Multi-methodical study of the Ti, Fe2+ and Fe3+ distribution in chevkinite-subgroup minerals: X-ray diffraction, neutron diffraction, 57Fe Mössbauer spectroscopy and electron-microprobe analyses

Abstract

Three non-metamict chevkinite-subgroup minerals, space group P21/a, from Cape Ashizuri, Japan, (No. 1), Tangir Valley, Diamar District, Pakistan (No. 2) and Haramosh Mts., Skardu district, Pakistan, (No. 3) were studied by crystal chemical techniques. Powder X-ray diffraction and transmission electron microscopic observations confirmed well crystalline samples. Electron-microprobe analyses indicated the general composition [(REE, Ca)4Fe2+(Fe2+, Fe3+, Ti)2Ti2(Si2O7)2O8] known for chevkinite-(Ce). Site scattering values determined by single-crystal X-ray structure refinements suggested assignment of subordinate Nb to the octahedral M3 and M4 sites, minor Th to M1 for the Ashizuri sample and minor Mg to M1 for both samples from Pakistan. Neutron time-of-flight powder diffraction studies were applied to determine the Ti/Fe distribution among octahedral sites for all samples and Mossbauer spectroscopy served for the Fe valence assignment at the four octahedral sites. Combined results gave the simplified formulas with Ce as dominant REE: No. 1 A(REE,Ca)4M1(Fe2+,Ti)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+,Nb)2(Si2O7)2O8, No. 2 A(REE,Ca)4M1(Fe2+,Fe3+,Mg)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+)2(Si2O7)2O8 and No. 3 A(REE,Ca)4M1(Fe3+,Fe2+,Mg,Ti)M2(Ti,Fe3+)2M3,M4(Ti,Fe2+)2(Si2O7)2O8. The dominant iron valence at M1 of the Haramosh sample (No. 3) is ferric whereas for samples Nos. 1 and 2 iron is ferrous. The structure of chevkinite-subgroup minerals consists of octahedral layers (M2, M3 and M4) alternating with an intermediate layer containing disilicate groups, REE polyhedra and strongly distorted M1 octahedra. Increased O atomic displacement parameters within the intermediate layers indicate disorder and/or strain to fit the extension of the embracing octahedral layers. A statistical analysis of chevkinite-subgroup structures (space groups P21/a, C2/m and P21/m) indicates a positive correlation between bond length and M1 bond angle variance. A negative correlation is found between bond length and the disilicate bending angle Si–O–Si, varying between ca. 160° and 175°. Outliers of this trend are delhuyarite-(Ce) Ce4Mg(Fe3+2W)□(Si2O7)2O6(OH)2, mainly due to octahedral W6+ and vacancies in the octahedral layers associated with OH groups, and synthetic Nd end-members with strong bonds between Nd and the intersecting O of the disilicate group. The observed trends reflect the main mechanisms in the intermediate layer to adopt to the size of the embracing rigid octahedral layers formed by M2, M3 and M4 octahedra.