Montmorillonite (Na,Ca)0.3(Al,Mg)2Si4O10(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.