Formula: Be2SiO4 Hexagonal
Description: Phenakite is a rare mineral found as vitreous, sometimes gemmy, crystals in beryllium-rich granite pegmatites.
MARATHON COUNTY: Phenakite occurs in pockets in pegmatite dikes in the Nine-Mile Pluton exposed in the “rotten granite” quarries near Wausau. The phenakite forms colorless, yellow or sherry colored crystals that are locally gemmy. Most crystals are a few millimeters across, but some may be as large as 3.6 cm (Falster, 1987; Falster et al., 2000). Falster (1977) describes phenakite-bearing pockets in small pegmatite dikes briefly exposed in a pit just north of Highway 10 in the town of Rib Mountain. One pocket produced over 70 crystals, both loose and perched on quartz. All of the crystals were elongated parallel to the c axis and show penetration twinning. Associated minerals were smoky quartz, pale blue beryl, siderite, goethite, anatase, biotite and microcline. Phenakite is reported as rarely found in the Koss Pit (SW sec. 2 T27N R.5E), the Red Rock Granite North Pit (along Spring Brook Road), and the Ladick Quarry (sec. 19 and 20 T. 27N. R.6E.) (Buchholz and Simons, 2002).
SHAWANO COUNTY: Phenakite occurs in masses a few millimeters across in a complex pegmatite exposed along the Embarass River south of the Tigerton Dells (NE SW NE sec. 15 T.26N. R.12E). Associated minerals include beryl, potassium feldspar, gadolinite, chlorite, and allanite (Falster, unpublished report in the WGNHS files).