Uedaite-(Ce), a new member of the epidote group with Mn at the A site, from Shodoshima, Kagawa Prefecture, Japan

Abstract

Uedaite-(Ce), ideally Mn 2+ CeAl 2 Fe(Si 2 O 7 )(SiO 4 )O(OH), occurs in granite at Shodoshima Island in Seto Inland sea, Kagawa Prefecture, Japan. It is monoclinic, P 2 1 / m , a = 8.939(8) A, b = 5.742(4) A, c = 10.187(8) A, β = 115.10(6)°, V = 473.5(6) A 3 , Z = 2. The four strongest lines in the powder XRD pattern [ d (A), I / I 0 , hkl ] are (2.92, 100, −302); (3.53, 54, −211); (2.71, 43, 013 120 300) and (2.62, 39, −311). Electron microprobe analysis gave (wt.%); SiO 2 29.94, Al 2 O 3 16.02, FeO 16.01, MnO 6.01, MgO 0.07, CaO 2.42, La 2 O 3 3.09, Ce 2 O 3 10.75, Pr 2 O 3 1.83, Nd 2 O 3 6.44, Sm 2 O 3 1.35, Gd 2 O 3 0.54, Y 2 O 3 0.72, ThO 2 0.51, and lead to the empirical formula Mn 0.51 Ca 0.26 Ce 0.39 Nd 0.23 La 0.11 Pr 0.07 Sm 0.05 Y 0.04 Gd 0.02 Th 0.01 Al 1.89 Fe 1.34 Mg 0.01 (Si 2 O 7 )(SiO 4 )O 0.85 (OH) on the basis of 3 silicon and 1 hydrogen atoms per formula unit. The estimated content of H 2 O is 1.50 and the total is 97.20. The variation in concentration of Mn in uedaite-(Ce) and associated allanite-(Ce) showed negative linear correlation with that of Ca, suggesting isomorphous substitution of Mn for Ca. The crystal structure of uedaite-(Ce) was refined to wR 1 = 0.0337 with single-crystal X-ray diffraction data. The refinement of occupancy parameters for the A and M sites yielded the structural formula A 1 (Mn 0.650 Ca 0.350 ) A 2 (Ce 0.914 Fe 0.086 ) M 1 (Al 0.873 Fe 0.127 ) M 2 (Al 0.988 Fe 0.012 ) M 3 (Fe 0.875 Al 0.125 )(Si 2 O 7 )(SiO 4 )O(OH), and confirmed the replacement of Ca with Mn at the A site. Uedaite-(Ce) is a Mn 2+ -analogue of allanite-(Ce). The calculated density of uedaite-(Ce) is 4.19 g/cm 3 . Uedaite-(Ce) occurs as short prismatic crystals less than 1 mm length. Crystals are black to dark brown in color. Uedaite-(Ce) is associated with allanite-(Ce), monazite-(Ce), zircon and thorite, and is often altered to bastnasite-(Ce). The new mineral is biaxial negative and transparent with pleochroism from brown to yellow. However, optical properties are too similar to those of allanite-(Ce), thus distinction by crystal optics is not possible.