Gahnite Composition,
Crystallization & Structure

A zinc spinel, ZnA1₂O₄ or ZnO.A1₂O₃, with iron and manganese isomorphous with the zinc and ferric iron with the aluminum. Isometric. Commonly octahedral, also rarely showing dodecahedrons and cubes. HY. = 7.5-8. G = 4.55. Vitreous luster. Dark green color. Infusible. The fine powder fused with sodium carbonate on charcoal gives a white nonvolatile coating of zinc oxide. A rare mineral. Found in the United States in notable crystals at Franklin, New Jersey, and Rowe, Massachusetts.