Formula: NaAlSi3O8 – CaAl2Si2O8 Triclinic
Description: Plagioclase is a family of feldspar minerals forming a solid solution that ranges, relatively continuously, from pure sodium aluminum silicate (the end member called albite) to pure calcium aluminum silicate (the end member called anorthite). Names such as andesine, labradorite and bytownite refer to progressively more calcium-rich (or anorthite-rich) compositions. All the plagioclase minerals are described together here. One reason for “lumping” them together is that not all workers have reported which type of plagioclase they have found. Another reason is that plagioclase grains are often complexly zoned with many different compositions possible within the same grain. Where the type of plagioclase has been determined, it is reported below.
The plagioclase feldspar family is the most common mineral family in the earth’s crust. It is almost ubiquitous in igneous rocks. Gabbros, basalts and anorthosites tend to have more calcium-rich plagioclase, generally labradorite. Andesites and diorite tend to have moderately sodium-rich plagioclase, usually in the andesine range. Granitic or syenite rocks tend to have more sodium-rich plagioclase. Nearly pure albite occurs in granitic pegmatites, some times as platy variety called “clevelandite”. Plagioclase does not survive extensive weathering, generally breaking down to clay minerals. Some plagioclase, however, may occur in greywacke or arkoses formed by rapid erosion and burial of material. Metamorphic rocks may be rich in plagioclase, particular gneisses and granulate and amphibolites. Nearly pure calcium-rich plagioclase may occur in some marbles. Space permits only a few of the many occurrence of plagioclase in Wisconsin to be reported below.
ASHLAND COUNTY: Plagioclase is abundant in the Mineral Lake Intrusion of the Mellen complex, associated with olivine and pyroxene. Local coarse pegmatitic layers exist, such as in a quarry in the NW NW sec. 4 T.44N. R.3W, in SE sec. 11 T.44N. R.3W., west of Mellen. Good exposures also occur in roadcuts along a gravel road in SE NE sec. 11 T.44N. R.3W. and in an old gravel pit in the NW sec. 3 T.44N. R.4W. (Klewin, et al., 1989).
— Labradorite and andesine are common in gabbro exposed in sec. 20, 21, 28 and 33 in T.45N. R.4W. (Leighton, 1954).
— Dark gray to black plagioclase showing a marked parallel orientation occurs with pyroxene and magnetite in quarries south of Loon Lake, near Mellen (SE and NE SW sec. 29 T.45N. R.2W.) (Cordua, field notes) Wilcox (1936) reports a pegmatite exposed here with coarse plagioclase grains associated with hornblende, biotite and zircon.
BAYFIELD COUNTY: Plagioclase (labradorite and andesine) is abundant in gabbros exposed throughout sec. 1 through 9, 11 through 13, and 16 through 18 T.44N. R.5W. and in sections 25, 35 and 36 T.45N. R.5W. Large plagioclase crystals occur in gabbroic pegmatites and porphyries in s 1/2 sec. T.45N. R.4W. and SE sec. 11 T.44N. R.5W. (Leighton, 1954).
— Plagioclase is common as phenocrysts in the Keweenawan basalts exposed throughout the county. Generally the originally calcic plagioclase is replaced by a mixture of albite, epidote and clinozoisite as a result of low-grade burial metamorphism. Some notable localities given by Ali (1982) are: SE NE sec. 8 T.42N. R.9W.; SW NW sec. 15 T.44N. R.7W.; SW NE sec. 21 T.44N. R.9W.; and NW NW sec. 12 T.44N. R.9W.
CHIPPEWA COUNTY: Plagioclase is abundant in the rocks below Wissota Dam in T.28N., R.8W. It makes up a majority of the trondhjemite gneiss exposed there, where it is associated with quartz and biotite. It also occurs as phenocrysts up to 8 cm. long in a gabbro dike cross-cutting the gneisses (UW Eau Claire, 1977).
DOUGLAS COUNTY: Plagioclase is abundant in the Keweenawan basaltic lavas and gabbroic plutons exposed throughout the county. Generally the originally calcic plagioclase is replaced by a mixture of albite, epidote and clinozoisite as a result of low-grade burial metamorphism. Some typical and unusual localities are given below:
— Large red plagioclase crystals occur as phenocrysts in amygduloidal basalt along the south bank of the Moose River, sec. 2 T.44N. R.13W. (Grant, 1901).
— Plagioclase is common in the troctolite in the Amnicon pluton as exposed in the Buckley Quarry, NW sec. 32 T.48N. R. 12W and elsewhere in the Amnicon Falls area. It is associated with augite and ilmenite (Johnson and Mengel, 1969; Dickas and Mudrey, 1991).
