Formula: Na(Al,Fe,Li,Mg,Mn)3(Al,Cr,V,Fe)6(BO3)3Si6O18(O,OH,F)4 Hexagonal
Description:
Tourmaline is a complex group of minerals, all of which contain boron and 6 member rings of silica tetrahedra. The most common tourmaline mineral is schorl, which is coal black. Pastel colored tourmaline is often, but not always, the mineral elbaite. The exact identity of a particular tourmaline mineral requires detailed optical and chemical tests, which have not, generally, been performed on Wisconsin tourmalines. Therefore, all the tourmaline minerals are described together, with the particular mineral noted only where more definitive information exists.
Tourmaline most often forms in granitic igneous rocks, particularly pegmatites. It may survive as a heavy mineral in sediments, and has been found widely in the Paleozoic sandstones and Precambrian quartzites of Wisconsin (Ockerman, 1930; Becker, 1931; Geiger et al., 1982). It also forms during metamorphism in schists, quartzites and marbles. Schorl is found in all of these environments. Elbaite is restricted to complex granitic pegmatites, where it may form large gemmy crystals with gorgeous colors, often arranged in zones. Dravite is another tourmaline mineral found primarily as root-beer brown crystals in marbles and metamorphosed calc-silicate rocks.
ADAMS COUNTY: Tourmaline crystals 2 to 3 cm. long occur in small pegmatite dikes with quartz and K feldspar in the quarry on Hamilton Mound, SE NE sec. 36 T.20N. R.6E. (Taylor, 1983).
DODGE COUNTY: Tourmaline occurs locally in amounts of up to 10% in the pelitic rocks associated with the Waterloo Quartzite. It forms tiny blue-green prisms associated with apatite, rutile, zircon and epidote. (Geiger et al., 1982). Tourmaline is also found as euhedral colorless to blue-green crystals severl mm. long in quartz veins and pemgtitic bodies cutting cutting quartzite in the Michels Materials Waterloo Quarry, NE sec. 33 and NW sec. 34 T.9N. R.13E. Analyses show these to be close to the elbaite-schorl compostion boundary (Buchholz, 2002, 2003, 2005b).
FLORENCE COUNTY: Tourmaline is common in the many small complex pegmatites exposed in sec. 22 T.39N. R.17E. Some tourmaline is light pink to deep red elbaite (“rubellite”) crystals up to 6 cm long, locally zoned light or dark blue. Dark blue to nearly black tourmaline crystals occur in individuals and sprays with separate crystals up to 12 cm. long. The blue tourmalines may also be zoned, with light blue cores and dark blue rims. Associated minerals are lepidolite, albite (var. clevelandite), quartz, spodumene, lithiophyllite and columbite-tantalite. Black tourmaline occurs also in the schist surrounding the pegmatites (Koehler, 1989).
FOREST COUNTY: Schorl tourmaline occurs as “clumps and sprays” in several small pegmatites along state highway 55 south of Alvin. (T. Buchholz, 2003, personal communication)
IRON COUNTY: Tourmaline occurs as a widespread accessory mineral in chloritic gneiss exposed in sec. 5 T.44N. R.1W. (Van Hise and Irving, 1892).
MARATHON COUNTY: Green acicular tourmaline crystals up to 1.2 cm. long were found in a single cavity in one pegmatite in the Nine Mile pluton near Wausau. X-ray analysis showed it to be elbaite. (Falster et al., 2000). Black tourmaline is also found rarely as crystal segments up to 1 cm. long in pegamtites of this pluton. (Falster, 1987).
MARINETTE COUNTY: Tourmaline-bearing veins occur in the Hoskins Lake Granite at numerous localities. A particularly well-exposed zone is in railroad cuts north of Hwy N in the SW sec. 7 T.38N. R.20E. (Sims et al., 1984).
— Veins of tourmaline in gray or smoky quartz occurs along the contact of the Spikehorn Granite and the Quinnesec volcanics, outcropping in NW sec. 1 T.37N. R.20E. , north of Pembine. (Sims et al., 1984).
— Small sprays of black tourmaline occur with sulfides in the outcrop at Pine Rapids (AKA “LaSalle Falls”) NW SE sec. 30 T.39N. R.18E. (Schulz and LaBerge, 2003).
PIERCE COUNTY: Black to dark blue tourmaline is common in granitic boulders in the glacial drift south of River Falls. It is speculated that these may have originated from the pegmatite bodies east of Winnipeg, Ontario. Such occurrences of tourmaline should thus be widespread throughout Wisconsin (Cordua, field notes, Pete Rodewald, personal ommunication).
SAUK COUNTY: Tourmaline is found in quartz veins cutting the Baraboo Quartzite at Baxter Hollow and Pine Hollow (Gates, 1942). Recent work has determined that thse are the specific tourmaline foitite (Medaris, Fournelle and Henry, 2003)
SHAWANO COUNTY: Tourmaline occurs in the complex pegmatite exposed south of the Tigerton Dells in the NE SW NE sec. 15 T.26N. R.12E. It is here associated with blue-green K feldspar, beryl, bertrandite, phenakite and other rare minerals (WGNHS files, 1986).