Formula: Ca2Al2Si3O10(OH)2 Orthorhombic
Description: Prehnite is most often found as a vein or amygdule mineral in basaltic volcanic rock subjected to low grade burial metamorphism or hydrothermal activity. It may also occur in hydrothermal veins in other rocks or in marbles or calcsilicate metamorphic rocks. All of the reported Wisconsin occurrences are in the Keweenawan basaltic lavas found in Douglas, Bayfield, Ashland and Iron Counties in the north part of the state. Here it is associated with native copper, laumontite, epidote, chlorite, quartz and calcite. The absence of prehnite in similar rocks further south in Polk and Burnett Counties suggests those rocks were buried too deeply or subjected to too high a temperature for prehnite to form.
BAYFIELD COUNTY: Prehnite is found in pits and shafts in amygdaloidal basalts at the old Montrose copper prospect near Upper Eau Claire Lake in the NW NW sec. 12 T.44N. R.9W. Associated minerals are native copper, quartz, calcite, epidote, chlorite and laumontite (Grant, 1901).
DOUGLAS COUNTY: Prehnite is found with epidote and quartz in basalts outcrops along the Brule River in NE sec. 23 T.48N. R.10W. (Sweet, 1880).
— Prehnite is found in veins cutting hydrothermally altered gabbro at the Buckley Quarry, NW sec. 32 T.48N. R.12W. near Amnicon Falls. Associated minerals include analcime, apophyllite, datolite and laumontite. (Cordua, 1991; Johnson and Mengel, 1969).
— Prehnite is found with epidote and quartz at the Wisconsin copper mine in the NE and SE sec. 2 T.47N. R.13W. (Sweet, 1880).
— Prehnite is found in veins and amygdules in basalt flows at the old Chippewa copper prospects in the SW SW sec. 3 T.47N. R.12W, north of Middle River. Associated minerals are chlorite, epidote, quartz and laumontite (Holliday, 1955).
— Prehnite is found with native copper, calcite and quartz in veins, amygdules and breccia cement at the Copper Creek Prospect (SW sec. 14 and SE sec. 15 T.47N. R.14W.). The locality is now within Pattison State Park (Grant, 1901; Dutton, 1972).
— Prehnite is found as pale green to yellow green masses and rosettes associated with quartz, calcite, chlorite, K feldspar and chalcopyrite at the South Range Quarry near the Flannagan Lookout tower (NW NW sec. 1 T.47N. R.13W.) (W. S. Cordua field notes).
— Prehnite occurs with calcite, epidote, quartz and native copper at the Fond du Lac copper prospect in the SE NE sec. 8 T.47N. R.13W, west of South Range (Grant, 1901).
— Prehnite is found with quartz, chlorite, K spar, calcite and native copper in amygduloidal porphyritic basalts along the south side of the Moose River in the NW sec. 2 T.44N. R.13W. west of Gordon. (Irving, 1883; Grant, 1901).
— Prehnite is found in amygdules with native copper along Crotty Brook near the center of sec. 30 T.44N. R.13W. (Grant, 1901).
— Prehnite is found in amygdules in basalt in roadcuts along Hwy. 35 about 1 km. south of Dairyland along the boundary between sec. 17 and 18 T.44N. R.14W. (W. S. Cordua field notes).
— White to green to pale pink radial vein fillings of prehnite are common in the amygduloidal basalt at the old Weyerhauser copper mine near Chittamo (NW SE sec. 12 T.43N. R.10W.) The prehnite encloses small clusters of epidote crystals. Other minerals found at the mine include native copper, chlorite, calcite, K feldspar and quartz. (Dutton,1972; White, 1978; Smith, 1947, W. S. Cordua field notes.)
IRON COUNTY: Prehnite occurs in veins and amygdules in basaltic rocks exposed along the Montreal River Gorge in sec. 20 and 21 T.47N. R.1E. near Saxon Falls (Irving, 1883).
WASHBURN COUNTY: Prehnite is found in brecciated and epidotized basalt at the Mudge copper exploration pits (NE sec. 5 T.42N. R.10W.) northeast of Chittamo. Associated minerals are native copper, quartz and calcite (Grant, 1901).