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

Abstract

The crystal structure of jinshajiangite, ideally BaNaTi 2 Fe 4 2+ (Si 2 O 7 ) 2 O 2 (OH) 2 F, a 10.6785(8), b 13.786(1), c 20.700(2) A, β 94.937(1)°, V 3035.93(6) A 3 , sp. gr. C 2/ 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 > 4σF). Electron microprobe analysis gave (wt%): SiO 2 27.56, Nb 2 O 5 0.12, TiO 2 18.36, ZrO 2 0.51, FeO 23.42, Fe 2 O 3 2.89 [the Fe 3+ /Fe tot ratio of 0.10(9) was determined by Mossbauer spectroscopy], MnO 5.13, MgO 0.44, CaO 2.52, BaO 10.24, K 2 O 1.95, Na 2 O 2.27, F 2.33, H 2 O 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 0.57 Ca 0.39 ) ∑0.96 (Fe 2.84 2+ Mn 0.63 Fe 0.32 3+ Mg 0.10 Zr 0.04 Na 0.07 ) ∑4.00 (Ti 2.00 Nb 0.01 ) ∑2.01 (Si 2 O 7 ) 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 Fe 2+ and Mn 2+ , with minor Fe 3+ , Mg, Zr and Na with O –O> = 2.175 A. Five M O sites give ideally Fe 2+ 4 pfu . In the H sheet, there are three [6]-coordinated M H sites occupied solely by Ti (Ti = 2 apfu ), with H –O>. = 1.953 A, and four [4]-coordinated Si sites occupied solely by Si, with . = 1.619 A. The M H octahedra and (Si 2 O 7 ) groups constitute the H sheet. Linkage of H and O sheets via common vertices of M H octahedra and (Si 2 O 7 ) 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 P –O> = 2.98 A and three [10]-coordinated Na-dominant B P sites with P –O> = 2.600 A. 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 0.57 Ca 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 Fe 2+ analogue of perraultite, ideally BaNaTi 2 Mn 4 2+ (Si 2 O 7 ) 2 O 2 (OH) 2 F, and its crystal structure is topologically identical to that of perraultite.