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Mineralogical Magazine; December 2007; v. 71; no. 6; p. 593-610; DOI: 10.1180/minmag.2007.071.6.593
© 2007 Mineralogical Society of Great Britain and Ireland
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From structure topology to chemical composition. VI. Titanium silicates: the crystal structure and crystal chemistry of bornemanite, a group III Ti-disilicate mineral

F. Cámara1,* and E. Sokolova1,2,3

1 CNR – Istituto di Geoscienze e Georisorse, Unità di Pavia, Via Ferrata 1, I-27100 Pavia, Italy
2 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
3 Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Moscow, 119017, Russia

* E-mail: camara{at}crystal.unipv.it

The crystal structure of bornemanite, ideally Na6{square}BaTi2Nb(Si2O7)2(PO4)O2(OH)F, a = 5.4587(3), b = 7.1421(5), c = 24.528(2) Å, {alpha} = 96.790(1), β = 96.927(1), {gamma} = 90.326(1)°, V = 942.4(2) Å3, space group PFormula, Z = 2, Dcalc. = 3.342 g cm–3, from the Lovozero alkaline massif, Kola Peninsula, Russia, has been solved and refined to R1 = 6.36% on the basis of 4414 unique reflections (Fo >4{sigma}F). Electron microprobe analysis yielded the empirical formula (Na6.07 Mn0.232+Ca0.06{square}0.64){sum}7.00 (Ba0.73K0.13Sr0.06{square}0.08){sum}1.00(Ti2.05Nb0.80Zr0.02Ta0.015+Fe0.033+Al0.02Mn0.062+Mg0.01){sum}3.00(Si2O7)2(P0.97O4)O2 [F1.27(OH)0.74]{sum}2.01. The crystal structure of bornemanite 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 TS block exhibits linkage and stereochemistry typical for Group III (Ti = 3 a.p.f.u.) of Ti-disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to the trans edges of a Ti octahedron of the O sheet. The O sheet cations give Na3Ti (4 a.p.f.u.). The TS block has two different H sheets, H1 and H2, where (Si2O7) groups link to [5]Ti and [6]Nb polyhedra, and there are two peripheral sites which are occupied by Ba and Na, respectively. There are two I blocks: the I1 block is a layer of Ba atoms; the I2 block consists of Na polyhedra and (PO4) tetrahedra.

KEYWORDS: bornemanite, Ti-disilicate minerals, crystal structure, EMP analysis, chemical formula, TS block, Group III






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