The crystal structure of natural Fe-rich chevkinite-(Ce)

Chemical composition and crystal structure of natural non-metamict Fe-rich chevkinite-(Ce) from the aegirine-alkali granite, Mianning, Sichuan Province, China, were investigated. Microprobe analyses indicated that FeO (total iron) content in Ferich chevkinite(Ce) ranges from 10.97 % to 11.59 % (wt.) and Ce₂O₃ from 23 % to 25 %. There are deficient A- and (B + C)-type cations in the formula. The crystal structure of Fe-rich chevkinite-(Ce) was refined from CCD data in the space-group type C2/m (R = 2.41 %). Cell parameters are a= 13.456(3) Ã…, b = 5.728(1) Ã…, c = 11.083(2) Ã…, β = 100.60(3)°, V = 839.7(3) Ã…3, Z = 2. The structure of Fe-rich chevkinite-(Ce) is made up of two kinds of sheets, one is a sheet of Si₂O₇ groups and B-octahedra, and the other is a sheet of C-octahedra, both parallel to the ab plane.The two kinds of sheets alternate along c axis and form a three dimensional framework. The rare earth ions lie between the disilicate groups and the sheets of octahedra to form (8+3)-coordinated A-polyhedra and 10-coordinated A2-polyhedra. The Fe and Ti are partially ordered among the octahedral sites of B, C1, C2A and C2B. The Charge Distribution analysis confirms Fe3+/ΣFe = 0.61 and a mixture of Fe²⁺ and Fe³⁺ in the B site. The chemical formula for the natural Fe-rich chevkinite-(Ce) is (Ce³⁺_1.76La_0.89Ca_0.46Nd_0.22Pr_0.18Sm_0.04Gd_0.04Th_0.03Ba_{0.01 □0.37)Σ4.00}(Fe³⁺_0.29Ti_0.21Fe²⁺_0.18Mn²⁺_0.13Mg_{0.08□0.11)σ1.00}(Ti_2.16Fe³⁺_0.81F²⁺_0.52Nb_0.16Al_{0.06□0.08)Σ4.00} Si₄O_20.72(OH)_1.28.