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 × 10–6 °C–1.