• journal_title：Mineralogical Magazine
• Contributor：R. Oberti ; M. Boiocchi ; F. C. Hawthorne ; P. Robinson
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2010.074.6.951
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：74
• issue：6
• firstpage：951
• section：Articles

Fluoro-potassic-magnesio-arfvedsonite, ideally ^AK^BNa₂^C(Mg₄Fe³⁺)^TSi₈O₂₂^WF₂, has been found in a dyke ∼25 km southwest of Monte Metocha, Xixano region, northeastern Mozambique. Fluoro-potassic-magnesio-arfvedsonite and low sanidine form a fine-grained mafic, ultrapotassic, peralkaline igneous rock without visible phenocrysts. The amphibole is brittle, has a Mohs hardness of 6 and a splintery fracture; it is non-fluorescent with perfect {110} cleavage and no observable parting, and has a calculated density of 3.174 gcm⁻³. In plane-polarized light, it is pleochroic, X = pale grey-green, Y = blue-green, Z = palegrey; X ^ c = 23.6° (in β obtuse), Y || b, Z ^ c = 66.4° (in β acute). Fluoro-potassic-magnesio-arfvedsonite is biaxial negative, α = 1.652(2), β = 1.658(2), γ = 1.660(2); 2V_obs = 22.5(7)°, 2V_calc = 30.2°. The unit-cell dimensions are a = 9.9591(4), b = 17.9529(7), c = 5.2867(2) Å, β = 104.340(1)°, V = 919.73(10) ų, Z = 2. The nine strongest X-ray diffraction lines in the experimental powder pattern are: [d in Å(I)(hkl)]: 2.716(100)(151), 3.410(70)(131), 8.475(50)(110), 3.178(50)(310), 3.309(30)(240), 2.762(20)(31), 2.549(20)(260), 2.351(10)(51), 2.269(10)(331). Electron microprobe analysis gave: SiO₂ 54.25, Al₂O₃ 0.03, TiO₂ 1.08, FeO 6.69, Fe₂O₃ 8.07, MgO 13.99, MnO 0.32, ZnO 0.05, CaO 1.16, Na₂O 6.33, K₂O 5.20, F 2.20, H₂O_calc 0.74, sum 99.18 wt.%. The formula unit, calculated on the basis of 24 (O,OH,F) with (OH+F) = 2–(2 Ti), is ^AK_0.98^B(Na_1.81Ca_0.18)_Σ1.99^C(Mg_3.07Mn_0.04Ti_0.12Zn_0.01)_Σ=4.98 ^TSi₈O₂₂^W[F_1.03(OH)_0.73O_0.24]_Σ2.00 and confirms the usual pattern of cation order in the amphibole structure. The presence of a significant oxo component (locally balanced by Ti at the M(1) site) is related to the crystallization conditions. The presence of Fe³⁺ at the T sites, originally suggested for the holotype specimen, is discounted for this amphibole composition.