Chromium Geochemistry of Serpentine Soils

Serpentine soils derived from the weathering of ultramafic rocks, mainly ophiolitic serpentinites, are typically characterized by Cr concentrations in excess of 200 mg kg-1, comparatively higher than non-serpentine soils. We review the chemistry of Cr in serpentine soils and their protoliths, focusing on serpentine soils collected from New Caledonia, Oregon, and California. Overall, serpentine soils are slightly acidic (average pH of ∼6), contain a variety Fe(III) oxides (magnetite and hematite), Fe(III) (oxy)hydroxides, phyllosilicates (serpentine and chlorite), and clays (smectites and vermiculites), and contain concentrations of Cr (> 200 mg kg-1), Ni (> 1,000 mg kg-1), and Mn (> 200 mg kg-1) exceeding values of non-serpentine soils. Although Cr concentrations in serpentine soils have been reported as high as 6 wt% in New Caledonia, Cr values in New Caledonia, Oregon, and California serpentine soils evaluated in this study range from 827 to 9,528 mg kg-1. Chromium(III) is the only valence state observed in the serpentine soil solids; however, Cr(VI) has been identified in New Caledonia and California serpentine soil solutions at concentrations below 30 μM. The enrichment and range of Cr concentrations in serpentine soils are directly related to the presence of Cr-spinels, specifically chromite and Cr-magnetite. These phases are resistant to weathering and are preserved in the soil environment; however, oxidation of Cr(III) from Cr-spinels by high-valent Mn oxides, or other strong oxidants, is a potential source of Cr(VI) identified in serpentine soil solutions. Due to the weathering resistant nature of the Cr-spinels, Cr-bearing silicates including clay minerals, Cr-chlorite, Cr-garnet, Cr-mica, and Cr-epidote are more viable sources of Cr identified in vegetation, soil extractions, soil solutions, and related waters.