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TitleBeltrandoite, a new root-name in the hogbomite supergroup: the Mg end-member magnesiobeltrandoite-2N3S
AuthorCamara, FORCID logo; Cossio, R; Regis, D; Cerantola, V; Ciriotti, M E; Compagnoni, R
SourceEuropean Journal of Mineralogy vol. 30, 3, 2018 p. 545-558, https://doi.org/10.1127/ejm/2017/0029-2692
Image
Year2018
Alt SeriesNatural Resources Canada, Contribution Series 20200189
PublisherE Schweizerbartsche Verlagsbuchhandlung
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
AreaValtournenche; Aosta Valley; Italy
Lat/Long WENS 7.5000 7.6000 45.8333 45.7500
Subjectsmineralogy; Nature and Environment; Science and Technology; minerals; morphology, crystal; crystallinity; electron probe analyses; electron microscope analyses
Illustrationslocation maps; tables; spectrograms; spectra; sections; digital images; distribution diagrams; structure stereograms
Released2017 09 22
AbstractMagnesiobeltrandoite-2N3S, ideally Mg6Al20Fe23+O38(OH)(2), is a new member of the hogbomite 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 (similar to 50-400 mu 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 approximate to 6-61/2. D-calc = 3.93 g.cm(-3) . It shows no fluorescence under LTV radiation and no cathodoluminescence. The mineral is optically uniaxial (-) with an estimated mean refractive index of ca. 1.80. Pleochroism is weak with epsilon = deep reddish brown (along c axis) and omega = reddish brown (perpendicular to 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 Mossbauer source spectrometry yielded the following empirical formula based on 40 anions per formula unit (pfu) [Al18.36Mg3.96Fe2.522+Fe2.083+Ti0.56Cr0.40Zn0.06V0.033+Mn0.02](Sigma 28)O-38(OH)(2). The ideal formula is Mg6Al20Fe23+O38(OH)(2).The eight strongest lines in the X-ray powder diffraction pattern are [d(obs)/angstrom (l) (h k l)]: 2.858 (42) (110), 2.735 (51) (107), 2.484 (46) (018), 2.427 (100) (1 15), 1.568 (29) (128), 1.514 (30) (0212), 1.438 (42) (2013), and 1.429 (72) (220). The crystal structure of magnesiobeltrandoite-2N3S [P (3) over bar ml, a = 5.7226(3), c = 23.0231(9) angstrom, V = 652.95(5) angstrom(3)] was refined from X-ray single-crystal data to RI = 0.022; it is isostructural with magnesiohogbomite-2N3S.
GEOSCAN ID326601

 
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