FLORENCE COUNTY: Albite, variety clevelandite, is abundant in platy crystals up to 5 cm. across in the complex pegmatites exposed near Fern in sec. 22 and sec. 29 T.39N. R.7E. It is associated with elbaite, lepidolite, spodumene, columbite-tantalite, lithiophyllite and many other rare minerals. (Koehler, 1989, personal communication).
— Plagioclase megacrysts up to 7 cm. long occur in the Hoskins Lake Granite, such as is exposed in the railroad cut in SW SE sec. 7 T.38N. R.20E. and in outcrop in SE SE sce. 28 T.38N. R.19E. (Sims et. al., 1992). Banks and Cain (1969) also report coarse plagioclase in the Hoskins Lake Granite outcropping in the NE sec. 24 T.38N. R.19E
— Plagioclase is an abundant component of the Dunbar Gneiss, forming megacrysts up to 2 cm. long associated with K feldspar, biotite and quartz. It is well-exposed as augen gneiss in the SE NW sec. 36 T.38N. R.18E. (Sims et al., 1992).
— Plagioclase is abundant in the Marinette Quartz Diorite with quartz and biotite. It is well exposed in the county, including in a railroad cut in NE SE sec. 18 T.38N. R.20E. ,in outcrop in SE SW sec. 33 T.38N. R.20E. and in outcrop in SE SE sec. 23 T.38N. R.19E. (Sims et al., 1992).
FOREST COUNTY: Plagioclase occurs as a relict mineral in metavolcanic country rocks at the Crandon massive sulfide deposit near Little Sand Lake (Lambe and Rowe, 1989).
IRON COUNTY: Plagioclase is abundant in the gabbroic rocks of the Potato River Intrusion of the East Mellen igneous complex. The plagioclase is commonly as “normally zoned lath-shaped to blocky grains” varies from 36% to 71% anorthite and occurs with olivine, augite, orthopyroxene, magnetite and apatite (Klewin, et al., 1989). Some typical outcrops reported by Tabet and Mangham(1978) and Klewin et. al. (1989) are: along Hwy 122 N. of Upson, NW NW sec. 32 T.46N. R.1E.; in olivine gabbro outcropping north of Upson at SW NW sec. 11 T.45N. R.1W; in cumulate layered gabbro and troctolite outcropping in SW SW sec. 2 and SW SW sec. 3 T.45N. R.1W; in olivine gabbro outcropping along a gravel road in SE SW sec. 35 T.46N. R.1W; and in quartz gabbro outcropping in SW SE sec. 34 T.46N. R.1W.
MARATHON COUNTY: Plagioclase, particularly albite is an abundant constituent of the granitic and syenitic rocks of the Stettin and Wausau areas (Patton et. al., 1989). Falster (1985, 1986,1987) reports it as abundant in colorless, pink, red or brown crystals in pockets in the pegmatite cavities, as exposed in the rotten granite quarries south of Rib Mountain. Falster et al. (2000) note that crystals can be up to 40 cm. long. Much plagioclase in the pluton is a deep red color. Albite is also an important component of the nepheline syenite in quarries and outcrops in sec. 22, 26 and 27 T.29N. R.6E. near Stettin. Here it occurs with nepheline, aegirine, zircon and K feldspar (Stobbe and Murray, 1956).
— Relict plagioclase phenocrysts up to 1 cm. long occur in metamorphosed andesites in Big Sandy Park (sec. 19 T.29N. R.9E), along the Rib River near Athens in the W 1/2 sec. 9 T.29N. R.5E. (LaBerge and Myers, 1983).
— Coarse, complexly zoned plagioclase occurs with hornblende in quartz diorite exposed in the Wisconsin River valley near Mosinee and Rib Falls (LaBerge and Myers, 1983).
— Twinned phenocrysts up to 20 cm. long of andesine rimmed by oligoclase occur in a mafic pluton exposed in the Red Rock quarry #510 (Koss Pit) in Sw sec. 2 T.27N. R.6E. (Cordua, 2004)
— Enormous crystals of plagioclase as much as 40 cm. on a side occur in a diorite pluton cross-cuting granite pegmatite and aplite in the Ladick West Quarry near the intersection of Hwy 107 and 153 near Wausau. The crystals are often compound crystals with overgrowths and contain many includions of hornblende and biotite. Occassionally one will have small vugs with acicular green amphibole crystals. (Buchholz, Tom, personal communication, 2005).
