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

R. Oberti*, M. Boiocchi, F. C. Hawthorne, P. Robinson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

9 Citations (Scopus)

Abstract

Fluoro-potassic-magnesio-arfvedsonite, ideally AK BNa 2 C(Mg 4Fe 3+) TSi 8O 22 WF 2, 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 = pale grey; 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) Å 3, 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)(3̄31), 2.549(20)(260), 2.351(10)(3̄51), 2.269(10)(331). Electron microprobe analysis gave: SiO 2 54.25, Al 2O 3 0.03, TiO 2 1.08, FeO 6.69, Fe 2O 3 8.07, MgO 13.99, MnO 0.32, ZnO 0.05, CaO 1.16, Na 2O 6.33, K 2O 5.20, F 2.20, H 2O 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 3.07Fe 2+ 0.83Mn 0.04Al 0.01Fe 3+ 0.90Ti 0.12Zn 0.01) Σ=4.98 TSi 8O 22 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 Fe 3+ at the T sites, originally suggested for the holotype specimen, is discounted for this amphibole composition.

Original languageEnglish
Pages (from-to)951-960
Number of pages10
JournalMineralogical Magazine
Volume74
Issue number6
DOIs
Publication statusPublished - Dec 2010
Externally publishedYes

Keywords

  • crystal-structure refinement
  • electron-microprobe analysis
  • fluoro-potassic-magnesio-arfvedsonite
  • Mount Metocha
  • Mozambique
  • optical properties
  • Xixano region

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