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

Authors: Oberti, R.; Cámara, F.; Hawthorne, F.C.; Ball, N.A.

Source: Mineralogical Magazine, Volume 73, Number 5, October 2009 , pp. 817-824(8)

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Abstract:

Fluoro-aluminoleakeite, ideally ^ANa^BNa₂^C(Mg₂Al₂Li)^TSi₈O₂₂^WF₂, is a new mineral of the amphibole group from Norra Kärr, Sweden (IMA-CNMMNC 2009-012). It occurs ina 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); 2V_obs. = 98.0(4)°, 2V_calc. = 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), (331, 151); 4.435, (80), (021, 040); 3.377, (80), (131); 2.527, (60), (202); 8.342, (50), (110); 3.096, (40), (310); 2.259, (40), (171, 312) 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, H₂O_calc. 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(Na_0.65K_0.43)_Σ=1.09^B(Na_1.99Ca_0.01)_Σ=2.00 _C(Mg_1.77Fe²⁺_0.38Mn_0.09Zn_0.04Fe³⁺_0.63Al_1.16 Ti_0.03Li_0.90)_Σ=5.00 ^TSi_8.00O₂₂^W(F_1.47OH_0.53)_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, ^ANa^BNa₂^C(Mg₂Fe³⁺₂Li)^TSi₈O₂₂^W(OH)₂, by the substitutions ^CFe³⁺ → ^CAl and ^WF → ^W(OH).

Keywords: FLUORO-ALUMINOLEAKEITE; NEW AMPHIBOLE; LITHIUM; ELECTRON MICROPROBE ANALYSIS; OPTICAL PROPERTIES; CRYSTAL-STRUCTURE REFINEMENT; NORRA KARR; SWEDEN

Document Type: Research article

DOI: http://dx.doi.org/10.1180/minmag.2009.073.5.817

Publication date: 2009-10-01

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