Fluoroleakeite, NaNa2(Mg2Fe3+2Li)Si8O22F2, is a new mineral of the amphibole group from the Verkhnee Espe deposit, Akjailyautas mountains, eastern Kazakhstan district, Kazakhstan. The granites and their host rocks have been intensely reworked by post-magmatic and host-rock fluids, resulting in intense recrystallization, enrichment in F, Li and rare elements, and replacement of primary biotite and sodic-calcic amphiboles by Li-bearing riebeckite, aegirine, astrophyllite and other sodic minerals including fluoroleakeite. Crystals are prismatic parallel to  with 100 and 110 faces and cleavage surfaces, and the prism direction is terminated by irregular fractures. Grains are up to 3 mm long, and occur as isolated crystals, as small aggregates, and as inclusions in cámaraite. Crystals are black with a very pale grey to colourless streak. Fluoroleakeite is brittle, has a Mohs hardness of 6 and a splintery fracture; it is non-fluorescent with perfect 110 cleavage, no observable parting, and has a calculated density of 3.245 g cm-3. In plane-polarized light, it is pleochroic, X = pale grey-green, Y = medium grey, Z = grey-brown; Xa = 14.1° (in β obtuse), Y b, Zc = 75.9° (in β acute). Fluoroleakeite is biaxial negative, α = 1.663(2), β = 1.673(2), γ = 1.680(2); 2Vobs = 80.9(6)°, 2Vcalc = 79.4° Fluoro-leakeite is monoclinic, space group C2/m, a = 9.8927(3), b = 17.9257(6), c = 5.2969(2) Å, β = 103.990(1)°, V = 905.7(1) Å3, Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d in Å (I)(hkl)]: 2.718(100)(151), 8.434(40)(110), 4.464(30)(021), 3.405(30)(131), 3.137(20)(310), 2.541(20)(202), 2.166(20)(261), 2.325(15)(351), 2.275(15)(312) and 2.806(10)(330). Analysis by a combination of electron microprobe and crystal-structure refinement gives SiO2 53.34, Al2O 3 0.62, TiO2 1.27, V2O3 0.05, Fe2O3 15.10, FeO 6.00, MnO 2.04, ZnO 0.18, MgO 6.40, CaO 0.13, Na2O 9.08, K2O 1.98, Li2O 1.10, F 3.33, H2Ocalc 0.16, sum 99.39 wt.%. The formula unit, calculated on the basis of 23 O, is A(Na0.64K0.38) (Na1.98Ca0.02)(Li0.66Mg1.42Fe 2+0.75Mn2+0.26Zn 0.02Fe3+1.69V3+0.01Ti4+0.14Al0.03)(Si 7.93Al0.07)O22(F1.57OH 0.16O0.27). Crystal-structure refinement shows Li to be completely ordered at the M(3) site. Fluoroleakeite, ideally NaNa 2(Mg2Fe3+2Li)Si8O 22F2, is related to end-member leakeite, NaNa 2(Mg2Fe3+2Li)Si8O 22(OH)2 by the substitution F → (OH).
|Titolo:||Fluoroleakeite, NaNa2(Mg2Fe3+2 Li)Si8O22F2, a new mineral of the amphibole group from the Verkhnee Espe deposit, Akjailyautas Mountains, Eastern Kazakhstan District, Kazakhstan : description and crystal structure|
CAMARA ARTIGAS, FERNANDO (Corresponding)
|Parole Chiave:||crystalstructure refinement; electron-microprobe analysis; fluoroleakeite; Kazakhstan; leakeite; new amphibole; optical properties; Verkhnee Espe deposit|
|Settore Scientifico Disciplinare:||Settore GEO/06 - Mineralogia|
|Data di pubblicazione:||giu-2010|
|Digital Object Identifier (DOI):||10.1180/minmag.2010.074.3.521|
|Appare nelle tipologie:||01 - Articolo su periodico|