Tonalite cobbles from moraines in the Tobacco Root Mountains, Montana, have weathering rinds in which the feldspars are altered to clays along microcracks. Energy-dispersive X-ray and X-ray diffraction analyses have shown that microcracks in orthoclase contain only kaolinite but that microcracks in oligoclase contain a central zone of kaolinite surrounded by a zone of smectite. These observations are in accord with predictions based on activity diagrams for the appropriate chemical systems. Feldspar dissolution was probably the rate-determining step in the development of the microcracks and their clay fillings.

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