Jinshajiangite: structure, twinning and pseudosymmetry.

Abstract

The crystal structure of jinshajiangite based on a sample from its original discovery location is studied using single-crystal X-ray diffraction and transmission electron microscopy methods. Jinshajiangite is a titanium silicate mineral with an ideal chemical formula of BaNaFe4Ti2(Si2O7)2O2(OH)2F. The structure of jinshajiangite is of P\bar 1 symmetry (triclinic system), with a = 8.7331 (2) Å, b = 8.7366 (2) Å, c = 11.0404 (3) Å, α = 81.477 (1)°, β = 110.184 (1)°, γ = 104.384 (1)° and V = 764.03 (3) Å3, instead of the previously proposed C\bar 1 cell [a = 10.7059 (5) Å, b = 13.7992 (7) Å, c = 20.760 (1) Å, α = 90.008 (1)°, β = 94.972 (1)°, γ = 89.984 (1)°, V = 3055.4 (4) Å3]. The basic topology of the new structure is similar to the previously proposed C\bar 1 structure, except there is only one type of titanium silicate and intermediate cation layer in the structure (instead of two types), which are all related by the translation along the c-axis. Even though there is a significant amount of Mn in the chemical composition, no obvious ordering between Fe and Mn is observed in the structure. All the mineral species of the perraultite-type structure (jinshajiangite, perraultite, surkhobite and bobshannonite) should have the same P\bar 1 structure as jinshajiangite with ∼10 Å d001 spacing, and all the previously proposed monoclinic space groups were pseudosymmetry generated by nanoscale polysynthetic twinning on the (001) composition plane. The similar phenomenon observed in bafertisite is also discussed in the paper with an alternative polytype structure model proposed.