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

A B S T R A C T : Reactions undergone by leadhillite from the type locality on heating to 1000 ~ 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~ 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~ the mineral undergoes two decomposition reactions: PbO. PbCO 3 and PbO. PbSO 4 form during the first reaction (PbCO 3 may form in the initial stages) and 4PbO.PbSO 4 during the second. a-2PbO. PbSO 4 appears at 650~ due to solid-state reaction between the other lead oxysulphate products. Melting occurs above 850~ The reaction products are discussed in relation to the phase diagrams for the systems PbO-CO 2 and PbO-PbSO 4.