Abstract: An unusual halloysite deposit occurs on the west side of the Lake Mountains in Utah County, Utah. A field and laboratory study was made of this Fox deposit to determine the paragenesis of the clay minerals, especially halloysite. Successive lenses of unaltered tuff, partially altered tuff, clay and travertine beds, indicate that the clays have resulted from alteration of siliceous volcanic tuffs in a Tertiary (?) calcareous hot spring environment. Clay minerals identified were montmorillonite, kaolinite, halloysite·4H2O (endellite) and halloysite·2H2O. Montmorillonite, which developed in a zone of less intensive silica leaching farthest from the hot spring vents, is the predominant clay mineral. Irregularly distributed pockets of halloysite and kaolinite developed nearest the hot spring vents in a high-calcium environment. In the clay, or associated with it, are calcite as travertine, quartz as rounded carbonate-corroded grains, feldspar, tridymite, biotite and glass.
Experiments that approximate the chemical environment prevalent in the halloysite alteration zone were conducted on phase relations in part of the system lime-alumina-silica-water. Results suggest the formation of halloysite·4H2O in this kinetic system from intermediate calcium aluminate or calcium silicate hydrates with halloysite-type structures, or both.