Formula: Ca2(Mg,Fe)5Si8O22(OH)2 Monoclinic
Description: Actinolite is a common member of the amphibole family. It typically forms during low-grade metamorphism of mafic igneous rocks (such as basalt and gabbro) or carbonate-rich rocks such as limey mudstones.
ASHLAND COUNTY: Van Hise and Irving (1892) report actinolite widespread in slates and schists associated with iron formation of the Gogebic Range. Huber (1959) believes that at most or all of these localities the actinolite is intergrown with or mistaken for minnesotaite or stilpnomelane. In Ashland County, these rocks strike from Marengo to Mellen, continuing east into Iron County. The rocks are usually relatively fine-grained, consisting of a mixture of actinolite, magnetite, quartz specular hematite, chlorite, almandine, and graphite. The actinolite appears as small needles and occasionally radiating clusters. The actinolite may locally be coarse enough to make attractive specimens.
Some specific localities in Ashland County given by Van Hise and Irving are:
—NW 1/4 SE 1/4 Sec. 26 T.44N. R.6W. where the actinolite occurs as “long dark green radiating blades.”
—N. 1/2 Sec. 14 T.44N. R.3W. on both sides of the Bad River at Penokee Gap, west of Mellen. The rock also contains almandine crystals.
—Outcrops in the NE 1/4 Sec. 15 and SW 1/4 Sec. 17 T.44W. R.3W.
—Outcrops along the Marengo River in Sec. 23 T.44N. R.5W.
BAYFIELD COUNTY: Common as a secondary mineral forming small felted grains replacing augite and matrix in Keweenawan basalts. It can be found in outcrops in a number of localities such as SE SW Sec. 8 and SE SE Sec. 9 T.44N. R.5W., SE SE Sec. 11 T.44N. R.6W. and NE SW Sec. 32 T.45N. R.4W. (LaPoint, 1976)
CLARK COUNTY: Flattened green crystals in gneisses at the Long Branch Quarry, south of Hwy 29, west of Thorpe, may be actinolite, according to EDS data (Buchholz, 2001, personal communication).
FLORENCE COUNTY: Minute prisms and radiating clusters may be found with talc and chlorite in greenstone of the Michigamme slate in Sec. 28 T.39N. R.18E. (Dutton, 1971).
IRON COUNTY: The actinolite-magnetite-quartz slates and fine schists associated with the iron formation of the Gogebic Range continue from Ashland into Iron County (see Ashland County). Such actinolite-rich rocks can be traced from Upson to Hurley.
—Actinolite occurs with calcite, chlorite and quartz in rocks exposed along the west branch of the Montreal River in Sec. 14 T.44N. R.3W. (Van Hise and Irving, 1892).
JACKSON COUNTY: Actinolite and ferroactinolite are common in coarse elongated to radial crystals in iron formation and associated talc schist at the Jackson County Iron Mine (SW 1/4 Sec. 15 T.21N. R.3W) east of Black River Falls (Jones, 1978). The actinolite is found with other amphiboles, including cummingtonite, anthophyllite, grunerite, and hastingsitic hornblende. Jones describes the actinolite often forming intergrowths, rims and exsolution patches associated with grunerite. Some of the crystals embedded in talc may exceed 8 cm in length with clusters over 20 cm in length.
MARATHON COUNTY: Compact masses of finely fibrous actinolite was found in metavolcanic rock in the vicinity of Little Chicago along the Little Rib River. This material was briefly marketed, with little success, as nephrite jade. G.N. Hanson, state geologist at the time of the discovery, was skeptical of its economic value, describing the material as “opaque, somewhat brittle and the only hint of green is rather drab olive patches.” The rock also contained minor zoisite. (WGNHS files).
—Found with epidote replacing original minerals in pillowed metavolcanic rocks at a number of places such as near Artus Creek (NW 1/4 NE 1/4 Sec. 29 T.29N. R.6E.), south of Rozellville (NE 1/4 SE 1/4 Sec. 29 T.26N. R.4E.) and along Troy Avenue in Wausau (SE 1/4 SE 1/4 Sec. 12 T.29N. R.7E.) (LaBerge and Meyers, 1983).
—Actinolite masses up to 3 cm across occur with chlorite in gash veins cutting cataclastic tonalite in outcrops south of Marathon in the SW NW Sec. 18 T.28N. R.6E. (WGNHS outcrop descriptions.)
MARINETTE COUNTY: Common with chlorite and epidote as a fine-grained component of the Quinnesec volcanic rocks throughout the county. A typical locality is in the center of Sec. 1 T.37N. R.20E. along Hwy. 141 north of Pembine (Inst. Lake Superior Geol., 1973). Actinolite also occurs with chlorite, epidote and stilpnomelane in a large quarry in Quinnesec Group rocks at Kremlin (Depangler, 1982).
—Actinolite and ferroactinolite occur in metabasalts and iron formation associated with the Duval massive sulfide deposit, Sec. 2 and 3 T.35N. R.18E. and Sec. 28 T.36N. R.18E. (Hollister and Cummings, 1982).
ONEIDA COUNTY: Found with chlorite, chloritoid and stilpnomelane in the metamorphosed volcanic rocks with wall-rock alteration zone at the Pelican River massive sulfide deposit (Bowden, 1978).
POLK COUNTY: Actinolite is found as tiny acicular crystals intergrown with epidote, chlorite, quartz and albite in the metamorphosed basaltic rocks of Keweenawan age throughout Polk County. These are well-exposed in Interstate Park in the trap rock quarry at Dresser (sec. 5 and 6 T.33N. R.18W). (Cordua, 1989b).
PRICE COUNTY: Actinolite-tremolite is a component of the metavolcanic tuff at the Ritchie Creek massive sulfide deposit (DeMatties, 1990).
RUSK COUNTY: Actinolite is a major component of the schists associated with the Flambeau massive sulfide deposit, south of Ladysmith (sec. 9 T.34N. R.6W.). It occurs as small prismatic crystals with chlorite and sericite in schists, particularly in the footwall zone. (May, 1977).
WOOD COUNTY: Long fibrous actinolite crystals are found in the talc deposits east of Rudolph, associated with talc (var. soapstone), serpentine, brucite, magnetite, and almandine (Crowns, 1976).
—Fibrous actinolite in radiating clusters with talc and chlorite schist is found at the Something Mine (old Czaikowski property) near Milladore, in NE SW Sec. 15 T.25N. R.5E. (U.S. Bureau of Mines, 1963).
—Actinolite occurs as “sprays of fine acicular crystals in pockets in quartz-epidote-pyrite rich veins” at the Tork and Haessley Quarries, Wisconsin Rapids (Buchholz, 1996).