words: percleveite-(Ce), new mineral, REE, disilicate, crystal structure, Bastnäs, Sweden..

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