WGNHS
Wisconsin Geological and Natural History Survey

Bertrandite

Twinned crystal of bertrandite in cavity in granitic rock. Field of view is 0.6 mm left to right. (Specimen from Tom Buchholz, photo by Dan Behnke.)

Formula: Be4Si2O7(OH)2 Orthorhombic

Description:

Bertrandite twin, Nine Mile pluton, near Wausau, Marathon County, Wisconsin. Scanning electron microscope image. (Photo by Al Falster.)

Bertrandite is a rare mineral found in beryllium rich pegmatites and hydrothermal veins.

MARATHON COUNTY: Relatively common as small (0.5 to 7.0 mm.) colorless, white or light pink crystals in the vugs of the pegmatites of the Wausau pluton, such as granite quarries in Sec. 19 and 20 T.28N. R. 7E. Here the mineral is late stage and associated with cheralite and titanium oxides such as anatase and brookite. Some excellent twins and trillings have been found (Falster, 1987; Buchholz, Falster and Simmons, 1999; Falster, et al., 2000). Bertrandite is reported as occurring at the Koss Pit (SW sec. 2 T27N R.5E), the Ladick Quarry (sec. 19 and 20 T. 27N. R.6E.) and the Wimmer #3 pit (NW sec. 19 T.28N R7E) (Buchholz and Simons, 2002).

Bertrandite from the Nine Mile pluton near Wausau in Marathon County, Wisconsin. Scanning electron microscope image. (Photo by Al Falster.)
Bertrandite, trilling twinned crystal (a triplet of twins) from the Nine Mile pluton near Wausau in Marathon County, Wisconsin. Scanning electron microscope image. (Photo by Al Falster.)

SHAWANO COUNTY: Very small crystals of bertrandite (0.3mm maximum dimension) have been found in mariolitic cavities with quartz, albite, and K-feldspar in the complex pegmatite south of the Tigerton Dells near Tigerton (NE 1/4 SW 1/4 NE1/4 Sec. 15 T.26N. R.12E.). Beryl, phenakite, columbite-tantalite and other rare minerals occur in this pegmatite (Wisc. Geol. Nat. History Survey files, 1986).