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

¹ Department of Geology, National Museum of Nature and Science, 3-23-1, Hyakunincho, Shinjuku, Tokyo 169-0073, Japan
² School of Science, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan

^* Corresponding author, e-mail: miyawaki{at}kahaku.go.jp

Uedaite-(Ce), ideally Mn²⁺CeAl₂Fe(Si₂O₇)(SiO₄)O(OH), occurs in granite at Shodoshima Island in Seto Inland sea, Kagawa Prefecture, Japan. It is monoclinic, P2₁/m, a = 8.939(8) Å, b = 5.742(4) Å, c = 10.187(8) Å, β = 115.10(6)°, V = 473.5(6) ų, Z = 2. The four strongest lines in the powder XRD pattern [d(Å), I/I₀, 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.%); SiO₂ 29.94, Al₂O₃ 16.02, FeO 16.01, MnO 6.01, MgO 0.07, CaO 2.42, La₂O₃ 3.09, Ce₂O₃ 10.75, Pr₂O₃ 1.83, Nd₂O₃ 6.44, Sm₂O₃ 1.35, Gd₂O₃ 0.54, Y₂O₃ 0.72, ThO₂ 0.51, and lead to the empirical formula Mn_0.51Ca_0.26Ce_0.39Nd_0.23La_0.11Pr_0.07Sm_0.05Y_0.04Gd_0.02Th_0.01Al_1.89Fe_1.34Mg_0.01(Si₂O₇)(SiO₄)O_0.85(OH) on the basis of 3 silicon and 1 hydrogen atoms per formula unit. The estimated content of H₂O 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 wR₁ = 0.0337 with single-crystal X-ray diffraction data. The refinement of occupancy parameters for the A and M sites yielded the structural formula ^A1(Mn_0.650Ca_0.350) ^A2(Ce_0.914Fe_0.086) ^M1(Al_0.873Fe_0.127) ^M2(Al_0.988Fe_0.012) ^M3(Fe_0.875Al_0.125)(Si₂O₇)(SiO₄)O(OH), and confirmed the replacement of Ca with Mn at the A site. Uedaite-(Ce) is a Mn²⁺-analogue of allanite-(Ce). The calculated density of uedaite-(Ce) is 4.19 g/cm³. 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 bastnäsite-(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.

Key-words: uedaite, new mineral, crystal structure, Shodoshima, epidote group, allanite, manganese, REE.