Soils and Mineral Weathering on Phyllite Colluvium and Serpentinite in Northwestern California

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Abstract
Haplohumults and Argixerolls have formed in close proximity on phyllite colluvium and serpentinite residuum, respectively, in the Klamath Mountains of northwestern California. This study was conducted to compare their contrasting properties and to investigate soil mineral weathering. Thick, dark A horizons containing 20 to 50 g kg−1 organic C have formed in the Argixeroll under the influence of perennial grass cover with sparse conifers and shrubs. The Haplohumult, under dense Douglas fir [Pseudotsuga menziesii (Mirbel) Franco] forest, has a thin A horizon containing 10 to 20 g kg−1 organic C. Soil pH is in the range of 5.6 to 5.8 in the A horizons of both soils. In the Haplohumult, pH decreases slightly with depth, whereas pH increases to 6.5 in the Crt horizon of the Argixeroll. The mineralogy of the Haplohumult is relatively uniform throughout its 260‐cm depth. Chlorite, the dominant mineral in the phyllite parent rock, has altered to interstratified chlorite/vermiculite and poorly crystalline kaolin in the soil fine‐sand fraction. Chlorite/vermiculite and kaolin dominate the clay fraction, accompanied by gibbsite, talc, and Fe oxides. Serpentine weathering prevails in the Argixeroll. The serpentine content of the fine‐sand fraction decreases upward in the profile, whereas magnetite and chlorite resist weathering and are concentrated. Serpentine is the most abundant mineral in the clay fraction. Clay‐size smectite, a product of serpentine weathering, is absent from A horizons, but increases with depth to a maximum in the fine‐earth material from cracks in the Crt horizon. Chlorite in the clay fraction is inherited from the parent rock. Both soils have experienced minor surficial additions of amorphous silica phytoliths and volcanic glass shards.

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