Bulachite, [Al6(AsO4)3(OH)9(H2O)4]⋅2H2O from Cap Garonne, France: crystal structure and formation from a higher hydrate

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Bulachite, [Al 6 (AsO 4 ) 3 (OH) 9 (H 2 O) 4 ]2H 2 O from Cap Garonne, France_ Crystal structure and formation from a higher hydrate.pdf(1.04 MB)
Date
2020-06-30
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Cambridge University Press
Abstract
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 powder X-ray diffraction pattern (PXRD) 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. © 2020 The Mineralogical Society of Great Britain and Ireland .
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Preprint version is attached.
Keywords
Minerals, Crystal structure, Arsenates, Hydrates, Hydration, Electron diffraction, X-ray diffraction, Synchrotrons
Citation
Grey, I. E., Yoruk, E., Kodjikian, S., Klein, H., Bougerol, C., Brand, H. E. A., Bordet, P., Mumme, W. G., Favreau, G. & Mills, S. J. (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(4), 608-615. doi:10.1180/mgm.2020.52
URI
https://doi.org/10.1180/mgm.2020.52
 https://apo.ansto.gov.au/dspace/handle/10238/12170
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