From structure topology to chemical composition. VI. Titanium silicates: the crystal structure and crystal chemistry of bornemanite, a group III Ti-disilicate mineral

Abstract

The crystal structure of bornemanite, ideally Na 6 &BaTi 2 Nb(Si 2 O 7 ) 2 (PO 4 )O 2 (OH)F, a = 5.4587(3), b = 7.1421(5), c = 24.528(2) Å , a = 96.790(1), b = 96.927(1), g = 90.326(1)º, V = 942.4(2) Å 3 , space group P1, Z = 2, D calc. = 3.342 g cm À3 , from the Lovozero alkaline massif, Kola Peninsula, Russia, has been solved and refined to R 1 = 6.36% on the basis of 4414 unique reflections (F o >4sF). Electron microprobe analysis yielded the empirical formula (Na 6 .0 7 Mn 2 + 0 . 2 3 Ca 0 .0 6 & 0 . 6 4 ) S 7 . 0 0 . 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 (Si 2 O 7 ) groups link to the trans edges of a Ti octahedron of the O sheet. The O sheet cations give Na 3 Ti (4 a.p.f.u.). The TS block has two different H sheets, H 1 and H 2 , where (Si 2 O 7 ) 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 I 1 block is a layer of Ba atoms; the I 2 block consists of Na polyhedra and (PO 4 ) tetrahedra.

Veblenite, K2☐2Na(Fe2+5Fe3+4Mn2+7☐)Nb3Ti(Si2O7)2(Si8O22)2O6(OH)10(H2O)3, a new mineral from Seal Lake, Newfoundland and Labrador: mineral description, crystal structure, and a new veblenite Si8O22ribbon

“…These distances are within the observed distances for Nb and Ti at the M H sites in the H sheet in Ti-disilicates: e.g. epistolite (Sokolova and Hawthorne, 2004), vuonnemite (Ercit et al, 1998), bornemanite (Cámara and Sokolova, 2007), nechelyustovite (Cámara and Sokolova, 2009) and kazanskyite (Cámara et al, 2012).…”

Cámaraite, Ba3NaTi4(Fe2+,Mn)8(Si2O7)4O4(OH,F)7. I. A new Ti-silicate mineral from the Verkhnee Espe Deposit, Akjailyautas Mountains, Kazakhstan

“…The TS block is characterized by a planar cell based on translation vectors, t 1 and t 2 , with t 1~5 .5 and t 2~7 Å and t 1^t2 close to 90º. For cámaraite, we chose the unconventional space group C1 and corresponding unit-cell parameters a = 10.6965 (2 t 1 ) and b = 13.7861(2 t 2 ) for better comparison with the related Group-II structures of bafertisite, jinshajiangite, hejtmanite, perraultite and surkhobite (Table 3) and 22 other minerals with the TS block, detailed structural descriptions of which can be found in Sokolova (2006), Cámara and Sokolova (2007), Sokolova and Hawthorne (2008) and Sokolova et al (2009). Here, we also report the (Niggli) reduced unit cell: a = 8.724 (2) (100) 3.44 (100) 2.726 (100) 3.474 (100) 3.454 (100) 2.79 (90) 2.11 (40) 3.15 (80) 5.472 (32) 10.43 (42) 2.592 (70) 1.721 (70) 1.72 (40) 2.630 (80) 3.241 (21) 2.606 (40) 2.074 (40) 3.39 (50) 2.52 (30) 2.570 (80) 3.215 (21) 3.186 (15) 10.39 (20) 3.18 (50) 2.23 (30) 10.2 (70) 3.455 (17) 2.084 (15) 3.186 (15) 2.101 (50) 2.07 (30) 1.715 (50 broad) 2.180 (16) 2.867 (13) 2.862 (15) 1.578 (50) 1.75 (30) 2.202 (40) 2.535 (14) 3.573 (11) 1.728 (15) Optical class …”

Kazanskyite, Ba & TiNbNa3Ti(Si2O7)2O2(OH)2(H2O)4, a Group-III Ti-disilicate mineral from the Khibiny alkaline massif, Kola Peninsula, Russia: description and crystal structure

“…(PO 4 ) 4 O 4 F 2 (Sokolova and Hawthorne, 2001), polyphite, Na 10 (Na 4 Ca 2 ) 2 Ti 4 (Si 2 O 7 ) 2 (PO 4 ) 6 O 4 F 4 (Sokolova et al, 2005), bornemanite (Cámara and Sokolova, 2007) and nechelyustovite (Cámara and Sokolova, 2009). Consider last the peripheral A P (1,2) sites, with refined site scattering values of 42.9 and 11.2 e.p.f.u., respectively ( Table 9).…”

“…Kazanskyite is the fourth Ti-disilicate mineral [after bornemanite (Cámara and Sokolova, 2007), cá maraite, ideally Ba 3 NaTi 4 (Fe 2 + ,Mn) 8 (Si 2 O 7 ) 4 O 4 (OH,F) 7 (Cámara et al, 2009) and nechelyustovite (Cámara and Sokolova, 2009)] and the third Group-III mineral (after bornemanite and nechelyustovite) with two different types of I block. Kazanskyite is closely related to nechelyustovite ( Fig.…”