MARINETTE COUNTY: Banks and Cain (1969) report coarse plagioclase in the Hoskins Lake Granite outcropping on the west side of U.S. Highway 141 in the center of sec. 1 T.37N. R.20E.
— Plagioclase is an abundant component of the Dunbar Gneiss, forming megacrysts up to 2 cm. long associated with K feldspar, biotite and quartz. It is well-exposed in outcrops in NW NW sec. 13 and along Hwy UU in NW NW sec. 14, both in T.37N. R.18E. (Sims et al., 1992). Plagioclase is abundant in the Marinette Quartz Diorite with quartz, hornblende and biotite. It is outcrops in a number of places in the county, including NW SW sec. 20 T.37N. R.19E., NW SE sec. 10 T.37N. R.19E., SE SW sec. 25 T.37N. R.18E. in railroad cuts in the S 1/2 NE sec. 18 T.38N. R.20E. (Prinz, 1965, Sims et al., 1992).
Plagioclase is abundant in the Newington Tonalite, exposed on Highway 8 in the NW sec. 5 T.36N. R.20E. and in NW NW sec. 2 T.36N. R.19E. (Sims et al., 1992).
— Albite is a component of the granite at the Camp Five molybdenite deposit, north of Middle Islet, sec. 18 T.33N. R.20E. (Fisher, 1965).
— Albite occurs with quartz, pyroxene and carbonates in the siliceous varioles in pillow basalt at Quiver Falls on the west side of the Menomoninee River, east of Pembine (S 1/2 sec. 24 T.37N. R.21E.) (Sims, Schulz and Peterman, 1984).
POLK COUNTY: Plagioclase is abundant in the Keweenawan basaltic lavas throughout the county. Generally the originally calcic plagioclase is replaced by a mixture of albite, epidote and clinozoisite as a result of low-grade burial metamorphism.
— Plagioclase forms large pinkish phenocrysts in the Eagle Peak Flow exposed in many outcrops and small quarries in the Dresser -Interstate Park- St Croix Falls area. Some good outcrops are on the top of Eagle Peak and a nearby quarry (NW SE and SE NE SW sec. 36 T.34N R.19W) . It is also traceable through sec. 1 and 12 in T.33N. R.19W. (Cordua, 1989b).
— Plagioclase is abundant in the basalt in the Dresser Trap Rock Quarry, sec. 5 and 6 T.33N. R.18W. It is most prominent as large grains intergrown with augite in pegmatitic zones within the basalt (Cordua, 1989b).
SAUK COUNTY: Albite, largely replaced with fine sericite mica, is a common constituent of the Baxter Hollow Granite near Sampler (SW sec. 35 T.11W. R.6E. Platy clevelandite also occurs at this locality in a dike cross-cutting the granite (Gates, 1942).
SHAWANO COUNTY: Plagioclase is abundant in the Tigerton Anorthosite, a complex series of blocks enclosed by the Wolf River Batholith and exposed over a 100 sq mile area. The plagioclase is the most common mineral in the anorthosite, ranges in composition from 40- 60% anorthite and may be in crystals up to 20 cm. long. It is associated with hornblende, biotite, pyroxene, ilmenite and magnetite. The anorthosite is exposed, among other places, along the Embarass River and its tributaries in the SW SW and NW sec. 23, SE sec. 22 and SE SE sec. 36 T.27N. R.12E. Outcrops near Shepley (SW NE sec. 33 T.28N. R.11E.) contain enough ilmenite and magnetite with the plagioclase to sway a compass needle. The anorthosite also outcrops east of Homme Pond in SE SW sec. 3 T.27N. R.11E. and along Highway 45 about 2 miles north of Wittenberg. (Read and Weiss, 1962; Inst. Lake Sup. Geol., 1973; Greenberg et al., 1986)
— Albite is a common component of the complex pegmatite found along the Embarass River south of the Tigerton Dells (NE SW NE sec. 15 T.26N. R.12E). It occurs as massive material and as euhedral crystals in miarolitic cavities. It is associated with quartz, beryl, K feldspar, bertrandite and other minerals (WGNHS, 1986).
WAUPACA COUNTY: Sodic plagioclase occurs as rims on K feldspar in a spectacular Rapakivi texture throughout the rocks of the Wolf River Batholith. This is particularly well-exposed in a quarry in the Waupaca Adamellite NW NW sec. 4 T.22N. R.12E. and in outcrops SW of Big Falls (SE SE sec. 26 T.25N. R.12E. (Greenberg et al., 1986)