Secondary mineral genesis from chlorite and serpentine in an ultramafic soil toposequence

The origin of secondary phyllosilicates in serpentinitic soils of differing moisture regimes is incompletely understood. The objective of this study was to determine the genesis of weathering products in serpentinitic soils along a moisture regime gradient using conventional x-ray diffraction (XRD) methods and high-resolution transmission electron microscopy (HRTEM). The samples studied were obtained from an Aquic Argixeroll and a Cumulic Endoaquoll on the Trinity ophiolite, in the Klamath Mountains, California. The soils are from backslope and toeslope landscape positions associated with a 3.2-ha wetland on a stabilized landslide bench. Chlorite and serpentine are the major primary minerals in the soils. Chlorite is relatively stable and was found in the clay fraction of all horizons studied. Serpentine was observed in all horizons except the Aquic Argixeroll Cr2 horizon. The soil mineral assemblages indicate that chlorite transforms to vermiculite and both randomly and regularly interstratified chlorite/vermiculite by loss of the hydroxide-interlayer sheet. The vermiculite then alters to a high-charge smectite that was found only in the lower horizons of the backslope landscape position. Smectite is the predominant secondary mineral in all horizons. Serpentine transformation products could not be directly identified, but the prevalence of a low-charge smectite in the Cumulic Endoaquoll is interpreted as a precipitate from serpentine dissolution products. Thus, the abundant smectite in these serpentinitic soils is of two origins: (i) a high-charge phase derived from chlorite transformation that is found in the backslope landscape positions, and (ii) a low-charge phase neoformed by precipitation of elements released by serpentine weathering.