IMA 2022-057 = bernardevansite

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Marco E. Ciriotti
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IMA 2022-057 = bernardevansite

Messaggio da Marco E. Ciriotti » mer 25 gen, 2023 21:56

Prossima pubblicazione.

Referenza:
▪ Yang, H., Gu, X., Jenkins, R.A., Gibbs, R.B., Downs, R.T. (2023): Bernardevansite, Al2(Se4+O3)3⋅6H2O, dimorphous with alfredopetrovite and the Al-analogue of mandarinoite, from the El Dragón mine, Potosí, Bolivia. Mineralogical Magazine, 87, (in press).

Abstract:
A new mineral species, bernardevansite (IMA 2022-057), ideally Al2(Se4+O3)3·6H2O, has been discovered from the El Dragón mine, Potosí Department, Bolivia. It occurs as aggregates or spheres of radiating bladed crystals on a matrix consisting of Co-bearing krut’aite–penroseite. Associated minerals are Co-bearing krutˊaite–penroseite, chalcomenite, and clinochalcomenite. Bernardevansite is colorless in transmitted light, transparent with white streak and vitreous luster. It is brittle and has a Mohs hardness of 2 3 Cleavage i not o erve . The measured and calculated densities are 2.93(5) and 2.997 g/cm3 , respectively. Optically, bernardevansite is biaxial (+), with α = 1.642(5), β = 1.686(5), γ = 1.74(1) (white light). An electron microprobe analysis yielded an empirical formula (based on 15 O apfu) (Al1.26Fe3+0.82)Σ2.08(Se0.98O3)36H2O, which can be simplified to (Al,Fe3+)2(SeO3)36H2O. Bernardevansite is the Al-analogue of mandarinoite, Fe3+2(SeO3)36H2O ordimorphous with P-62c alfredopetrovite. It is monoclinic, with space group P21/c and unit-cell parameters a = 16.5016(5), b = 7.7703(2), c = 9.8524(3) Å, β = 98.258(3) °, V = 1250.21(6) Å3, and Z = 4. The crystal structure of bernardevansite consists of a cornersharing framework of M3+O6 (M = Al, Fe) octahedra and Se4+O3 trigonal pyramids, leaving large voids occupied by the H2O groups. There are two unique M3+ positions: M1 is octahedrally coordinated by (4O + 2H2O) and M2 by (5O + H2O). The structure refinement indicates that Al preferentially occupies M1 (= 0.692Al + 0.308Fe) over M2 (= 0.516Al + 0.484Fe). The substitution of the majority of Fe in mandarinoite by Al results in a significant reduction in its unit-cell volume from 1313.4 Å3 to 1250.21(6) Å3 for bernardevansite. The discovery of bernardevansite begs the question whether the Fe3+-end-member, Fe3+2(SeO3)3·6H2O, has two polymorphs as well, one with P21/c symmetry, as for mandarinoite, and the other P-62c, as for alfredopetrovite.

NdR: Mandarinoite group
Alfredopetrovite Al2(Se4+O3)3·6H2O hex P62c 8.818 8.818 10.721 90.00 120.00 90.00 4
Bernardevansite Al2(Se4+O3)3∙6H2O mon P21/c 16.5016 7.7703 9.8524 90.00 98.258 90.00 4
Mandarinoite Fe3+2(Se4+O3)3·6H2O mon P21/c 16.81 7.88 10.02 90.00 98.3 90.00 4
Telluromandarinoite Fe3+2(Te4+O3)3·6H2O mon P21/c 16.9356 7.8955 10.1678 90.00 98.0064 90.00 4


Non è che, per qualche motivo, i dati relativi alla alfredopetrovite sono fasulli? O-D? Il lato a (parametro di cella) della alfredopetrovite è la metà degli altri lati a.
Potrebbe essere di interessamento per il Prof. Merlino. Ma, magari, invece è tutto OK. Stranezze naturali!
In effetti gli spettri Raman di alfredopetrovite e di bernardevansite non sono simili e porterebbero a dire che è tutto OK.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 31675
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: IMA 2022-057 = bernardevansite

Messaggio da Marco E. Ciriotti » gio 06 lug, 2023 10:31

Pubblicazione effettuata.

Referenza:
▪ Yang, H., Gu, X., Jenkins, R.A., Gibbs, R.B., Downs, R.T. (2023): Bernardevansite, Al2(Se4+O3)3⋅6H2O, dimorphous with alfredopetrovite and the Al-analogue of mandarinoite, from the El Dragón mine, Potosí, Bolivia. Mineralogical Magazine, 87, 407-414.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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