| Chromium persists in the environment as Cr(III), a non-hazardous species, or as Cr(VI), a strong oxidant that is toxic to living cells and a human carcinogen. Serpentinites (metamorphosed ultramafic rocks) are distributed worldwide and contain Cr concentrations typically greater than 200 mg kg−1 . Additionally, serpentine soils derived from the weathering of serpentinites contain Cr concentrations often exceeding their corresponding protolith, ranging up 60,000 mg kg−1. Using a multidisciplinary approach combining field and laboratory studies, Cr geochemistry related to serpentinites and serpentine soils was evaluated and in particular their potential for generating Cr(VI). In serpentinites, Cr-bearing minerals are primarily oxides (chromite and Cr-magnetite) and silicates (Cr-muscovite, Cr-garnet, Cr-epidote, and Cr-chlorite) in which Cr is present as Cr(III). The alteration and tectonic history of serpentinites as well as the soil-forming factors climate, biota, topography, and time are responsible for the physical and chemical characteristics of serpentine soils. Weathering resistant chromite and Cr-magnetite are present in serpentine soils and they are responsible for a majority of the Cr enrichment; however, Cr-bearing silicates with comparatively less Cr than chromite and Cr-magnetite potentially provide the most accessible source of Cr in soil solutions and in plant uptake. Although Cr(VI) was not identified in serpentinite and serpentine soil solids, it was present in groundwater and soil solutions at concentrations <30 μg L−1 in New Caledonia and California. Chromium release from chromite and oxidation is promoted by Mn-oxides such as birnessite, and thus represents a pathway for Cr(VI) generation in ultramafic sediments and serpentine soils. |
