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

The crystal structure of jinshajiangite, ideally BaNaTi2Fe2þ 4 ðSi2O7Þ2 O2ðOHÞ2F, a 10.6785(8), b 13.786(1), c 20.700(2)A ° , b 94.937(1) , V 3035.93(6) A° 3, sp. gr. C2/m, Z ¼ 8, Dcalc. 3.767 g/cm3, from Norra Ka¨rr, To¨nko¨ping province, Sweden, has been refined to R1 5.69 % on the basis of 3193 unique reflections (FO . 4sF). Electron microprobe analysis gave (wt%): SiO2 27.56, Nb2O5 0.12, TiO2 18.36, ZrO2 0.51, FeO 23.42, Fe2O3 2.89 [the Fe3þ/Fetot ratio of 0.10(9) was determined by Mo¨ssbauer 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 þ F ¼ 3 apfu), O ¼ F 0.98, total 98.76. The empirical formula is ðBa0:58K0:36Þ 0:94ðNa0:57Ca0:39Þ 0:96 ðFe2þ 2:84Mn0:63Fe3þ 0:32Mg0:10Zr0:04Na0:07Þ 4:00ðTi2:00Nb0:01Þ 2:01 ðSi2O7Þ2O2:12ðOHÞ1:93F1:07, 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 Fe2þ and Mn2þ, with minor Fe3þ, Mg, Zr and Na with ,MO–O. ¼ 2.175A° . Five MO sites give ideally Fe2þ4 pfu. In the H sheet, there are three [6]-coordinated MH sites occupied solely by Ti (Ti ¼ 2 apfu), with ,MH–O. ¼ 1.953 A ° , and four [4]-coordinated Si sites occupied solely by Si, with ,Si–O. ¼ 1.619 A ° . The MH octahedra and (Si2O7) groups constitute the H sheet. Linkage of H and O sheets via common vertices of MH octahedra and (Si2O7) groups with MO(1–5) octahedra results in a TS block. The topology of the TS block is as in Group II of the Ti disilicates (Ti ¼ 2 apfu). There are six interstitial sites, three [9–10]-coordinated Ba-dominant AP sites with ,AP–O. ¼ 2.98 A ° and three [10]-coordinated Nadominant BP sites with ,BP–O. ¼ 2.600 A ° . The total content of three AP sites sums to 1 apfu ¼ Ba0.58 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 ofMH octahedra (as in astrophyllite-group minerals) and AP and BP sites which constitute the I block. Jinshajiangite is an Fe2þ analogue of perraultite, ideally BaNaTi2Mn2þ 4 ðSi2O7Þ2O2ðOHÞ2F, and its crystal structure is topologically identical to that of perraultite.