Potassium Feldspar

Pink K feldspar crystal with quartz. Wimmer Pit, near Wausau, Marathon County, WI. Crystal is about 4 cm. (Photo by W. Cordua.)

Formula: KAlSi3O8 Monoclinic or Triclinic


Potassium feldspar refers to any of the three polymorphs microcline, orthoclase or sanidine. The distinction between these minerals can be made with certainty only after detailed X-Ray analysis. This has not kept the terms from being used freely in the literature without such analyses being made. In the following listings the particular polymorph is specified only if the proper analysis appears to have been made. Otherwise the more generic term “K spar” is used.

K spar is abundant in many geologic settings in Wisconsin. It is a major component of granitic and syenitic rocks in the central and northeastern portion of the state. These make up plutons of considerable size, such as the Wolf River Batholith, Nine-Mile Granite and Stettin Pluton. Crystals of K spar may reach considerable size in the pegmatites associated with these rocks. Red or pink K spar gives the state rock – red granite – its particular color. Some of these plutons contain Rapikivi textured K spar in which large K spar phenocrysts are mantled by plagioclase. Some K spar in the syenites has a moonstone play of colors. K spar is also found widely in the schists and gneisses of the state’s Canadian Shield Province. It occurs as pink or orange crystals in vein and amygdule fillings in the Keweenawan basalts in the northwestern portion of the state. The larger related plutons contain granitic rocks in which the K spar is in granophyric intergrowth with quartz. K spar occurs as a clastic and authigenic mineral in Upper Precambrian, Cambrian and Ordovician sandstones and siltstones. Much sandstone once thought to be pure quartz has been shown to contain significant K spar. In places, whole beds are virtually entirely K spar with authigenic overgrowths on clastic grains (Odom, 1975, 1978; Odom et. al., 1979). More rarely K spar is a component of bentonitic ash interlayered with Paleozoic strata (Allen, 1932). Finally, K spar has been reported as a product of wall rock alteration in the Upper Mississippi Valley zinc-lead district (Hostermann et. al., 1964).

Only some of the more representative or unusual of the state’s many occurrence of K spar are described below.

ADAMS COUNTY: “Bright orange-red phenocrysts” of K spar occur in granite exposed in the quarry on Hamilton Mound (SE NE sec. 36 T.20N. R.6E). (Greenberg et. al., 1986).

ASHLAND COUNTY: Both orthoclase and microcline are reported as abundant components of the Mellen Granite exposed widely in sec. 20-30 and 32-36 T.45N. R.3W. Crystals up to 3 cm long are found, some rimmed with plagioclase (Katzman, 1968).
— Granophyric intergrowths of red K spar and quartz are common in the upper portions of the Mineral Lake Intrusion of the Mellen Complex. A spectacular outcrop occurs at Morgan Falls (NW sec. 30 T.45N. R.4W.) where a 100 foot high waterfall cascades over the red granophyre (Klewin et al., 1989).

– Biotite occurs in granite pegmatites exposed in 2001 during road construction along State Hwy. 13 just north of its intersection with State Hwy. 169 (Sikkila, 2002).

CHIPPEWA COUNTY: Coarse pink to white Kspar is present in numerous pegmatite dikes as well as in the gneissic country rock along the Chippewa River at Jim Falls (sec. 30 T.30N. R.7W.) (Myers et al., 1974).

CLARK COUNTY: Microcline as augen clasts up to 5 cm. long occur in the Neillsville Gneiss exposed along the Black River in the NW NW sec. 26 T.24N. R.3W. (Maas and Van Schmus, 1981).
— K spar occurs in a sheared granite west of Christy in T.25N. R.2W. (Gore, 1968).

DODGE COUNTY: Potassium feldspar, largely altered to clay minerals occurs in pegamtites cutting quartzite in the Michal Materials Quarry, sec. 33 and 34, T.9N R.13E. (Brown et al, 2001; Buchholz et al., 2002).

