Fluoro-aluminoleakeite, Na Na2 (Mg2Al2Li) Si8O22 F2, a new mineral of the amphibole group from Norra Kärr, Sweden: Description and crystal structure

Fluoro-aluminoleakeite, ideally (NaNa2C)-Na-A-Na-B(Mg2Al2Li)(Si8O22F2)-Si-T-F-W, is a new mineral of the amphibole group from Norra Karr, Sweden(IMA-CNMMNC 2009-012). It occurs in a proterozoic alkaline intrusion that mainly comprises a fine-grained schistose agpaitic nepheline-syenite (grennaite). Fluoro-aluminoleakeite occurs as isolated prismatic crystals 0.10-2 mm long in a syenitic matrix. Crystals are light greenish-blue with a greenish-blue streak. It 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 3.14 g cm(-3). In plane-polarized light, it is pleochroic, X = pale green, Y = dark green, Z = pale green; X boolean AND a = 62.9 (in beta obtuse), Y parallel to b. Fluoro-aluminoleakeite is biaxial negative, alpha = 1.632(1), beta = 1.638(1), gamma = 1.643(1); 2V(obs.) = 98.0(4)degrees, 2V(calc.) = 95.5 degrees. Fluoro-aluminoleakeite is monoclinic, space group C2/m, a = 9.7043(5) angstrom, b = 17.7341(8) angstrom, c = 5.2833(3) angstrom, beta = 104.067(4)degrees, V = 882.0(2) angstrom(3), Z = 2. The eight strongest X-ray diffraction lines in the powder-diffraction pattern are [d in angstrom, (I), (hkl)]: 2.687, (100), ((3) over bar 31, 151); 4.435, (80), (021, 040); 3.377, (80), (131); 2.527, (60), ((2) over bar 02); 8.342, (50), (110); 3.096, (40), (310); 2.259, (40), ((1) over bar 71, (3) over bar 12) and 2.557, (30), (002, 061). Analysis, by a combination of electron microprobe and crystal-structure refinement, gives SiO2 58.61, Al2O3 7.06, TiO2 0.32, FeO 3.27, Fe2O3 6.05, MgO 8.61, MnO 0.73, ZnO 0.43, CaO 0.05, Na2O 9.90, K2O 2.43, Li2O 1.62, F 3.37, H2Ocalc. 0.50, sum 101.08 wt.%. The formula unit, calculated on the basis of 24 (O, OH, F, Cl) p.f.u. with (OH) + F = 2 a. p.f.u., is (A)(Na0.65K0.43)(Sigma=1.09)(B)(Na1.99Ca0.01)(Sigma=2.00)(C)(Mg1.77Fe0.382+Mn0.09Zn0.04Fe0.633+Al1.16 Ti0.03Li0.90)(Sigma=5.00)(Si8.00O22W)-Si-T(F1.47OH0.53)(Sigma=2.00). Crystal-structure analysis shows Li-C to be completely ordered at the M(3) site, and provided reliable site populations. Fluoro-aluminoleakeite is related to the end-member leakeite, (NaNa2C)-Na-A-Na-B(Mg2Fe23+Li)(Si8O22W)-Si-T(OH)(2), by the substitutions Fe-C(3+) --> Al-C and F-W --> (W)(OH).