Pedogenic weathering of tonalite in southern California

Abstract

Tonalite (quartz diorite) in southern California has weathered under aa mediterranean climate to depths of more than 30 m. In weathering, the rock was broken up by expansion of biotite as it weathered to vermiculite. The particle density of the resulting fragments is the same as that of the rock (2.75 g/cm³). Biotite comprises 14% of the rock and could produce a linear extensibility of 1.8% in such a uniform, medium-grained material if all the biotite weathered to vermiculite. Measured expansion (1–2%) of the fragmented rock agrees with the predicted value. Other minerals, mostly unweathered, in the rock are: andesine, 56%; hornblende, 13%; quartz, 9%; and orthoclase, 8%.