Brattforsite, Mn19(AsO3)12Cl2, a new arsenite mineral related to magnussonite, from Brattforsgruvan, Nordmark, Värmland, Sweden
| Autor: | Ulf Hålenius, Cristian Biagioni, Dan Holtstam |
|---|---|
| Jazyk: | angličtina |
| Rok vydání: | 2021 |
| Předmět: |
Materials science
010504 meteorology & atmospheric sciences Analytical chemistry Crystal structure Magnussonite engineering.material 010502 geochemistry & geophysics 01 natural sciences Skarn symbols.namesake Geochemistry and Petrology Formula unit Pleochroism 0105 earth and related environmental sciences Sweden New mineral Cleavage (crystal) Geokemi Geochemistry Geophysics Nordmark Jacobsite engineering symbols Phlogopite Raman spectroscopy Arsenite Brattforsite Monoclinic crystal system |
| Popis: | Brattforsite is an approved mineral (IMA2019-127), with ideal formula Mn19(AsO3)12Cl2. Associated minerals in the type specimen from the Brattfors mine, Nordmark (Värmland, Sweden) include jacobsite, alleghanyite, phlogopite, calcite and dolomite. Brattforsite, forming subhedral, mostly equant crystals up to 0.5 mm across, is orange to reddish-brown with a white streak, and translucent with a resinous to vitreous lustre. The fracture is uneven to subconchoidal, and no cleavage is observed. It is very weakly pleochroic in yellow, optically biaxial (–) with 2V = 44(5)° and has calculated mean refractive index of 1.981. Measured and calculated density values are 4.49(1) and 4.54(1) g·cm− 3, respectively. Chemical analyses yields (in wt%): MgO 0.62, CaO 1.26, MnO 48.66, FeO 0.13, As2O3 46.72, Cl 2.61, H2Ocalc 0.07, O ≡ Cl –0.59, sum 99.49, corresponding to the empirical formula (Mn17.67Ca0.58Mg0.40Fe0.05)∑18.70As12.17O35.90Cl1.90(OH)0.20, based on 38 (O + Cl + OH) atoms per formula unit. The five strongest Bragg peaks in the powder X-ray diffraction pattern are [d (Å), I (%), (hkl)]: 2.843,100, ($$ \overline{4} $$ 4 - 44); 2.828, 99, (444); 1.731, 32, (880); 2.448, 28, (800); 1.739, 25, (088). Brattforsite is monoclinic and pseudotetragonal, space group I2/a, with unit-cell parameters a = 19.5806(7), b = 19.5763(7), c = 19.7595(7) Å, β = 90.393(3)°, V = 7573.9(5) Å3 and Z = 8. The crystal structure was solved and refined to an R1 index of 3.4 % for 7445 reflections [Fo > 4σ(Fo)]. Brattforsite has the same overall structural topology as magnussonite (i.e., the species can be considered as homeotypic), but with 12 independent tetrahedrally coordinated As sites and 21 Mn sites with varying (4–8) coordination. The Mn-centered polyhedra, bonded through edge- and face-sharing, give rise to a three-dimensional framework. The (AsO3)3− groups are bonded to this framework through corner- and edge-sharing. Spectroscopic measurements (optical absorption, Raman, FTIR) carried out support the interpretation of the compositional and structural data. |
| Databáze: | OpenAIRE |
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