Cadmium associated with phosphate deposits in southern Tunisia

Cadmium values varying between ppm to tens of ppm have been noted in phosph orites from the Early Eocene of Tunisia. The heavy fraction (d greater than or equal to 3.1) (<2%) with higher Cd contents is composed of pellets with sulphide inclusions of free sphalerite and pyrite. The sphalerite XRD indi cates a crystallographic parameter with a value of 5.4492 Angstrom. Analyti cal data give the following Structural formula: (Zn-.875 Cd-.109 Fe-.002 Pb -.001) S-1.012 for J. M'Rata; (Zn-.959 Cd-.056)S-.995 for J.M'Dilla; (Zn-.9 52 Cd-.079 Fe-.004 Cu-.006 Pb-.005) S for J. Abdallah. SEM photographs show a heterogeneous Cd distribution within sphalerite. One transverse using 10 linear points across a grain of sphalerite gave a composition of S (48.47- 49.80 at.%), Zn (44.39-46.19 at.%), Cd (4.85-7.16 at.%) and Fe (0.02-0.19 at.%). Such high Cd values have not been found in natural sphalerite. A geo chemical process involving sulphate-reducing bacteria could explain the pre sence of sphalerite inside and outside the phosphate grains. Zinc and Cd ar e present in sediment abnormally rich in exclusively planktonic organic mat ter, which is known to specifically concentrate them. (C) 1999 Elsevier Sci ence Limited. All rights reserved.