CNR ExploRA

Articolo in rivista, 2009, ENG

Fluoro-aluminoleakeite, NaNa2(Mg2Al2Li)Si8O22F2, a new mineral of the amphibole group from Norra Kärr, Sweden: description and crystal structure.

Oberti R.; Camara F.; Hawthorne F.; Ball N.

CNR-IGG, Pavia; Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada

Fluoro-aluminoleakeite, ideally ANaBNa2C(Mg2Al2Li)TSi8O22WF2, is a new mineral of the amphibole group from Norra Ka¨rr, Sweden(IMA-CNMMNC 2009-012). It occurs ina proterozoic alkaline intrusion that mainly comprises a fine-grained schistose agpaitic nepheline-syenite (grennaite). Fluoroaluminoleakeite occurs as isolated prismatic crystals 0.102 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 cm3. In plane-polarized light, it is pleochroic, X = pale green, Y = dark green, Z = pale green; X ^ a = 62.9º (in b obtuse), Y || b. Fluoro-aluminoleakeite is biaxial negative, a = 1.632(1), b = 1.638(1), g = 1.643(1); 2Vobs. = 98.0(4)º, 2Vcalc. = 95.5º. Fluoro-aluminoleakeite is monoclinic, space group C2/m, a = 9.7043(5) A ? , b = 17.7341(8) A ? , c = 5.2833(3) A ? , ß = 104.067(4)º, V = 882.0(2) A ? 3, Z = 2. The eight strongest X-ray diffraction lines in the powder-diffractionpatternare [d inA? , (I), (hkl)]: 2.687, (100), (3¯31, 151); 4.435, (80), (021, 040); 3.377, (80), (131); 2.527, (60), (2¯02); 8.342, (50), (110); 3.096, (40), (310); 2.259, (40), (1¯71, 3¯ 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(Na0.65 K0.43)S=1.09 B(Na1.99Ca0.01)S=2.00C(Mg1.77Fe2+0.38Mn0.09Zn0.04Fe3+0.63Al1.16 Ti0.03Li0.90)S=5.00 TSi8.00O22W(F1.47OH0.53)S=2.00. Crystal-structure analysis shows CLi to be completely ordered at the M(3) site, and provided reliable site populations. Fluoro-aluminoleakeite is related to the end-member leakeite, ANaBNa2C(Mg2Fe3+2 Li)TSi8O22W(OH)2, by the substitutions CFe3+ ? CAl and WF ? W(OH).

Mineralogical magazine (Print) 73 , pp. 817–824