From structure topology to chemical composition. VII. Titanium silicates: the crystal structure and crystal chemistry of jinshajiangite

The crystal structure of jinshajiangite, ideally BaNaTi2Fe42+ (Si2O7)(2) O-2(OH)(2)F, a 10.6785(8), b 13.786(1), c 20.700(2) angstrom, beta 94.937(1)degrees, V 3035.93(6) angstrom(3), sp. gr. C2/m, Z = 8, D-calc. 3.767 g/cm(3), from Norra Karr, Tonkoping province, Sweden, has been refined to R-1 5.69% on the basis of 3193 unique reflections (F-O > 4 sigma F). Electron microprobe analysis gave (wt%): SiO2 27.56, Nb2O5 0.12, TiO2 18.36, ZrO2 0.51, FeO 23.42, Fe2O3 2.89 [the Fe3+/Fe-tot ratio of 0.10(9) was determined by Mossbauer spectroscopy.], MnO 5.13, MgO 0.44, CaO 2.52, BaO 10.24, K2O 1.95, Na2O 2.27, F 2.33, H2O 2.00 (calc. from structure refinement: OH + F = 3 apfu), O = F - 0.98, total 98.76. The empirical formula is (Ba0.58K0.36)(Sigma 0.94)(Na0.57Ca0.39)(Sigma 0.96) (Fe2.842+Mn0.63Fe0.323+Mg0.10Zr0.04Na0.07)(Sigma 4.00)(Ti2.00Nb0.01)(Sigma 2.01) (Si2O7)(2)O-2.12(OH)(1.93)F-1.07, calculated on the basis of 4 Si apfu. The crystal structure of jinshajiangite can be described as a combination of a TS block and an I block. The TS (titanium silicate) block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is a component of 27 Ti-disilicate minerals. In the O sheet, there are five [6]-coordinated M-O sites occupied mainly by Fe2+ and Mn2+, with minor Fe3+, Mg, Zr and Na with < M-O-O > = 2.175 angstrom. Five M-O sites give ideally Fe-4(2+) pfu. In the H sheet, there are three [6]-coordinated M-H sites occupied solely by Ti (Ti = 2 apfu), with < M-H-O > = 1.953 angstrom, and four [4]-coordinated Si sites occupied solely by Si, with < Si-O > = 1.619 angstrom. The M-H octahedra and (Si2O7) groups constitute the H sheet. Linkage of H and O sheets via common vertices of M-H octahedra and (Si2O7) groups with M-O(1-5) octahedra results in a TS block. The topology of the TS block is as in Group II of the Ti disilicates (Ti = 2 apfu). There are six interstitial sites, three [9-10]-coordinated Ba-dominant A(P) sites with < A(P)-O > = 2.98 angstrom and three [10]-coordinated Na-dominant B-P sites with < B-P-O > = 2.600 angstrom. The total content of three A(P) sites sums to similar to 1 apfu = Ba-0.58 K-0.36 or ideally 1 Ba pfu. The total content of the three B-P sites is Na0.57Ca0.39 or ideally 1 Na pfu. Along c, the TS blocks link via common vertices of M-H octahedra (as in astrophyllite-group minerals) and A(P) and B-P sites which constitute the I block. Jinshajiangite is an Fe2+ analogue of perraultite, ideally BaNaTi2Mn42+ (Si2O7)(2)O-2(OH)(2)F, and its crystal structure is topologically identical to that of perraultite.