Iron‐Rich Montmorillonite Formation in Soils Derived from Serpentinite

Abstract

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% SiO 2 ; 17.2, 18.4, 17.6, and 7.5% combined Fe 2 O 3 (by chemical analysis after removal of free Fe 2 O 3 ); 7.2, 4.3, 8.7, and 25.0% MgO; and 4.3, 10.4, 1.0, and 6.1% Al 2 O 3 . 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 1.8–2.7 Al 0.2–1.8 Mg 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 SiO 2 (43.3, 38.1, 40.0, and 39.8%) and Fe 2 O 3 (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 Al 2 O 3 (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.