Crystal chemistry of uedaite-(Ce) from the Karasugawa mine, Fukushima, Japan, and the Heftetjern pegmatite, Norway

The crystal structure of uedaite-(Ce), ideal formula A1Mn2+A2CeM1-2Al2M3Fe2+(SiO4)(Si2O7)O(OH), was studied using single crystals from the Karasugawa mine, Fukushima, Japan (hereafter, KG), and the Heftetjern pegmatite, Norway (HJ), by means of electron microprobe and X-ray diffraction. Both the specimens have similar composition, and are rich in MnO (8.1-8.5 wt%) and FeO (13.8-14.8 wt%). The Fe2+/total Fe ratios were estimated as 0.93. Structure refinements converted to R1 = 1.70 and 1.55% for KG and HJ, respectively. The unit-cell parameters are a = 8.8200(2), b = 5.69346(12), c = 9.9745(3) Å, β = 114.016(3)° and V = 457.52(2) Å3 for KG, and a = 8.8212(3), b = 5.69723(11), c = 9.9821(3) Å, β = 114.083(2)° and V = 458.00(2) Å3 for HJ. The cation assignments of the A and M sites are as follows: A1Mn2+0.71Fe2+0.17Ca0.12, A2Ce0.40Nd0.27La0.09Pr0.06Sm0.06Y0.05Gd0.03Na0.01Ca0.03, M1Al0.85Fe3+0.09Fe2+0.05Ti0.01, M2Al1.00, and M3Fe2+0.91Mg0.02Al0.07 for KG, and A1Mn2+0.70Fe2+0.16Ca0.14, A2Ce0.39Nd0.28La0.09Sm0.07Pr0.06Y0.06Gd0.04Na0.01, M1Al0.83Fe3+0.09Fe2+0.07Ti0.01, M2Al1.00, and M3Fe2+0.90Mg0.02Al0.08 for HJ. The unit-cell volume of uedaite is the smallest among allanite-group minerals, resulting from the smallest combination of dominant cations at the A1 and M3 sites, Mn2+ and Fe2+. In the uedaite structure, the substitution of Ca for Mn2+ induces a topological change at the A1 site, as reported in previous studies, and the structure also shows an elongation of the A1-O5 bond. This elongation results from the edge-sharing coordination polyhedra of Fe2+-dominated M3 and Al-dominated M1 sites, being slightly tilted to maintain bonding with adjacent SiO4 tetrahedra. Moreover, a positive correlation between the β angle and the O4-M3-O8 angle was observed. This correlation is suggested to be a general feature across the entire epidote supergroup minerals.