A physical and mineralogical description of Theisenschlamm is presented. This material was generated as a by-product during extraction of copper from the Kupferschiefer formation a mineralized Permian black shale in Sachsen-Anhalt, Germany. The Theisenschlamm consists of sulphide grains which are a complex solid solution of Pb, Zn, Cu and Fe. The crystal structures are modified forms of galena and sphalerite/wurtzite. Variations in the X-ray diffraction peak positions of these minerals reflect the intensive ionic substitution that underwent the lattice structures of these phases. Anglesite is a secondary phase associated with the alteration through oxidation of the lead sulphides in the slurry. The original particles have a median diameter of 1.25 μm, however these particles are themselves aggregates of still smaller particles, typically of sub-micron in size. In this chapter we describe the physical properties of the Theisenschlamm, including the mineralogy, radioactivity and grain size as well as the micro-chemical composition and variation.