Wodegongjieite, ideally KCa₃(Al₇Si₉)O₃₂, a new sheet silicate isostructural with the feldspar polymorph kokchetavite, KAlSi₃O₈

Abstract Wodegongjieite occurs in the Cr-11 chromitite orebody of the Luobusa ophiolite in the Kangjinla district, Tibet, China. It is found in two inclusions in corundum: (1) as a partial overgrowth (holotype) up to 1.5 μm thick around a spheroid 20 μm across of wenjiite (Ti 10 (Si,P,□) 7 ), kangjinlaite (Ti 11 (Si,P) 10 ), zhiqinite (TiSi 2 ) and badengzhuite (TiP), and (2) as pools up to 0.25 μm wide filling interstices between wenjiite, jingsuiite (TiB 2 ), osbornite–khamrabaevite (Ti[N,C]) and corundum. Energy dispersive analyses gave Al 2 O 3 34.09, SiO 2 49.11, K 2 O 2.56, CaO 11.71, SrO 2.53, total 100.0 wt.%, corresponding to K 0.58 Sr 0.26 Ca 2.25 Al 7.20 Si 8.80 O 31.20 , ideally KCa 3 (Al 7 Si 9 )O 32 , for Si + Al = 16 cations. Single-crystal studies were carried out with three-dimensional electron diffraction providing data for an ab initio structure solution in the hexagonal space group P 6/ mcc (#192) with a = 10.2(2) Å, c = 14.9(3) Å, V = 1340(50) Å 3 and Z = 2. Density (calc.) = 2.694 g⋅cm –3 . The refinement, which assumes complete Si–Al disorder, gives average T 1–O and T 2–O bond lengths both as 1.65 Å. It was not practical to use unconstrained refinement for the occupancies of the large cation sites 6 f and 2 a . The ab initio model shows clearly that the two cation sites have different sizes and coordination. Consequently, we imposed the condition (1) that all the K occupies the 2 a site as the average K–O bond length of 3.07 Å is close to the average K–O bond lengths reported in kokchetavite and (2) that all the Ca occupies the 6 f site as the average Ca–O bond length of 2.60 Å (2.36 Å and 2.84 Å for Ca–O1 and Ca–O3, respectively) is reasonable for Ca–O. Assuming that all K and all Ca are located at the 2 a site and 6 f site, respectively, Sr occupancies of these sites could be refined. Thermal parameters are positive and in a reasonable range. The structure is a sheet silicate isostructural with the K-feldspar polymorph kokchetavite, with two crystallographically distinct sites for K, but not with the topologically identical anorthite polymorph dmisteinbergite (CaAl 2 Si 2 O 8 ) with only a single site for Ca. Substitution of K by Ca at the 6 f site is associated with marked rotation of the Si,Al tetrahedra and a collapse of the structure to accommodate the smaller Ca ion. The spheroid of intermetallic phases is believed to have formed from the interaction of mantle-derived CH 4 + H 2 fluids with basaltic magmas at depths of ~30–100 km, resulting in precipitation of corundum that entrapped intermetallic melts. Associated immiscible silicate melt of granodioritic composition crystallised metastably to wodegongjieite instead of a mixture of anorthite and K-feldspar.