Beltrandoite, a new root-name in the hogbomite supergroup; the Mg end-member magnesiobeltrandoite-2N3S
Beltrandoite, a new root-name in the hogbomite supergroup; the Mg end-member magnesiobeltrandoite-2N3S
European Journal of Mineralogy (September 2017) 30 (3): 545-558
- 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
- Marmore Stream
- Valtourneche
- beltrandoite
Magnesiobeltrandoite-2N3S, ideally Mg6Al (sub 20) Fe (sub 2) (super 3+) O (sub 38) (OH) (sub 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 + or - dolomite veins. It occurs as subhedral to euhedral black crystals ( approximately 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 approximately 6-61/2. D (sub calc) = 3.93 g . cm (super -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 epsilon = deep reddish brown (along c axis) and omega = reddish brown (perpendicular to c). Absorp> is E > O. The Raman spectrum shows a weak and strongly polarized broad OH-characteristic absorption centered at 3364 cm (super -1) . Electron microprobe analysis combined with Synchrotron Mossbauer source spectrometry yielded the following empirical formula based on 40 anions per formula unit (pfu) [Al (sub 18.36) Mg (sub 3.96) Fe (super 2+) (sub 2:52) Fe (super 3+) (sub 2:08) Ti (sub 0.56) Cr (sub 0.40) Zn (sub 0.06) V (super 3+) (sub 0:03) Mn (sub 0.02) ] (sub Sigma 28) O (sub 38) (OH) (sub 2) . The ideal formula is Mg (sub 6) Al (sub 20) Fe (sub 2) (super 3+) O (sub 38) (OH) (sub 2) . The eight strongest lines in the X-ray powder diffraction pattern are [d (sub obs) / A(l) (I) (hkl)]: 2.858 (42) (110), 2.735 (51) (107), 2.484 (46) (018), 2.427 (100) (115), 1.568 (29) (128), 1.514 (30) (0212), 1.438 (42) (2013), and 1.429 (72) (220). The crystal structure of magnesiobeltrandoite-2N3S [P3 (super -) m1, a = 5.7226(3), c = 23.0231(9)A, V = 652.95(5)A (super 3) ] was refined from X-ray single-crystal data to R1 =0.022; it is isostructural with magnesiohogbomite-2N3S.