Original paper
From structure topology to chemical composition. VII. Titanium silicates: the crystal structure and crystal chemistry of jinshajiangite
Sokolova, Elena Cámara; Hawthorne, Frank C.; Abdu, Yassir
European Journal of Mineralogy Volume 21 Number 4 (2009), p. 871 - 883
published: Aug 31, 2009
Abstract
The crystal structure of jinshajiangite, ideally BaNaTi2Fe2+4 (Si2O7)2O2(OH)2F, a 10.6785(8), b 13.786(1), c 20.700(2) Å, β94.937(1) , V 3035.93(6) Å 3, sp. gr. C2/ m, Z = 8, D calc. 3.767g/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, Nb2 O5 0.12, TiO2 18.36, ZrO2 0.51, FeO 23.42, Fe2 O3 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, K2 O 1.95, Na2 O 2.27, F 2.33, H2 O 2.00 (calc. from structure refinement: OH + F = 3 apfu), O = F 0.98, total 98.76. The empirical formula is (Ba0.58K0.36)Σ 0.94 (Na0.57Ca0.39)Σ0.96 (Fe2+2.84 Mn0.63 Fe3+ 0.32Mg0.10Zr0.04Na0.07)Σ4.00(Ti2.00Nb0.01)Σ2.01 (Si2O7)2O2.12(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 = 2.175 Å . Five MO sites give ideally Fe2+4pfu. In the H sheet, there are three [6]-coordinated MH sites occupied solely by Ti (Ti = 2 apfu), with 1.953 Å, and four [4]-coordinated Si sites occupied solely by Si, with >Si-O< = 1.619 å . the mhoctahedra and (si2o7) groups constitute the h sheet. linkage of h and o sheets via common vertices of mh octahedra and (si2 o7) 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 = 2apfu). there are six interstitial sites, three [9-10]-coordinated ba-dominant ap sites with 2.98 Å and three [10]-coordinated Nadominant BP sites with = 2.600 Å. The total content of three AP sites sums to 1apfu= Ba0.58 K0.36 or ideally 1 Ba pfu. The total content of the three BP sites is Na0.57 Ca0.39 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 BaNaTi2Mn2+4(Si2O7)2O2(OH)2F, and its crystal structure is topologically identical to that of perraultite.
Keywords
jinshajiangite • structure refinement • mö • ssbauer spectroscopy • emp analysis • titanium silicate • structure topology • chemical formula