Coarse Columbia River sand, with neither silt- nor clay-size grains present, altered to sediment composed of 82 wt. % sand, 5% silt, and 13% montmorillonitic clay in a 5-month hydrothermal experiment. Run conditions were 200°C and 200 psi in a brine solution as the fluid phase. The original sand was composed largely of volcanic detritus from andesitic sources. Hypersthene grains were severely etched during the runs, and other mineral grains and lithic fragments probably also participated in the reaction to form clay. The clay, which coated various parts of the hydrothermal apparatus, appeared to form at least partly as a precipitate. The fluid phase showed a slight increase in Na and K, a slight decrease in Ca and Mg, and saturation with Si. The pH remained constant t 3.5. Oxidizing conditions probably prevailed throughout the experiment.

A similar experiment was run with Columbia River silt as starting material. Montmorillonitic clay was also produced, and the fluid phase showed the same changes.

Similar reactions undoubtedly take place in natural systems. Clay matrix in sandstones must, at least in part, be formed by chemical alteration of thermodynamically unstable clasts.

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