THE CRYSTAL CHEMISTRY OF EPISTOLITE

• Main Authors: Elena Sokolova, Frank, C. Hawthorne
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Greenland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.561.2203; http://rruff.info/doclib/cm/vol42/CM42_797.pdf

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), 96.01(3), 89.98(3)°, V 455.4(5) Å3, space group P¯1, 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 ≥ 4F | 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 Ti4+0.04 Fe3+0.04)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 prima-rily 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 = 2.39 Å, and the A(3) site is [8]-coordinated, with = 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