The crystal structure and crystal chemistry of jinshajiangite, a Group-II Ti disilicate mineral

Currently, the crystal structures of about 120 Ti-O compounds minerals are known, half of them Ti silicates and half of them Ti oxides. Ti disilicates constitute more than 30% of Ti silicates. Sokolova [1] considered structural hierarchy and stereochemistry for twenty-four titanium disilicate minerals. Those structures contain the TS (titanium-silicate) block, a trioctahedral central (O) sheet and two adjacent heteropolyhedral (H) sheets consisting of different polyhedra including (Si2O7) groups. She divided them into four groups, characterized by different topology and stereochemistry of the TS block. Each group of structures has a different linkage of H and O sheets in the TS block, and a different arrangement of Ti (= Ti + Nb) polyhedra. In a structure, the TS block can alternate with another block, an intermediate (I) block, so called as it is intercalated between two TS blocks. In Groups I, II, III and IV, Ti equals 1, 2, 3 and 4 apfu, respectively. Jinshajiangite [2] was not considered by Sokolova [1] as its structure was unknown. We will report on the structure of jinshajiangite as a continuation of our work on Ti disilicate minerals with the TS block. The crystal structure of jinshajiangite, ideally Ba Na Ti2 Fe2+ 4 (Si2 O7)2 O2 (OH)2 F, a 10.6785(8), b 13.786(1), c 20.700(2) Å, β 94.937(1)o, V 3035.93(6) Å3, space group C2/m, Z = 8, Dcalc. 3.703 g/cm3, from Norra Kärr, Tönköping province, Sweden, has been refined to R1 5.7% on the basis of 3193 unique reflections (Fo > 4σF). Electron microprobe analysis gave (wt.%): SiO2 27.56, Nb2O5 0.12, TiO2 18.36, FeO 23.42, Fe2O3 2.89 [the Fe3+/Fetot ratio of 0.10(9) was determined by Mössbauer spectroscopy], MnO 5.13, MgO 0.44, CaO 2.52, BaO 10.24, K2O 1.95, Na2O 2.27, Cs2O 0.03, F 2.33, H2O 2.00 (calc. from structure-topology requirements: OH + F = 3 apfu), O=F –0.98, total 98.79. The crystal structure of jinshajiangite can be described as a combination of a TS block and an I block. In the O sheet, there are five Fe2+-dominant MO sites which give ideally Fe2+ 4 pfu. In the H sheet, there are three [6]-coordinated MH sites solely occupied by Ti (they give Ti = 2 apfu) and four Si sites. The MH octahedra and (Si2O7) groups constitute the H sheet. Jinshajiangit is a Group-II mineral: Ti equals 2 apfu and it occurs in the H sheet.Linkage of H and O sheets via common vertices of MH octahedra and (Si2O7) groups with MO octahedra results in a TS block. The topology of the TS block is as in Group II of the Ti disilicate minerals (Ti = 2 apfu). There are six interstitial sites, three [10-9]-coordinated Ba-dominant AP sites and three [10]-coordinated Na-dominant BP sites. The total content of three AP and three BP sites is ideally 1 Ba and 1 Na pfu, respectively. Along c, the TS blocks link via common vertices of MH octahedra (as in astrophyllite-group minerals) and the AP and BP sites which constitute the I block. Jinshajiangite, ideally Ba Na Ti2 Fe2+ 4 (Si2 O7)2 O2 (OH)2 F, is a Fe2+ analogue of perraultite, ideally Ba Na Ti2 Mn2+ 4 (Si2O7)2 O2 OH2 F, space group C2 and its crystal structure is topologically identical to that of perraultite [3].The revised crystal chemistry of Group II of Ti disilicate minerals is discussed. [1] Sokolova, E. (2006): Can. Mineral, 44, 1273-1330. [2] Hong, W. & Fu, P. (1982): Geochemistry (China), 1, 458- 464. [3] Yamnova, N.A., Egorov-Tismenko, Yu.K., Pekov, I.V. (1998): Crystallogr. Rep., 43, 401-410.