DOUGLAS COUNTY: Coarse pink K spar occurs with prehnite, quartz, epidote and other minerals in the Weyerhauser mine dumps near Chittamo (NW SE sec. 12 T.43N. R.10W. (Smith, 1947; White, 1978).
— Cleavable masses of pink and orange K spar occur in amygdules with quartz, epidote, chlorite and chalcopyrite at the South Range quarry (NW NW sec. 1 T.47N. R.3W.) (Cordua, 1990).
— K spar occurs with chlorite, quartz and prehnite in the NW sec. 2 T.44N. R.13W. (Irving, 1883).

EAU CLAIRE COUNTY: K spar is common in granite and pegmatite where County K crosses the Eau Claire River at Little Falls (SW NW sec. 19 T.27N. R.8W.) (Myers et al., 1974).

FLORENCE COUNTY: K spar is abundant in the Hoskins Lake Granite where it may form megacrysts up to 7 cm. long. A good outcrop can be found in a railroad cut in the SW SE sec. 7 T.38N. R.20E. (Sims et al., 1992; Banks and Cain, 1969).
— K spar occurs abundantly in the Bush Lake Granite such as is exposed in the NW NW sec. 13 T.38N. R.17E and SE SE sec 18 T.38N. R.18E. (Sims et al., 1992).
— Both orthoclase and microcline are abundant in the complex zoned granitic pegmatites in sec. 22 T.39N. R.17E. in Fern Township. Many rare minerals such as elbaite, spodumene and lepidolite are associated. (Koehler, 1989 and personal communication).

IRON COUNTY: “Spectacular radiating sheaves of intergrown quartz and alkali feldspar” occur in the granophyre rocks near the top of the Potato River Intrusion of the Mellen Complex. Good outcrops are plentiful north of Upson. (Klewin et. al., 1989).
– Granophyric intergrowths of K spar and quartz occur in the igneous rocks of the East Mellen Complex, as in the NE NW sec. 35 T.46N. R.1W. and the NE NE sec. 16 T.46N. R.1E. (Tabet and Mangham, 1978).
— Pink K spar occurs in amygdules with calcite and prehnite in Keweenawan basalts exposed along the Montreal River in sec. 20 and 21 T.47N. R.1E. near Saxon Falls (Irving, 1883).

JACKSON COUNTY: K spar occurs in gneisses outcropping below the dam at Arbutus Lake (Gore, 1968).
— K spar occurs abundantly in the granitic rocks exposed along the Black River in downtown Black River Falls (Gore, 1968).

JUNEAU COUNTY: Small euhedral xls. of K spar occur in quartz veins cutting quartzite breccia in the quarries on Necedah Bluff (NE sec. 24 T.18N. R.3E. and NW sec. 19 T.24N. R.4E. (Greenberg et al., 1986).

LAFAYETTE COUNTY: K spar is reported as a wall rock alteration product at the Thompson-Temperly zinc mine in New Diggings (Hosterman et al., 1964).

