| Popis: |
Percleveite-(Ce), (Ce, La, Nd) 2 Si 2 O 7 , is a new mineral species from the Bastnas Fe-Cu-REE deposit, Skinnskatteberg, Vastmanland, Sweden. It occurs closely associated with mainly cerite-(Ce), bastnasite-(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: La 2 O 3 14.66, Ce 2 O 3 31.36, Pr 2 O 3 3.41, Nd 2 O 3 12.97, Sm 2 O 3 2.69, Gd 2 O 3 2.26, Dy 2 O 3 0.53, Ho 2 O 3 0.07, Er 2 O 3 0.21, Yb 2 O 3 0.04, Y 2 O 3 2.93, CaO 0.10, FeO 0.01, SiO 2 26.55, sum 97.79, yielding the empirical formula (Ce 0.87 La 0.41 Nd 0.35 Y 0.12 Pr 0.09 Sm 0.07 Gd 0.06 Dy 0.01 Ca 0.01 ) Σ=1.99 Si 2.01 O 7 based on 7 O atoms. The mineral is shown to be isostructural with the synthetic, low-temperature lanthanide disilicates (general formula Ln 2 Si 2 O 7 ) with tetragonal symmetry (space group P 4 1 ). Unit-cell parameters are a = 6.7805(8) and c = 24.689(4) A (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 SiO 2 activities, and lower concentrations of Ca and Mg, than conditions favourable for the crystallization of cerite-(Ce). |
