Certain aspects of the geochemistry of cadmium

Abstract

Our knowledge about the geochemistry of cadmium is incomplete in several respects. The "enrichment factor" for cadmium in sphalerite, as can be calculated from published data, is lower than the theoretical one by about two orders of magnitude. Further, the absence of CdS as a primary mineral in hydrothermal ore deposits is itself a problem. Experiments conducted at low temperatures demonstrate that the theoretical value of the "enrichment factor" is obtainable when dealing with the pure components. But presence of Cl⁼ ions (2.0-3.0 M) in the system remarkably depresses the value to the range actually found in ore deposits. Since NaCl is a ubiquitous constituent of the fluid inclusions (= 0.8-2.5 M), it is argued that formation of chloride complexes of cadmium, whose stability constants are much higher than those of zinc, causes a remarkable relative lowering of the concentration of free Cd⁺⁺ ions, thereby depressing the value of the partitioning coefficient and hindering the appearance of CdS as a separate phase.