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Clay Mineralogy of Soils Formed on Mazama Pumice¹

ABSTRACT

Amorphous components predominated in the clay size fraction of samples taken across a climatic transect of soils from Mazama pumice in central Oregon. The amounts were greatest in the less intensely weathered horizons, i.e., in lower horizons and at drier sites of the transect. Crystalline minerals were present in relatively small percentages and were unexpectedly complex. The phyllosilicates included beidellite, montmorillonite, vermiculite, a micaceous component, chloritic intergrades, and chlorite. Non-phyllosilicate minerals of the suite were gibbsite, plagioclase feldspars, and quartz. The amount and distribution of the individual clay size minerals varied within and between profiles.

Results of clay mineral identification were evaluated with respect to environmental data available for the transect sites. The factors primarily controlling the processes responsible for 2:1 clay mineral genesis were hypothesized to be the vesicular structure and chemical composition of the pumice material. Other factors such as climate and vegetation were believed to govern local weathering intensity.

¹ Technical paper no. 2470, Oregon Agr. Exp. Sta., Corvallis. Financial support through National Science Foundation Grant GP-2449 is gratefully acknowledged. Presented before Div. S-5, Soil Science Society of America, August 1966, Stillwater, Okla.

² Research Soil Scientist, US Soils Laboratory, SWC, ARS, USDA, Beltsville, Md. (formerly Assistant in Soils, Oregon State University), and Professors, Oregon State University, respectively.

Received for publication April 26, 1968. Accepted for publication August 14, 1968.