Crystal structure and crystal chemistry of fluoro-potassic-magnesioarfvedsonite from Monte Metocha, Xixano region, Mozambique, and discussion of the holotype from Quebec, Canada

Fluoro-potassic-magnesio-arfvedsonite, ideally (super A) K (super B) Na (sub 2) (super C) (Mg (sub 4) Fe (super 3+) ) (super T) Si (sub 8) O (sub 22) (super W) F (sub 2) , has been found in a dyke approximately 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 g cm (super -3) . In plane-polarized light, it is pleochroic, X=pale grey-green, Y=blue-green, Z=pale grey; X c=23.6 degrees (in beta obtuse), Yb, Z c=66.4 degrees (in beta acute). Fluoro-potassic-magnesio-arfvedsonite is biaxial negative, alpha =1.652(2), beta =1.658(2), gamma =1.660(2); 2V (sub obs) =22.5(7) degrees , 2V (sub calc) =30.2 degrees . The unit-cell dimensions are a=9.9591(4), b= 17.9529(7), c=5.2867(2) Aa, beta =104.340(1) degrees , V=919.73(10) Aa (super 3) , Z=2. The nine strongest X-ray diffraction lines in the experimental powder pattern are: [d in Aa(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)(331), 2.549(20)(260), 2.351(10)(351), 2.269(10)(331). Electron microprobe analysis gave: SiO (sub 2) 54.25, Al (sub 2) O (sub 3) 0.03, TiO (sub 2) 1.08, FeO 6.69, Fe (sub 2) O (sub 3) 8.07, MgO 13.99, MnO 0.32, ZnO 0.05, CaO 1.16, Na (sub 2) O 6.33, K (sub 2) O 5.20, F 2.20, H (sub 2) O (sub 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 (super A) K (sub 0.98) (super B) (Na (sub 1.81) Ca (sub 0.18) ) (sub Sigma 1.99) (super C) (Mg (sub 3.07) Fe (super 2+) (sub 0.83) Mn (sub 0.04) Fe (super 3+) (sub 0.90) Ti (sub 0.12) Zn (sub 0.01) ) (sub Sigma =4.98) (super T) Si (sub 8) O (sub 22) (super W) [F (sub 1.03) (OH) (sub 0.73) O (sub 0.24) ] (sub Sigma 2.00) 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 Fe (super 3+) at the T sites, originally suggested for the holotype specimen, is discounted for this amphibole composition.