Formation of basic lead phases during fire-setting and other natural and man-made processes

Abstract

Basic lead phases are relatively rare compounds occurring in various natural and anthropogenically influenced environments, most importantly those related to fire-setting (FS). The medieval FS mining method and subsequent alteration processes lead to a complex set of basic lead phases including caledonite, hydrocerussite, leadhillite, and lanarkite. Although basic lead phases have been known for over 100 years, their mode of formation and stability relations are only insufficiently known. In this study, the formation of this interesting phase assemblage is described in detail including textures, genesis, and conditions of formation. Samples include ones collected in a medieval mining district in SW-Germany and ones that underwent short-term (50 days) experiments mimicking FS and subsequent mine dump processes. The mode of occurrence and the stability relation of basic lead phases formed during FS is discussed using thermodynamic models that are adapted to also explain their occurrence in other anthropogenic and in natural environments.