Magnesiobeltrandoite-2 N 3 S, ideally Mg6 Al20 Fe3+
2O38(OH) 2, is a new member of the högbomite supergroup of minerals. It occurs in magnesian chloritites of a metamorphosed layered mafic complex in
the Etirol-Levaz continental slice, middle Valtournenche, Aosta Valley, Italy. Magnesiobeltrandoite-2 N 3 S grows in a fine-grained chlorite matrix associated as inclusions to relict pre-Alpine hercynite spinels and dolomite in cm- to dm-long darker boudins, which are cut by
corundum + clinochlore ± dolomite veins. It occurs as subhedral to euhedral black crystals (~50 – 400μ m), dark reddish-brown in thin section. It shows dark brown streak and vitreous lustre. It is brittle, with no cleavage observed and uneven fracture. Mohs hardness ≈ 6–6½. D
calc = 3.93 g ·cm3. It shows no fluorescence under UV radiation and no cathodoluminescence. The mineral is optically uniaxial (–) with an estimated mean refractive index of ca. 1.80. Pleochroism is weak with ε = deep reddish brown (along
c axis) and ω = reddish brown (c). Absorption is E> O. The Raman spectrum shows a weak and strongly polarized broad OH-characteristic absorption centred at 3364 cm1. Electron microprobe analysis combined with Synchrotron Mössbauer source spectrometry
yielded the following empirical formula based on 40 anions per formula unit (pfu) [Al18.36 Mg3.96 Fe2+
O38 (OH)2. The ideal formula is Mg6 Al20 Fe3+
2O38(OH) 2. The eight strongest lines in the X-ray powder diffraction pattern are [d
obs/Å (I) (h k l)]: 2.858 (42) (1 1
0), 2.735 (51) (1 0 7), 2.484 (46) (0 1 8), 2.427 (100) (1 1 5), 1.568 (29) (1 2 8), 1.514 (30) (0 2 12), 1.438 (42) (2 0 13), and 1.429 (72) (2 2 0). The crystal structure of magnesiobeltrandoite-2N3S[P
m1, a = 5.7226(3), c = 23.0231(9)
Å, V = 652.95(5) Å3] was refined from X-ray single-crystal data to R 1 = 0.022; it is isostructural with magnesiohögbomite-2 N 3 S.
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ELECTRON MICROPROBE ANALYSIS;
MAGNESIOBELTRANDOITE-2 N 3 S;
Document Type: Research Article
Publication date: June 1, 2018
This article was made available online on September 22, 2017 as a Fast Track article with title: "Beltrandoite, a new root-name in the högbomite supergroup: the Mg end-member magnesiobeltrandoite-2N3S".
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