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

The crystal structure of jinshajiangite, ideally BaNaTi2Fe 2+ (Si2O7)2 O2(OH) 2F, a 10.6785(8), b 13.786(1), c 20.700(2) Å, β 94.937(1)°, V 3035.93(6) Å3, sp. gr. C2/m, Z = 8, D calc. 3.767 g/cm3, from Norra Kärr, Tönköping province, Sweden, has been refined to R1 5.69 % on the basis of 3193 unique reflections (FO > 4σF). Electron microprobe analysis gave (wt%): SiO2 27.56, Nb2O 5 0.12, TiO2 18.36, ZrO2 0.51, 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, F 2.33, H 2O 2.00 (calc. from structure refinement: OH + F = 3 apfu), O = F - 0.98, total 98.76. The empirical formula is (Ba0.58K 0.36)Σ0.94(Na0.57Ca0.39) Σ0.96 (Fe2.842+, Mn0.63Fe 0.323+Mg0.10Zr0.04Na 0.07)Σ4.00(Ti2.00Nb0.01) Σ2.01 (si2O7)2O 2.12(OH)1.93F1.07, 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 MO sites occupied mainly by Fe2+ and Mn2+, with minor Fe3+, Mg, Zr and Na with <MO-O> = 2.175 Å. Five MO sites give ideally Fe2+4 pfu. In the H sheet, there are three [6]-coordinated MH sites occupied solely by Ti (Ti = 2 apfu), with <MH-0> = 1.953 Å, and four [4]-coordinated 5/sites occupied solely by Si, with <Si-O> = 1.619 Å. The MH 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 Ap sites with <AP-O> = 2.98 Å and three [10]-coordinated Na-dominant Bp sites with <Bp-O> = 2.600 Å. The total content of three Ap sites sums to ∼1 apfu = Ba 0.58 K0.36 or ideally 1 Ba pfu. The total content of the three Bp sites is Na0.57Ca0.39 or ideally 1 Na pfu. Along c, the TS blocks link via common vertices of MH octahedra (as in astrophyllite-group minerals) and Ap and Bp sites which constitute the I block. Jinshajiangite is an Fe2+ analogue of perraultite, ideally BaNaTi2Mn42+ (Si 2O7)2O2(OH)2F, and its crystal structure is topologically identical to that of perraultite.