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Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
E-mail address: elena_sokolova{at}umanitoba.ca
The crystal structure of epistolite, ideally Na4 Nb2 Ti4+ (Si2O7)2 O2 (OH)2 (H2O)4, a 5.460(1), b 7.170(1), c 12.041(2) Å, 
103.63(3), ß 96.01(3), 
89.98(3)°, V 455.4(5) Å3, space group P
, Z = 1, Dcalc 2.987 g.cm–3 from the Ilímaussaq alkaline complex, South Greenland, has been solved by direct methods and refined to R1 9.8% on the basis of 1986 unique reflections |Fo
4
F| collected on a Bruker P4 diffractometer with a CCD 1K Smart detector and MoK radiation. An electron-microprobe analysis gave (wt.%): SiO2 29.59, Nb2O5 31.43, TiO2 10.24, MnO 0.34, Fe2O3 0.35, Ta2O5 0.28, Na2O 14.45, CaO 1.89 K2O 0.27, P2O5 0.36, F 1.32, H2O (calculated from structure refinement) 10.46, total 100.43. The empirical formula is (Na3.79 Ca0.27 Mn0.04)
4.06 (Nb1.92 Ti0.04 4+ Fe0.04 3+)2.00 Ti4+ (Si2O7)2 O2 (OH1.44 F0.56) (H2O), calculated on the basis of 4 Si (apfu). In the crystal structure, there are two tetrahedrally coordinated sites occupied by Si, with a grand <Si–O> distance of 1.617 Å. The (SiO4) tetrahedra link together to form [Si2O7] groups. There are two octahedrally coordinated M sites. The M(1) site is occupied primarily by Nb with minor amounts of Fe3+ and Ti4+, with <M(1)–O, H2O> = 2.01 Å. The M(2) site is occupied solely by Ti4+, with <M(2)–O,OH> = 1.962 Å. There are three A sites, occupied primarily by Na: the A(1) and A(2) sites are octahedrally coordinated, with <A(1,2)–O,OH> = 2.39 Å, and the A(3) site is [8]-coordinated, with <A(3)–O,OH,H2O> = 2.57 Å. The A(1) site is occupied by Na, the A(2) site is occupied 92% by Na, and the A(3) site is approximately half-occupied by Na: 0.87 Na + 0.82 
+ 0.27 Ca + 0.04 Mn2+ apfu. The M(2) and A(1,2) octahedra each share six common edges to form a close-packed sheet. This sheet of octahedra is the central part of a TS (titanium-silicate) block. Two adjacent sheets of heteropolyhedra consist of [Si2O7] groups and M(1) octahedra with large hexagonal voids that incorporate [8]-coordinated A(3) polyhedra. A sheet of heteropolyhedra is connected to a sheet of octahedra through vertices of (SiO4) tetrahedra, M(1) octahedra and A(3) polyhedra. Within one TS block in epistolite, two [Si2O7] groups, one from each sheet of heteropolyhedra, link to the M(2) octahedron of the central sheet, and sheets of heteropolyhedra are approximately related by a pseudo-mirror plane, mz. The TS blocks repeat along (001) and are connected through hydrogen bonds involving (H2O) groups and acceptor O atoms of the TS blocks. Previously, the crystal structure of murmanite, ideally Na4 Ti4 4+ (Si2O7)2 O4 (H2O)4, a 5.383(4), b 7.053(4), c 12.170(3) Å, 93.16(2), ß 107.82(2), 90.06(2)°, V 439.14 Å3, space group P1, Z = 1, Dcalc 2.86 g cm–3, has been considered as a topological analogue of epistolite. It has a TS block as a fundamental building block, but its bond topology is different from that in epistolite. This difference results from different linkages between the central sheet of octahedra and the two adjacent sheets of heteropolyhedra. In murmanite, sheets of heteropolyhedra are shifted relative to each other in the (001) plane. Thus epistolite and murmanite are not isostructural. Murmanite and epistolite are related by the substitution Ti3 4+ + O2 2– 
Nb2 5+ + + (OH)2 –.
Keywords: epistolite, crystal structure, disorder, murmanite, Ilímaussaq alkaline complex, South Greenland.
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