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Dehydration phase transitions in new aluminium arsenate minerals from the Penberthy Croft mine, Cornwall, UK

Published online by Cambridge University Press:  02 January 2018

Ian E. Grey ,
Helen E. A. Brand  and
John Betterton
Ian E. Grey*
Affiliation:
CSIRO Mineral Resources, Private bag 10, Clayton South, Victoria 3169, Australia
Helen E. A. Brand
Affiliation:
Australian Synchrotron, 800 Blackburn Rd, Clayton, VIC 3168, Australia
John Betterton
Affiliation:
Haslemere Educational Museum, 78 High Street, Haslemere, Surrey GU27 2LA, UK
*
*E-mail: ian.grey@csiro.au
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Abstract

Bettertonite, [Al6(AsO4)3(OH)9(H2O)5]•11H2O and penberthycroftite, [Al6(AsO4)3(OH)9(H2O)5].8H2O, two new minerals from the Penberthy Croft mine, Cornwall, have flexible layer structures based on corner-connected heteropolyhedral columns. Their response to dehydration on heating was studied using in situ synchrotron powder X-ray diffraction at temperatures in the range -53 to 157°C. The bettertonite sample transforms to penberthycroftite in a narrow temperature range of 67 to 97°C with a large (8%) contraction of the layer separation and a 6 Å sliding of adjacent layers relative to each other. Above 100°C a second phase transition occurs to a DL (displaced layer) phase, involving another 8% inter-layer contraction combined with a rotation of the columns. On heating the penberthycroftite sample the phase transition to the DL phase occurs at a lower temperature of ∼80°C. The DL phase is stable to a temperature of ∼120°C. At higher temperatures, increased rotation of the columns is accompanied by a progressive amorphization of the sample. Bettertonite, penberthycroftite and the DL phase exhibit negative thermal expansion (NTE) along all three axes with large NTE coefficients, of the order of-100 x 10 -6 °C-1.

Keywords

penberthycroftitebettertonitehydrated aluminium arsenatesdehydration phase transitionsin situ synchrotron powder diffraction
Type
Research Article
Information
Mineralogical Magazine , Volume 80 , Issue 7 , December 2016 , pp. 1205 - 1217
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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