Fluoro-sodic-ferropedrizite, NaLi2( Fe22+Al2Li)Si8O22F2, a new mineral of the amphibole group from the Sutlug River, Tuva Republic, Russia: description and crystal structure

Fluoro-sodic-ferropedrizite, ideally (NaLi2C)-Na-A-Li-B(Fe22+Al2Li)(TSi8O22F2)-F-W, is a new mineral of the amphibole group from the Sutlug River, Tuva Republic, Russia. It occurs at the endogenic contact of a Li-pegmatite with country rocks near to a diabase dyke and formed by reaction of the pegmatitic melt with the country rock. Fluoro-sodic-ferropedrizite occurs as prismatic to acicular crystals, ranging in length from 0.1-3 cm and widths of up to 50 mm. Crystals occur inparallel to sub-parallel aggregates up to 5 mm across in a matrix of calcite and plagioclase feldspar. Crystals are pale bluish-grey with a greyish-white streak. Fluoro-sodic-ferropedrizite is brittle, has a Mohs hardness of similar to 6 and a splintery fracture; it is non-fluorescent with perfect {110} cleavage, no observable parting, and has a calculated density of 3.116 g cm(-3). In plane-polarized light, it is pleochroic, X = pale purple-grey, Y = light grey, Z = colourless; X boolean AND a = 71.2 degrees (in beta acute), Y vertical bar vertical bar b, Z boolean AND c = 83.4 degrees (in b obtuse). Fluoro-sodic-ferropedrizite is biaxial positive, alpha = 1.642(1), beta = 1.644(1), gamma = 1.652(1); 2V((obs)) = 68.0(3)degrees, 2V((calc)) = 56.4 degrees. Fluoro-sodic-ferropedrizite is monoclinic, space group C2/m, a = 9.3720(4) angstrom, b = 17.6312(8) angstrom, c = 5.2732(3) angstrom, beta = 102.247(4)degrees, V = 851.5(2) angstrom(3), Z = 2. The strongest ten X-ray diffraction lines in the powder patternare (d in angstrom,(I),(hkl)): 8.146,(10),(110); 2.686,(9),(151); 3.008,(8),(310); 4.430,(7),(021); 2.485,(6),((2) over bar 02); 3.383,(4),(131); 2.876,(3),((1) over bar 51, (3) over bar 11); 2.199,(3),((3) over bar 12); 4.030,(2),(111) and 3.795,(2),((1) over bar 31). Analysis by a combination of electron microprobe and crystal-structure refinement gives SiO2 59.81, Al2O3 12.66, TiO2 0.09, FeO 10.32, MgO 5.56, MnO 0.73, ZnO 0.17, CaO 0.20, Na2O 2.81, Li2O 4.80, F 2.43, H2Ocalc 1.10, sum = 99.65 wt.%. The formula unit, calculated on the basis of 24(O, OH, F) is (A)(Na-0.68)(B)(Li1.92Na0.05Ca0.03)(C)(Fe1.162+Mg1.10Mn0.082+Zn0.02Al1.97Ti0.01Li0.66) (T)(Si7.98Al0.02)O-22(W)(F1.03OH0.97). Crystal-structure refinement shows Li to be completely ordered at the M(3) and M(4) sites. Fluoro-sodic-ferropedrizite, ideally (NaLi2C)-Na-A-Li-B(Fe22+Al2Li)(Si8O22F2)-Si-T-F-W, is related to the theoretical end-member 'sodic-pedrizite', (NaLi2C)-Na-A-Li-B(Mg2Al2Li)(Si8O22W)-Si-T(OH)(2), by the substitutions Fe-C(2+) -> Mg-C and F-W -> (W)(OH).