Publication Date:
2016-12-01
Description:
Bettertonite, [Al 6 (AsO 4 ) 3 (OH) 9 (H 2 O) 5 ]·11H 2 O and penberthycroftite, [Al 6 (AsO 4 ) 3 (OH) 9 (H 2 O) 5 ]·8H 2 O, 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 .
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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