Crystal structure and crystal chemistry of fluorannite and its relationships to annite

This contribution deals about the crystal chemical characterization of fluorannite from Katugin Ta-Nb deposit, Chitinskaya Oblast’, Kalar Range, Transbaikalia, Eastern-Siberian Region, Russia. The mineral chemical formula is (K0.960Na0.020Ba0.001) (Fe2+2.102Fe3+0.425Cr3+0.002Mg0.039Li0.085Ti0.210Mn0.057) (Al0.674 Si3.326) O10 (F1.060OH0.028O0.912). This mica belongs to 1M polytype (space group C2/m) with layer parameters a = 5.3454(2) Å, b = 9.2607(4) Å, c = 10.2040(5) Å, beta = 100.169(3)°. Structure refinement, using anisotropic displacement parameters, converged at R = 0.0384. When compared to annite, fluorannite shows a smaller cell volume (Vfluorannite = 497.19 Å3; Vannite = 505.71 Å3), because of its smaller lateral dimensions and its reduced c parameter. The flattening of the tetrahedral basal oxygen atoms plane decreases with F content, together with the A-O4 distance (i.e., the distance between interlayer A cation and the octahedral anionic position) because of the reduced repulsion between the interlayer cation and the anion sited in O4

Chlorites crystal chemistry: new data and crystal structure refinements

n

Crystal structure refinement of IIb-4 clinochlore

n

Poppiite, the V3+ end-member of the pumpellyite group: Description and crystal structure

Poppiite, a new mineral from Gambatesa mine (Val Graveglia, Genova, Northern Italy), is the V3+ end-member of the pumpellyite group [([VII])(Ca7.68Na0.27K0.03Rb0.02)(Sigma 8.00)([VI])(V1.263+Fe1.023+Mg0.78Mn0. 2+59Al0.31Cu0.042+)(Sigma 4.00)([VI])(V6.893+Al1.07Ti0.04)(Sigma 8.00)([IV])(Si11.69Al0 (.31))(Sigma 12.00)O-42(OH)(14); C2/m, a = 19.2889(6), b = 6.0444(2), c = 8.8783(3) angstrom, beta = 97.328(2)degrees, V = 1026.66(6) angstrom(3), D-meas = 3.36(2) g/cm(3), and D-calc = 3.44 g/cm(3)]. Poppiite crystals, with size varying from 0.1 to 0.6 mm, are minute, greenish-brown, and prismatic, and are associated with roscoelite, ganophyllite, manganaxinite, goldmanite, and calcite. The strongest lines in the X-ray powder diffraction pattern [d(abs) (angstrom), I-rel, (hkl)] are: 2.930, 100, (511); 3.817, 70, (202); 2.548, 65, ((3) over bar 13); 2.551, 62, (420); 1.612, 57, ((7) over bar 31,424); and 2.367, 51, (222, 403). Poppiite is optically negative, with 2V(calc) = 44 degrees, n(alpha) = 1.768(9), n(beta) 1.804(8), n(gamma) 1.810(9). The pleochroic scheme is a = light yellowish brown, = deep greenish brown, and gamma = brown to reddish brown. The crystal structure was refined using 1918 unique reflections to R = 0.0307. Like the other pumpellyite-group minerals the crystal structure of poppiite consists of chains of edge-sharing octahedra linked by SiO4, Si2O7, and CaO7 polyhedra

Trilithionite-2M2 and Polylithionite-2M2: a Crystal Chemical Study

n

Nb-containing titanite: new data and crystal structure refinement

Nb-containing titanite, Si-[4](1.000) ([6])(Ti-0.748 Al-0.095 Zr-0.045 Nb-0.041 Ta-0.001 Fe-0.067(3+) Sn-0.002 Sb-0.001)(1.000) ([7])(Ca-0.986 Ce-0.004 La-0.001 Mn-0.004 Na-0.005 K-0.001)(1.001) F-0.055 (OH)(0.065) O-4.880, from San Vito quarry, Mt. Somma, Naples, Italy, is characterised by extensive substitutions in the six- and seven-fold coordination sites. Single-crystal X-ray diffraction study indicates this niobian titanite to be monoclinic (a = 6.557(l), b = 8.701 (1), c = 7.077(l) Angstrom, beta = 113.86degrees) with C2/c symmetry. Crystal structure refinement (agreement factor, R = 0.027) suggests that the substitution mechanism (Al, Fe)(3+) + (Nb, Ta)(5+) = 2Ti(4+) is primarily responsible for the incorporation of pentavalent metal ions in the octahedral site

The crystal structure of Roscoelite-1M

n

The octahedral anionic site in micas: a comparison between fluorannite and annite

n