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

Published online by Cambridge University Press:  30 June 2020

Ian E. Grey*
Affiliation:
CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria3169, Australia
Emre Yoruk
Affiliation:
Université Grenoble Alpes and CNRS, Institut Néel, 38000, Grenoble, France
Stéphanie Kodjikian
Affiliation:
Université Grenoble Alpes and CNRS, Institut Néel, 38000, Grenoble, France
Holger Klein
Affiliation:
Université Grenoble Alpes and CNRS, Institut Néel, 38000, Grenoble, France
Catherine Bougerol
Affiliation:
Université Grenoble Alpes and CNRS, Institut Néel, 38000, Grenoble, France
Helen E.A. Brand
Affiliation:
Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria3168, Australia
Pierre Bordet
Affiliation:
Université Grenoble Alpes and CNRS, Institut Néel, 38000, Grenoble, France
William G. Mumme
Affiliation:
CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria3169, Australia
Georges Favreau
Affiliation:
421 avenue Jean Monnet, 13090 Aix-en-Provence, France
Stuart J. Mills
Affiliation:
Geosciences, Museums Victoria, GPO Box 666, Melbourne, Victoria3001, Australia
*
*Author for correspondence: Ian E. Grey, Email: ian.grey@csiro.au

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.

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Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2020

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Footnotes

Associate Editor: Giancarlo Della Ventura

References

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