Formula: CaAl2Si4O12.4H2O Monoclinic
Laumontite is a member of the zeolite family and occurs most frequently in veins and amygdule fillings in basaltic rocks. The mineral forms as the rocks are subjected to heating during relatively shallow burial. Laumontite readily dehydrates to a chalky material called leonhardite. All of the known Wisconsin localities for laumontite are in the Keweenawan volcanic terrain in the northern part of the state.
ASHLAND COUNTY: Pink crystals, tentatively identified as laumontite occur along shears in anorthositic gabbro in a quarry in NW NW Sec. 4 T.44N R.3W (W. S. Cordua notes).
BAYFIELD COUNTY: Laumontite is reported in amygduloidal basalts exposed in trenches and pits at the old Montrose copper prospects in the NW NW Sec. 12 T.44N R.9W. It is here associated with prehnite, epidote, calcite, quartz, chlorite and native copper (Grant, 1901).
— Laumontite is found in amygdules with calcite, pyrite, orthoclase and agate along the Nemakagon River in Sec. 21 T.43N R.7W (Strong, 1880).
DOUGLAS COUNTY: Found “frequently” in amygdules at the North Wisconsin (a.k.a. Chippewa) copper prospect pits in the SW Sec. 3 and the NW Sec. 10 T.47N R.12W northwest of Middle River. Associated minerals are prehnite, epidote, quartz and copper (Grant, 1901; Holliday, 1955).
— Laumontite reportedly occurs in small amounts in basalts in Sec. 35 T.47N R.12W (Irving, 1883).
— Laumontite was found at the Starkweather coppers prospects in the NW NW Sec. 2 T.47N R.13W associated with copper, epidote and calcite (Grant, 1901).
— Laumontite is common in amygdules in basalts at the Percival copper prospect in the N 1/2 SW NE Sec. 27 T.48N R.10W northeast of Blueberry. Copper, epidote and calcite are associated minerals (Grant, 1901).
— Numerous veins of laumontite and calcite cut “much altered diabase” on the south bank of the Middle River along the south edge of Sec. 24 T.48N R.12W (Irving, 1883).
— Pink friable laumontite xls. occur in veins and amygdules at Copper Creek Falls associated with epidote, prehnite, copper and quartz at SE SE Sec. 15 T.47N R.14W ( Sweet, 1880; WSC. field notes).
– Thin veins of friable laumontite are found at the Buckley quarry near Amnicon Falls, NW Sec. 32 T.48N R.12W (W. S. Cordua field notes).
IRON COUNTY: Laumontite occurs in amygdules and veins with prehnite and calcite in the basalts exposed in the gorge of the Montreal River, near Saxon Falls in Sec. 20 T.47N R.1E north of Hurley (Irving, 1883, W. S. Cordua field notes).
WASHBURN COUNTY: Strong (1880) reports laumontite in amygduloidal basalt with quartz, chlorite and epidote in the SE Sec. 13 T.42N R.11W near Chittamo.