Bulachite (from Cap Garonne) crystal structure

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Marco E. Ciriotti
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Bulachite (from Cap Garonne) crystal structure

Messaggio da Marco E. Ciriotti » mar 30 giu, 2020 10:12

Prossima pubblicazione.

▪ Grey, I.E., Yoruk, E., Kodjikian, S., Klein, H., Bougerol, C., Brand, H.E.A., Bordet, P., Mumme, W.G., Favreau, G., Mills, J.S. (2020): Bulachite, [Al6(AsO4)3(OH)9(H2O)4]⋅2H2O from Cap Garonne, France: Crystal structure and formation from a higher hydrate. Mineralogical Magazine, 84, (in press).

Bulachite specimens from Cap Garonne, France, comprise two intimately mixed hydrated aluminium arsenate minerals with the same Al:As ratio of 2:1 and with different water contents. The crystal structures of both minerals have been solved using data from low-dose electron diffraction tomography combined with synchrotron powder X-ray diffraction. One of the minerals has the same PXRD pattern as for published bulachite. It has orthorhombic symmetry, space group Pnma with unit cell parameters a = 15.3994(3), b = 17.6598(3), c = 7.8083(1) Å and Z = 4, with the formula [Al6(AsO4)3(OH)9(H2O)4]∙2H2O. The second mineral is a higher hydrate with composition [Al6(AsO4)3(OH)9(H2O)4]∙8H2O. It has the same Pnma space group and unit cell parameters a = 19.855(4), b = 17.6933(11) and c = 7.7799(5) Å. i.e. almost the same b and c parameters but a much larger a parameter. The structures are based on polyhedral layers, parallel to (100), of composition [Al6(AsO4)3(OH)9(H2O)4] and with H-bonded H2O between the layers. The layers contain [001] spiral chains of edge-shared octahedra, decorated with corner connected AsO4 tetrahedra that are the same as in the mineral liskeardite. The spiral chains are joined together by octahedral edge-sharing to form layers parallel to (100). Synchrotron PXRD patterns collected at different temperatures during heating of the specimen show that the higher-hydrate mineral starts transforming to bulachite when heated to 50°C, and the transformation is complete between 75 and 100°C.

NdR: La fase più idratata è una potenziale nuova specie e gli autori stanno lavorando per la presentazione del proposal all'IMA CNMNC.
Marco E. Ciriotti

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