FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. III. TITANIUM SILICATES: THE CRYSTAL CHEMISTRY OF BARYTOLAMPROPHYLLITE

ABSTRACT Here we report electron-microprobe data and unit-cell parameters for jinshajiangite, ideally NaBaFe2+4Ti2(Si2O7)2O2(OH)2F, from a new locality: the Oktyabr'skii massif in the coastal area of the Azov Sea, Donetsk region, Ukraine. Chemical analysis by electron microprobe gave Nb2O5 1.59, ZrO2 0.61, TiO2 17.07, SiO2 27.60, Al2O3 0.08, Fe2O3 2.04, FeO 16.42, BaO 9.81, ZnO 0.76, MnO 12.97, CaO 1.82, MgO 0.07, K2O 2.05, Na2O 2.51, F 2.48, H2O 1.92, O = F –1.04, sum 98.76 wt.%; H2O was determined in accord with the required number of monovalent anions for the Ti-dominant perraultite-type minerals: OH + F = 3 pfu; the Fe3+/Fe2+ ratio was assigned in accord with Mössbauer-spectroscopy results for jinshajiangite from a different locality. The empirical formula calculated on the basis of 19 (O + F) is (Na0.71Ca0.28□0.01)Σ1(Ba0.56K0.38□0.06)Σ1(Fe2+1.99Mn1.59Fe3+0.22Zn0.08Mg0.02Al0.01□0.09)Σ4 (Ti1.86Nb0.10Zr0.04)Σ2(Si4.00O14)O2[(OH)1.86F0.14]Σ2F1.00, Z = 4. Unit-cell parameters from the single-crystal data were determined by least-squares refinement of 9807 reflections with I > 10σI and are as follows: a = 10.726(8), b = 13.834(10), c = 11.065(8) Å, α = 108.172(5), β = 99.251(7), γ = 90.00(1)°, V = 1537.5(3.4) Å3, space group C .

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From structure topology to chemical composition. XX. Titanium silicates: the crystal structure of hejtmanite, Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, a Group-II TS-block mineral

Mineralogical Magazine ◽

10.1180/minmag.2016.080.026 ◽

2016 ◽

Vol 80 (5) ◽

pp. 841-853 ◽

Cited By ~ 3

Author(s):

E. Sokolova ◽

F. Cámara ◽

F. C. Hawthorne ◽

L. A. Pautov

Keyword(s):

Crystal Structure ◽

Chemical Composition ◽

Structure Refinement ◽

Central Province ◽

Crystal Structure Refinement ◽

Titanium Silicate ◽

Oh Groups ◽

Structure Topology ◽

Titanium Silicates ◽

Group Ii

AbstractThe crystal structure of hejtmanite, Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, from Mbolve Hill, Mkushi River area, Central Province, Zambia (holotype material) has been refined on a twinned crystal toR1= 1.88% on the basis of 4539 [|F| > 4|F|] reflections. Hejtmanite is triclinic,C1̅,a= 10.716(2),b= 13.795(3),c= 11.778 (2) , = 90.07(3), = 112.24(3), = 90.03(3),V= 1612(2)3. Chemical analysis (electron microprobe) gives: Ta2O50.09, Nb2O51.27, ZrO20.65, TiO214.35, SiO223.13, BaO 26.68, SrO 0.19, FeO 11.28, MnO 15.12, Cs2O 0.05, K2O 0.33, F 3.82, H2Ocalc. 1.63, O = F 1.61, total 97.10 wt.%, where the H2O content was calculated from the crystal-structure refinement, with (OH F) = 4 apfu. The empirical formula, calculated on the basis of 20 (O F) anions, is of the form(Si2O7)2(XO)4(XP)2, Z=4: (Ba1.82K0.07Sr0.02)Σ1.91(Mn2.33Zr0.04Mg0.03)Σ3.95(Ti1.88Nb0.10Zr0.02)Σ2(Si2.02O7)2O2[(OH)1.89F0.11]Σ2F2. The crystal structure is a combination of a TS (Titanium Silicate) block and an I (intermediate) block. The TS block consists of HOH sheets (H heteropolyhedral, O octahedral). The topology of the TS block is as in Group-II TS-block minerals: Ti ( Nb) = 2 apfu per (Si2O7)2[as defined by Sokolova (2006)]. In the O sheet, five[6]MOsites are occupied mainly by Mn, less Fe2and minor Zr and Mg, with <MOφ> = 2.198 (φ = O,OH), ideally giving Mn4apfu. In the H sheet, two[6]MHsites are occupied mainly by Ti, with <MHφ> = 1.962 (φ = O,F), ideally giving Ti2apfu; four[4]Sisites are occupied by Si, with < SiO> = 1.625 . The MHoctahedra and Si2O7groups constitute the H sheet. The two[12]Ba-dominant AP(1,2) sites, with <APφ> = 2.984 (φ = O, F), ideally give Ba2apfu. Two(1,2) and two(1,2) sites are occupied by O atoms and OH groups with minor F, respectively, ideally giving (XO)4= ()2()2=O2(OH)2pfu. Two(1,2) sites are occupied by F, giving F2apfu. TS blocks link via a layer of Ba atoms which constitute the I block. Simplified and end-member formulae of hejtmanite are Ba2(Mn,Fe2)4Ti2(Si2O7)2O2(OH,F)2F2and Ba2Mn4Ti2(Si2O7)2O2(OH)2F2,Z= 4. Hejtmanite is a Mn-analogue of bafertisite, Ba24 Ti2(Si2O7)2O2(OH)2F2.

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