Thermal stability of crandallite CaAl3(PO4)2(OH)5•(H2O) A 'Cave' mineral from the Jenolan Caves

Thermogravimetry combined with evolved gas mass spectrometry has been used to characterise the mineral crandallite CaAl3(PO4)(2)(OH)(5)center dot(H2O) and to ascertain the thermal stability of this 'cave' mineral. X-ray diffraction proves the presence of the mineral and identifies the products of the thermal decomposition. The mineral crandallite is formed through the reaction of calcite with bat guano. Thermal analysis shows that the mineral starts to decompose through dehydration at low temperatures at around 139 degrees C and the dehydroxylation occurs over the temperature range 200-700 degrees C with loss of the OH units. The critical temperature for OH loss is around 416 degrees C and above this temperature the mineral structure is altered. Some minor loss of carbonate impurity occurs at 788 degrees C. This study shows the mineral is unstable above 139 degrees C. This temperature is well above the temperature in the caves of 15 degrees C maximum. A chemical reaction for the synthesis of crandallite is offered and the mechanism for the thermal decomposition is given.