Crystal structure and crystal chemistry of fluoro-potassic-magnesio-arfvedsonite from Monte Metocha, Xixano region, Mozambique, and discussion of the holotype from Quebec, Canada
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Bibliographic record
Abstract
Abstract Fluoro-potassic-magnesio-arfvedsonite, ideally A K B Na 2 c (Mg 4 Fe 3+ ) T Si 8 O 22 w F 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)( 31), 2.549(20)(260), 2.351(10)( 51), 2.269(10)(331). Electron microprobe analysis gave: SiO 2 54.25, A1 2 O 3 0.03, TiO 2 1.08, FeO 6.69, Fe 2 O 3 8.07, MgO 13.99, MnO 0.32, ZnO 0.05, CaO 1.16, Na 2 O 6.33, K 2 O 5.20, F 2.20, H 2 O 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 A Ka 0.98 B (Na 1.18 Ca 0.18 ) ∑1.99 C (Mg 3.07 Fe 0.83 2+ Mn 0.04 Al 0.01 Fe 0.90 3+ Ti 0.12 Zn 0.01 ) ∑=4.98 T Si 8 O 22 W [Fi.o 3 (OH) 0.73 O 0.24 ] ∑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 3+ at the T sites, originally suggested for the holotype specimen, is discounted for this amphibole composition.
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Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
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Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it