HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Oze, C. Articles by Coleman, R. G. Search for Related Content Articles by Oze, C. Articles by Coleman, R. G. GeoRef GeoRef Citation

Chromium geochemistry in serpentinized ultramafic rocks and serpentine soils from the Franciscan complex of California

Christopher Oze^*,, Scott Fendorf, Dennis K. Bird and Robert G. Coleman

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA

^* Current Address: Department of Earth Sciences, Dartmouth College, 6105 Fairchild Hall, Hanover, New Hampshire 03755; oze{at}dartmouth.edu

Weathering of ultramafic rocks and serpentinites in the Franciscan Complex of California produces serpentine soils containing high concentrations of Cr as well as other potentially toxic elements including Ni, Co, and Mn. Chromium concentrations in serpentine soils from Jasper Ridge Biological Preserve in the Central Coast Range are as high as 4,760 mg kg^–1, nearly three times greater than the serpentinite protolith (1800 mg kg^–1). Chromium-containing minerals within the bedrock include chlorite (0.3 Cr wt. %), enstatite (0.4 Cr wt. %), augite (0.7 Cr wt. %), chromite (10.8 Cr wt. %), magnetite (8.2-10.3 Cr wt. %), and an ultra fine-grained mixture of spinel and a silicate phase containing 13.3 Cr weight percent. Chromium concentrations in serpentine soil profiles fluctuate between 1,725 to 4,760 mg kg^–1 and do not correspond to variations in soil pH, organic carbon, or electrical conductivity. The enrichment and variability of soil Cr is directly related to the modal abundance and weathering of chromite, Cr-magnetite, and the spinel-silicate mixture. By comparison, Cr silicates account for < 10 percent of the total soil Cr. Chemical analyses and X-ray microprobe maps demonstrate that Cr-spinels in these soils undergo incongruent dissolution progressively enriching the spinel toward a Cr-enriched end-member (FeCr₂O₄). Chromium occurs in the trivalent state in both the rock and soil samples. The apparent resistance of Cr-spinels to weathering, evident from extraction experiments, suggests that these minerals are not large inputs for Cr in soil solutions and vegetation associated with serpentine soils. Chromium-bearing igneous and metamorphic silicates in the protolith and Cr-bearing clay minerals in the soil are more likely sources of chemically mobile and bioavailable Cr.

This article has been cited by other articles:

				D. Fandeur, F. Juillot, G. Morin, L. Olivi, A. Cognigni, J.-P. Ambrosi, F. Guyot, and E. Fritsch Synchrotron-based speciation of chromium in an Oxisol from New Caledonia: Importance of secondary Fe-oxyhydroxides American Mineralogist, May 1, 2009; 94(5-6): 710 - 719. [Abstract] [Full Text] [PDF]

				C. Oze, D. K. Bird, and S. Fendorf Genesis of hexavalent chromium from natural sources in soil and groundwater PNAS, April 17, 2007; 104(16): 6544 - 6549. [Abstract] [Full Text] [PDF]