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Beltrandoite, a new root-name in the högbomite supergroup: the Mg end-member magnesiobeltrandoite-2N3S

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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+ 2.52Fe3+ 2.08Ti0.56Cr0.40Zn0.06V3+ 0.03Mn0.02]σ28 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 3 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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Keywords: AOSTA VALLEY; CHEMICAL FORMULA; CRYSTAL-STRUCTURE REFINEMENT; ELECTRON MICROPROBE ANALYSIS; HÖGBOMITE SUPERGROUP; ITALY; MAGNESIOBELTRANDOITE-2 N 3 S; NEW MINERAL; VALTOURNENCHE

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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  • The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered.
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