MARATHON COUNTY: K spar in abundant in the Nine Mile Pluton’s granitic rocks and cross-cutting pegmatites exposed in the many “rotten granite” quarries south of Rib Mountain. Crystals are perthitic, many be up to 30 cm. across and are colored white, gray, tan, flesh or rarely green (amazonite) (Falster, 1981, 1984, 1985, 1986, 1987, falster et al., 2000) Buchholz, (1999a). Martin and Falster (1986) verified the presence of high sanidine in the miarolitic cavities in the pegmatites. They speculate that its precipitation and persistence in the vugs was due to quenching and sudden decompression in an epizonal environment. The vugs were subsequently resealed so that recrystallization or reaction of the sanidine with later fluids could not occur.
— Coarse microperthitic K spar (anorthoclase) showing a schiller “moonstone” occurs in a quarry in pyroxene syenite in the Stettin pluton just north of Stettin Road in the SE sec. 14 T.29N. R.6E. Xls. up to 35 cm. long occur in this pluton. At last report, the quarry is water-filled, hazardous and posted “No Trespassing”. It should be avoided unless permission is granted by the owner. (LaBerge and Myers, 1983; Myers et al., 1984 Falster et al., 2000).
— Microperthitic K spar is common at many other outcrops of the syenitic rocks of the Stettin complex, such as at the old mines in sec. 22 and adjacent parts of sec. 26 and 27 T.29N. R.6E. Here they occur with a host of minerals including nepheline, zircon, fluorite, aegirine, eudialyte, pyrochlore, rutile and riebeckite. (Weidman, 1907; Stobbe and Murray, 1956; Sood et. al., 1980).
— Microperthitic K spar occurs in syenite and nepheline syenite exposed in outcrop and roadcuts along County Highway U 1 km. east of the Little Rib River in the SW SW sec. 18 T.29N. R.7E. (Myers, et al., 1987).
— K spar is abundant as tan grains several cm. long in quartz syenite well exposed in roadcuts and outcrops near the Employer’s Mutual Insurance Company offices near the intersection of Hwy 51 and Hwy 29, NW SE sec. 27 T.29N. R.7E. (LaBerge and Myers, 1983).
— Coarse microcline, locally in graphic intergrowths with quartz is found in leucogranite east of Rothschild (LaBerge and Myers, 1983).
— K spar is abundant in the quartz monzonite exposed near Unity where it occurs in xls. up to 2 cm. long. It can be found in Cherokee County Park (sec. 14 T.28N. R.2E.) and in old quarries in sec. 1, 2 and 3 T.27N. R.2E. and sec. 35 and 36 T.28N. R.2E. (LaBerge and Myers, 1983).
— Perthitic microcline is conspicuous in the “Wisconsin Ruby Red Granite” exposed in places such as Granite Heights (NW sec. 26 T.30N. R.7E), near Edgar and Wein (NE sec. 21 T.28N. R.4E.), east of Stratford (sec. 26 T.27N. R.4E.) and elsewhere. (LaBerge and Myers, 1983).
— Unusual micrographic intergrowths of quartz and K spar occur in granitic rocks exposed in a roadcut on Hwy 51 just north of the Mosinee Hwy 155 interchange. (LaBerge and Weis, 1968).

MARINETTE COUNTY: Pink perthitic microcline is abundant in the Athelstane quartz monzonite. The rock is well exposed in the NW SW sec. 10 T.35N. R.20E. (Institute on Lake Superior Geology, 1973).
— Megacrysts of perthitic K spar up to 4 cm. long occur in the Hoskins Lake Granite exposed at many spots in the county. One accessible outcrop in on the west side of Hwy 141 in the center of sec. 1 T.37N. R.20E. (Banks and Cain, 1969).
— Perthitic K spar grains up to 3 cm. long with local Rapakivi overgrowths occur in the Amberg granite exposed in the large quarry near Middle Inlet (NE sec. 30 T.33N. R.20E). Associated minerals are plagioclase, quartz, epidote, clinozoisite, biotite and zircon. (Banks and Cain, 1969).
— K spar occurs both in the granitic country rock and as tiny crystals in cross-cutting veins at the Camp Five Molybdenite Prospect in sec. 18 T.33N. R.20E. (Fisher, 1965; Greenberg, 1985).

MENOMONEE COUNTY: K spar xls. up to 3 cm. long, often with local Rapakivi overgrowths are abundant in the Wolf River Quartz Monzonite. Typical outcrops are at Beartrap Falls (sec. 4 T.28N. R.15E.) and at Ducknest Falls (sec. 27 T.30N. R.15E.). (Institute on Lake Superior Geology, 1973).

PIERCE COUNTY: K feldspar forms abundant authigenic overgrowths on detrital K feldspar grains in the Washington Road Sandstone within the Rock Elm disturbance (Cordua, 1987).

POLK COUNTY: Pink cleavable masses are found with quartz, epidote, and chlorite in numerous outcrops of Keweenawan basalt flows in the Dresser – St. Croix Falls area. It is notable at the Dresser Trap Rock Quarry in sec. 5 and 6 T.33N. R.18W, where it occurs as pink druses on quartz in massive quartz veins. (Cordua, 1989b). Other notable nearby localities are in the old quarries in Interstate Park in SW sec. 36 T.34N. R 19W. and along the pipeline in SW sec. 12 T.33N. R.19W. (W. S. Cordua, field notes).

