Abstract

Fluoro-aluminoleakeite, ideally ANaB

\(\mathrm{Na}_{2}^{C}\)
(Mg2Al2Li)TSi8
\(\mathrm{O}_{22}^{W}\)
F2, is a new mineral of the amphibole group from Norra Kärr, 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 ^ a = 62.9° (in β obtuse), Y ∥ b. Fluoro-aluminoleakeite is biaxial negative, α = 1.632(1), β = 1.638(1), γ = 1.643(1); 2Vobs. = 98.0(4)°, 2Vcalc. = 95.5°.

Fluoro-aluminoleakeite is monoclinic, space group C2/m, a = 9.7043(5) Å, b = 17.7341(8) Å, c = 5.2833(3) Å, ß = 104.067(4)°, V = 882.0(2) Å3, Z = 2. The eight strongest X-ray diffraction lines in the powder-diffraction pattern are [d in Å, (I), (hkl)]: 2.687, (100), (

\({\bar{3}}\)
31, 151); 4.435, (80), (021, 040); 3.377, (80), (131); 2.527, (60), (
\({\bar{2}}\)
02); 8.342, (50), (110); 3.096, (40), (310); 2.259, (40), (
\({\bar{1}}\)
71,
\({\bar{3}}\)
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(Na0.65K0.43)Σ=1.09B(Na1.99Ca0.01)Σ=2.00C(Mg1.77
\(\mathrm{Fe}_{0.38}^{2+}\)
Mn0.09Zn0.04
\(\mathrm{Fe}_{0.63}^{3+}\)
Al1.16 Ti0.03Li0.90)Σ=5.00TSi8.00O22W(F1.47OH0.53)Σ=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(Mg2
\(\mathrm{Fe}_{2}^{3+}\)
Li)TSi8O22W(OH)2, by the substitutions CFe3+CAl and WF → W(OH).

You do not currently have access to this article.