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

Abstract

Magnesiobeltrandoite-2N3S, ideally Mg6Al₂₀Fe₂³⁺O₃₈(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-2N3S 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 (∼50–400μm), 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 · cm^-3. 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 cm^-1. Electron microprobe analysis combined with Synchrotron Mössbauer source spectrometry yielded the following empirical formula based on 40 anions per formula unit (pfu) [Al_18.36Mg_3.96Fe²⁺_2:52Fe³⁺_2:08Ti_0.56Cr_0.40Zn_0.06V³⁺_0:03Mn_0.02]_Σ28O₃₈(OH)₂. The ideal formula is Mg₆Al₂₀Fe₂³⁺O₃₈(OH)₂. The eight strongest lines in the X-ray powder diffraction pattern are [d_obs/A (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 [P3^-m1, a = 5.7226(3), c = 23.0231(9)Å, V = 652.95(5)ų] was refined from X-ray single-crystal data to R1 = 0.022; it is isostructural with magnesiohögbomite-2N3S.