Black polycrase-(Y) mass on granitic matrix. From Fredericksberg, Vaxholm, Uppland, Sweden. Field of view is about 1 cm. (Photo courtesy Thames Valley Minerals; unauthorized reproduction is prohibited.)
Formula: (Y,U)(Ti,Nb)2O6 Orthorhombic
Description: Polycrase-(Y) is a rare black metallic mineral found as flattened prismatic crystals or masses in chemically complex pegmatites.
MARATHON COUNTY: Small grains of polycrase (Y) occur in pegmatite at the Koss Pit several miles east of Wausau, Wisconsin (Tom Buchholz, 2002, personal communication).
Properties of minerals 