Formula: (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2.nH2O Monoclinic
Description: Montmorillonite is one of a number of clay minerals within the smectite group. It forms by weathering or hydrothermal alteration of other aluminum-rich minerals, and is particularly common in altered volcanic ashes called bentonites. Bentonitic clay occurs at several stratigraphic horizons within the Ordovician Platteville and Decorah Formations, which are widely exposed in western and southern Wisconsin. Montmorillonite occurs intermixed with illite and other clay minerals. Its identification requires X-Ray analysis.
GRANT COUNTY: K-rich montmorillonite is a component of a bentonite layer found in the Decorah Formation near Platteville. It is associated with sanidine, quartz, apatite and zircon (Allen, 1932). Such material is doubtlessly found where ever this layer is exposed.
LAFAYETTE COUNTY: A zinc-bearing montmorillonite occurs as a yellow-brown tallow clay at the Helena-Roachdale Mine, 2 miles east of Leadmine (NE SW sec. 7 T.1N. R.2E.). Such a mineral likely occurs at other localities throughout the upper Mississippi Valley zinc-lead district. (Heyl, et al., 1959).
MARATHON COUNTY: Montmorillonite is a constituent of the clay fill in pockets in the pegmatites of the Wausau Pluton (Falster, 1987).