Cadmium (Cd) is a metallic element naturally occurring in trace amounts at the Earth��s surface. It is toxic to humans, animals and plants, and known to be a human carcinogen. The metal associates with sulphide ores, mainly the mineral sphalerite (ZnS), and its cycling can be highly influenced by accumulation in plants and organic debris. Its abundance in igneous and sedimentary rocks is generally low, not exceeding 0.3 mg/kg, although Cd can concentrate in metalliferous ore deposits, in argillaceous (fine grained) rocks and in coal. In England there is also an enrichment of Cd in the Chalk soils of the North and South Downs, but not further north. The reason for this is unknown.
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