Pretreating zinciferous surfaces

Abstract

Between Late Cretaceous and Miocene time the central part of Tunisia emerged. During this period a karstic system developed in Senonian limestones of the Ain Khamouda area. It is known that three facies occur in the resulting caves: (1) red mineralised crust containing goethite, hemimorphite and smithsonite; (2) white clay containing 0.1–8.5% ZnO; (3) Miocene sand and sandstone. The white clay has been studied using STEM (scanning transmission electron microscopy) in order to determine with which minerals Zn is associated. Two zinciferous phases were found, the first being a swelling clay of sauconite type, the second a nearly amorphous Zn-hydroxide gel. Halloysite which is the main component of the white clay seems totally devoid of Zn. The difference between Zn minerals contained in the crust and in the clay can be related to pH gradients. Minute particles (3–10 nm) of pure Ag, were found in the clay, indicating that the present mineralisation is probably related to the alteration of sulphides.