Thermal reactions of leadhillite; Pb (sub 4) SO (sub 4) (CO (sub 3) ) (sub 2) (OH) (sub 2)

Reactions undergone by leadhillite from the type locality on heating to 1000 degrees C have been followed by DTA, TG, DSC, evolved gas analysis, continuous-heating XRD and IR, and hot-stage microscopy. Intermediate decomposition products were identified by X-ray powder photography. At 80 degrees C, biaxial leadhillite inverts to a uniaxial phase with properties similar to those of susannite, a naturally occurring polymorph of leadhillite, but this higher-temperature modification only partially reverts to the original structure on cooling (up to 24 hours at room temperature is required for complete reversion). Between 250 and 600 degrees C the mineral undergoes two decomposition reactions: PbO.PbCO (sub 3) and PbO.PbSO (sub 4) form during the first reaction (PbCO (sub 3) may form in the initial stages) and 4PbO.PbSO (sub 4) during the second. alpha -2PbO.PbSO (sub 4) appears at 650 degrees C due to solid-state reaction between the other lead oxysulphate products. Melting occurs above 850 degrees C. The reaction products are discussed in relation to the phase diagrams for the systems PbO-CO (sub 2) and PbO-PbSO (sub 4) .