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Silica in Duric Soils: II. Mineralogy¹

ABSTRACT

Opal-A and opal-CT were identified by x-ray diffraction as the primary pedogenic silica minerals in two central Nevada duric soils. Opal-A was also present in the form of allogenic volcanic glass. Pedogenic microcrystalline quartz was not identified. Saturated paste extracts contained > 60 mg L^–1 Si(OH)₄ and opal-A precipitation is likely during soil dehydration. Opal-CT precipitates from soil solution once silica concentrations have been reduced through adsorption, precipitation, and/or increased solution ionic strength, or it may be produced by transformation from opal-A by mineral aging processes. High concentrations of silica in soil solution favor opal formation in preference to deposition of microcrystalline quartz.

¹ Contribution from the Departments of Geoscience and Soil and Water Science, Univ. of Arizona, Tucson, AZ 85721; and the USDA National Soil Survey Laboratory, Lincoln, NE 68508. Portions of the research were supported by USDI Geological Survey Earthquake Hazards Reduction Program contract no. 14080001G1205.

² Postdoctoral researcher, Dep. of Plant and Soil Biology, Univ. of California, Berkeley, CA 94720; Associate Professor, Dep. of Soil and Water Science, Univ. of Arizona, Tucson; and Research Pedologist, USDA Soil Conservation Service, Lincoln, respectively.

Received for publication April 21, 1986.