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 O₂(OH)₂F, a 10.6785(8), b 13.786(1), c 20.700(2) Å, β 94.937(1)°, V 3035.93(6) ų, 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 (F_O > 4σF). 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 Mössbauer 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 (Ba_0.58 K_0.36) _∑0.94(Na_0.57Ca_0.39)_∑0.96 (Fe_2.84²⁺Mn_0.63Fe_0.32³⁺Mg_0.10Zr_0.04Na_0.07)_∑4.00(Ti_2.00Nb_0.01)_∑2.01 (Si₂O₇)₂O_2.12(OH)_1.93F_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 Å. Five M^O sites give ideally Fe²⁺₄pfu. 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 Å, and four [4]-coordinated Si sites occupied solely by Si, with <Si–O>. = 1.619 Å. 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 Å and three [10]-coordinated Na-dominant B^P sites with <B^P–O> = 2.600 Å. The total content of three A^P sites sums to ~1 apfu = Ba_0.58 K_0.36 or ideally 1 Ba pfu. The total content of the three B^P sites is Na_0.57Ca_0.39 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)₂O₂(OH)₂F, and its crystal structure is topologically identical to that of perraultite.