Formula: Be4Si2O7(OH)2 Orthorhombic
Description: 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).
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).