Willemite

Phenakite Group. Crystals prismatic, both stout and slender, terminated by rhombohedra and {0001}, to 20 cm. Commonly tiny simple hexagonal prisms or radial tufts of acicular needles outside of the New Jersey occurrences. Coarse to finely granular, disseminated; in fibrous aggregates, botryoidal, and massive. A relatively common secondary zinc silicate in many Pb-Zn or Zn deposits which contain or contained sphalerite, from which it is commonly formed upon oxidation in a siliceous environment. While phenakite is frequently found as twinned crystals, twinning in willemite is rare. Metcalf-Johansen (1977) reported willemite trillings {1010} from Illimausaq, Greenland. At least one definite trilling twin of willemite crystals is known from Franklin, New Jersey.