Weathering products within microcracks in feldspars Available to Purchase

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.