Trimeric As3+3O6 clusters in walentaite: Crystal structure and revised formula

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Marco E. Ciriotti
Messaggi: 24698
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Trimeric As3+3O6 clusters in walentaite: Crystal structure and revised formula

Messaggio da Marco E. Ciriotti » lun 11 giu, 2018 12:14

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Referenza:
▪ Grey, I.E., Mumme, W.G., Hochleitner, R. (2018): Trimeric As3+3O6 clusters in walentaite: Crystal structure and revised formula. European Journal of Mineralogy, 30, (in press).
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: 24698
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Trimeric As3+3O6 clusters in walentaite: Crystal structure and revised formula

Messaggio da Marco E. Ciriotti » mar 04 set, 2018 17:34

Referenza:
▪ Grey, I.E., Mumme, W.G., Hochleitner, R. (2018): Trimeric As3+3O6 clusters in walentaite: Crystal structure and revised formula. European Journal of Mineralogy, 30, (in press).

Abstract:
The crystal structure of walentaite, originally reported as a calcium iron arsenate phosphate, has been determined and the As has been found to be predominantly in the trivalent state, as AsO3 trigonal pyramids and as As3O6 trimeric clusters. The mineral has body-centred orthorhombic symmetry with a = 26.188(5), b = 7.360(2), c = 10.367(2) Å. The structure was solved in space group Imma using synchrotron single-crystal diffraction data and refined to R1 = 0.054 for 1454 observed reflections. The walentaite structure is based on the stacking of heteropolyhedral layers along [100] with the layers held together by hydrogen bonds to interlayer hydrated cations M(H2O)6, M=Mn2+, Fe2+, Ca2+, Na+. The layers, of composition Fe3+3(P0.84As0.16O4)2(O,OH)6, have the same topology of corner-connected octahedra and tetrahedra as in the alunite-supergroup mineral kintoreite-1c. Unit-cell-scale twinning of the kintoreite-like layers gives a saw-tooth aspect to the layers. The AsO3, As3O6 and (Ca,Na)O6 groups are attached to the surface of the layers. Although the kintoreite-like heteropolyhedral layers are fully ordered in Imma, the surface-attached groups and the interlayer hydrated cations are highly disordered, so the Imma model represents the average structure. Models for the local ordering have been developed from interpretation of the partial site occupancies and splitting of the disordered atoms. The crystal structure results have been combined with electron microprobe analyses to obtain a new formula for walentaite, Fe3+3(P0.84As0.16O4)2(O,OH)6As3+2.56Ca0:42Na0:28Mn2+0.35Fe+0.30O6.1(OH)0.9(H2O)0.9, in which 89% of the As is present as As3+.
Marco E. Ciriotti

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