Phenacite Group – Willemite Composition, Crystallization & Structure
Composition. Zinc orthosilieate, Zn2Si04 = Silicea 27, zinc oxide 73; zinc = 58.6. Manganese often replaces a considerable part of the zinc (manganiferoús variety called troostite), iron also present at times in small amount.
Crystallization. Hexagonal-rhombohedral; tri-rhombohedral. In hexagonal prisms with rhombohedral terminations. Faces of third-order rhombohedrons rare.
Structure. Usually massive to granular. Rarely crystallized except in variety troostitc.
Physical Properties. H. = 5.5. G. = 3.89-4.18. Vitreous to resinous luster. Color white, yellow-green, blue, when pure; with increase of manganese becomes apple-green, flesh-red and brown. Transparent to opaque.
Tests. Willeniite infusible, troostite difficultly fusible (4.5-5). Soluble in hydrochloric acid and yields gelatinous silica on evaporation. Gives a coating of zinc oxide when heated with sodium carbonate on charcoal; coating yellow when hot, white when cold; if coating is moistened with cobalt nitrate and heated again it turns green. Troostite will give reddish violet color to the borax bead in O. F. (manganese).
Varieties. Ordinary. White or light colored.
Troostite. Apple-green, flesh-red or gray color. Contains a considerable amount of manganese. Found at Franklin Furnace, New Jersey, in quite large crystals.
Occurrence. Found at Altenberg, near Moresnet, Belgium, and at Franklin Furnace, New Jersey. At the latter locality it is associated with franklinite and zincite, often in an intimate mixture; also embedded in calcite. Also from Algeria, French Congo, Northern Rhodesia, South West Africa, Greenland. Occurs sparingly at Merritt Mine, New Mexico.
Use. A valuable zinc ore.
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