PRICE COUNTY: Coarse K feldspar occurs in granite pegmatites scattered through sec. 27, 31, 32 and 33 near Fifield (Hotchkiss, 1915). – K feldspar occurs in large grains in pegmatites cutting amphibolite along the east shore of Blockhouse Lake, east of Park Falls in sec. 11, 15 and 16 T.40N. R.1E. (Hotchkiss, 1915).
— K feldspar occurs in pegmatites near Park Falls in T. 40 N. R.1W. in the S 1/2 SE sec. 15, SW sec. 13 and SW SW sec. 35. (Hotchkiss, 1915).
— K feldspar xls. up to 1 inch in diameter occur in pink biotite granite outcropping on the section line between sec. 16 and 17 T.38N. R.2E., northeast of Phillips (Hotchkiss, 1915).

ROCK COUNTY: Euhedral authigenic monoclinic K feldspar is found in the St. Peter Formation in the Silica Sand quarry near Hanover (Odom, Willard and Lassin, 1979).

RUSK COUNTY: Euhedral clear to white orthoclase crystals occur in the carbonate zone of the Flambeau Copper Mine, near Ladtsmith (Jone, jones and LaBerge, 1999)

SAUK COUNTY: K feldspar is a common constituent of the Baxter Hollow Granite near Sampler in SW sec. 25 T.11W. R.6E. (Gates, 1942).

SAWYER COUNTY: K feldspar crystals up to 1 inch in diameter are common in gneisses along the Chippewa River east of Radison in sec. 12 and 13 T.38N. R.7W. (Hotchkiss, 1915).
— Coarse microcline occurs in hornblende granodiorite along the river channel downstream from Arpin Dam in the NW SE Sec. 23 T.38N. R.7W. (Mudrey, 1979).

SHAWANO COUNTY: K feldspar is common in the coarse pegmatite exposed south of the Tigerton Dells in the NE SW NE sec. 15 T.26N. R.12E. It often has a light blue-green color, and is associated with beryl, bertrandite, phenakite and other rare minerals. (WGNHS files, 1986)
— K feldspar grains up to 2 cm.. long are common in the Red River porphyritic quartz monzonite of the Wolf River Batholith. A typical locality is in the SW SE sec. 2 T.27N. R.14E. near Gresham. (Institute on Lake Superior Geology, 1973).
— Pink K feldspar occurs in granite by the Hwy 45 overpass of the Embarrass River, in Tigerton. (Carl Mauritz, 2003, personal communication).

WAUPACA COUNTY: Coarse K feldspar with excellent Rapakivi texture occurs in the granite quarry in NW NW sec. 4 T.22N. R.12E. about 6 km. north of Waupaca. Similar material is found along the Little Wolf River in SE SE sec. 26 T.25N. R.12E. (Greenberg, et al., 1986).
– Microcline occurs in granite, granite pegmatites and small miarolitic cavities at Cactus Rock, about 3 miles south of New London, SE SE sec. 26 T.22N. R.14E. (UW Oshkosh, 1989).
— Coarse potassium feldpsaris common in road cuts in pegmatite cutting granite along County G near Keller Lake, NE of Big Falls. Associated minerals include euxenite-(Y), biotite, fluorite, bastnaesite group minerals, allanite and various sulfides. The K feldspar fluoresces deep red under shortwave ultraviolet light. (Tom Buchholz, 2002, personal communication)

WOOD COUNTY: K spar is abundant in granite in the Frederick Schill Quarry northeast of Vesper. K spar can also be found as tiny euhedral crystals in veins cross-cutting the granite. (Buchholz, 1999).
— Small xls. occur in quartz veins cutting granite in the quarry on the east side of Highway B, sec. 25 T.24N. r.2E. It is also a major component of the granite in the quarry. (Buchholz,1994 personal communication).
— Occurs as excellent micrographic intergrowths with quartz in the granites around the Cary Rock area, including the John Cepress quarry (sec. 11 T.23N R. 2E), Wood County Quarry (NW sec. 1 T23N R2E) and Cary Rock Road Quarry NE sec. 34 T23N R2E).
— Occurs as excellent micrographic intergrowths with quartz in the granite of the Haske Quarry, (SW SE sec. 25 T.24N. R.2E.). It is also found as small xls. in vugs. (Buchholz et al., 1998)