Magnesiobeltrandoite-2N3S, ideally Mg6Al20O38(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 (∼50–400 µm), 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. Dcalc = 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) [Al18.36Mg3.96Ti0.56Cr0.40Zn0.06Mn0.02]Σ28O38(OH)2. The ideal formula is Mg6Al20O38(OH)2.The eight strongest lines in the X-ray powder diffraction pattern are [dobs/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 [, a = 5.7226(3), c = 23.0231(9) Å, V = 652.95(5) Å3] was refined from X-ray single-crystal data to R1 = 0.022; it is isostructural with magnesiohögbomite-2N3S.
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Research Article|
May 01, 2018
Beltrandoite, a new root-name in the högbomite supergroup: the Mg end-member magnesiobeltrandoite-2N3S
Fernando Cámara;
1
Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Luigi Mangiagalli 34, 10125 Torino
, Italy
Corresponding author, e-mail: fernando.camara@unimi.it
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Roberto Cossio;
Roberto Cossio
2
Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, 10125 Torino
, Italy
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Daniele Regis;
Daniele Regis
3
Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8
, Canada
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Valerio Cerantola;
Valerio Cerantola
4
European Synchrotron Radiation Facility, Division of Experiments, 38043 Grenoble Cedex 9
, France
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Marco E. Ciriotti;
Marco E. Ciriotti
5
Associazione Micromineralogica Italiana, via San Pietro 55, 10073 Devesi-Cirié
, Italy
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Roberto Compagnoni
Roberto Compagnoni
2
Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, 10125 Torino
, Italy
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European Journal of Mineralogy (2018) 30 (3): 545–558.
Article history
received:
01 Jun 2017
rev-recd:
03 Jul 2017
accepted:
29 Aug 2017
first online:
08 Jun 2018
Citation
Fernando Cámara, Roberto Cossio, Daniele Regis, Valerio Cerantola, Marco E. Ciriotti, Roberto Compagnoni; Beltrandoite, a new root-name in the högbomite supergroup: the Mg end-member magnesiobeltrandoite-2N3S. European Journal of Mineralogy 2018;; 30 (3): 545–558. doi: https://doi.org/10.1127/ejm/2017/0029-2692
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Index Terms/Descriptors
- crystal structure
- electron probe data
- Europe
- ferric iron
- ferrous iron
- formula
- hydroxides
- iron
- Italy
- metals
- Mossbauer spectra
- new minerals
- nomenclature
- optical properties
- oxides
- physical properties
- Raman spectra
- refinement
- Southern Europe
- spectra
- Valle d'Aosta Italy
- hogbomite supergroup
- magnesiobeltrandoite
- beltrandoite
- Valtourneche
- Marmore Stream
Latitude & Longitude