Wodegongjieite, ideally KCa3(Al7Si9)O32, a new sheet silicate isostructural with the feldspar polymorph kokchetavite, KAlSi3O8. (8th December 2022)

• Record Type: Journal Article
• Authors: Mugnaioli, Enrico; Xiong, Fahui; Xu, Xiangzhen; Gemmi, Mauro; Wirth, Richard; Yang, Jingsui; Grew, Edward S.
• Is Part Of: Mineralogical magazine. Volume 86:Number 6(2022)
• Page Start: 975
• Page End: 987
• Publication Date: 2022-12-08
• Subjects: wodegongjieite -- kokchetavite -- dmisteinbergite -- feldspar family -- 3-dimensional electron diffraction (3DED) -- ab initio structure solution -- Luobusa ophiolite -- Tibet -- China; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2022.107
• Languages: English
• ISSNs: 0026-461X

Abstract

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 (Ti10 (Si, P, □)7 ), kangjinlaite (Ti11 (Si, P)10 ), zhiqinite (TiSi2 ) and badengzhuite (TiP), and (2) as pools up to 0.25 μm wide filling interstices between wenjiite, jingsuiite (TiB2 ), osbornite–khamrabaevite (Ti[N, C]) and corundum. Energy dispersive analyses gave Al2 O3 34.09, SiO2 49.11, K2 O 2.56, CaO 11.71, SrO 2.53, total 100.0 wt.%, corresponding to K0.58 Sr0.26 Ca2.25 Al7.20 Si8.80 O31.20, ideally KCa3 (Al7 Si9 )O32, 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 (CaAl2 Si2 O8 ) 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 CH4 + H2 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.