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Iron-Rich Montmorillonite Formation in Soils Derived from Serpentinite¹

ABSTRACT

Montmorillonite, identified by X-ray diffraction, is the predominant clay mineral in the B horizons of four widely separated California soils (Henneke, Dubakella, and Fancher series and one from the New Idria area) derived from serpentinite. The < 0.2µ fractions contain respectively, 81, 68, 71, and 33% montmorillonite as determined by CEC determination. These fractions contain 57.5, 54.9, 62.5, and 50.0% SiO₂; 17.2, 18.4, 17.6, and 7.5% combined Fe₂O₃ (by chemical analysis after removal of free Fe₂O₃); 7.2, 4.3, 8.7, and 25.0% MgO; and 4.3, 10.4, 1.0, and 6.1% Al₂O₃. Much of the MgO can be attributed to < 0.2µ serpentine minerals, identified by electron microscopy. The montmorillonite has a high content of Fe in octahedral position, represented by the range in octahedral sheet formula, Fe_1.8–2.7Al_0.2–1.8Mg_0–1.1, calculated for the four soil clays. Mg is the predominant exchangeable cation in the < 2 mm soil (67–96% of the total exchangeable cations). The parent serpentinite contrasts with the soil clays in having lower SiO₂ (43.3, 38.1, 40.0, and 39.8%) and Fe₂O₃ (6.6, 3.6, 8.9, and 3.1%, after magnetite removal from the parent rock samples), higher MgO (33.7, 41.0, 42.2, and 40.3%), and lower Al₂O₃ (0.4, 2.4, 1.5 and 0.7%). Leaching of serpentinite removed much of the Mg; concurrently Si, Fe, and Al were enriched in the pedogenic montmorillonite which formed.

¹ Cooperative interinstitutional contribution from Wisconsin Agr. Exp. Sta., Madison, and California Agr. Exp. Sta., Davis. Supported in part by National Science Foundation Grant GP4144 and GA1108-Jackson. Presented before Div. S-2 and S-5, Soil Science Society of America, Stillwater, Okla., Aug. 25, 1966.

² Formerly Graduate Research Assistant, now Extension Soils Specialist, University of California, Davis; Professor of Soil and Water Sciences, University of Wisconsin; Professor of Soil Science, University of California.

Received for publication February 15, 1968. Accepted for publication June 24, 1968.