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.

Time-resolved synchrotron powder diffraction was used to follow the thermal transformation of cement-asbestos. Thermal transformation of asbestos fibers into nonfibrous crystalline phases is a promising solution for the elimination of these hazardous minerals. Time resolution offered by the use of an imaging plate detector with a high-brightness X-ray source allowed for the observation of metastable phases, commonly not detectable with conventional instrumentation. In addition, the use of a closed capillary as a sample holder mimicked the real, novel industrial reactor where cement-asbestos slates are sealed in a tunnel kiln. The changing gas atmosphere in the closed system was shown to affect the final composition of the recrystallized product. This study demonstrates the importance of advanced powder diffraction techniques in this field of applied research.