zincite

n.
A red to yellow-orange zinc ore, ZnO.

A mineral with composition ZnO (zinc oxide). It crystallizes in the hexagonal system with a wurtzite-type structure. Thus its principal axis is polar and different forms appear at top and bottom of crystals. Such crystals are rare and the mineral is usually massive. Its hardness is 4 and its specific gravity 5.6. The mineral has a subadamantine luster and a deep-red to orange-yellow color. Zincite is rare except at the zinc deposits at Franklin and Sterling Hill, N.J. There, associated with franklinite and willemite, it is mined as a valuable ore of zinc. See also Franklinite; Willemite; Zinc.

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ZnO
Hexagonal -- Dihexagonal pyramidal

Environment

Essentially a mineral of one locality--a metamorphosed weathered ore deposit in New Jersey.

Crystal description

Crystals rare, always pyramidal, showing interesting hexagonal hemimorphism, usually lying sideways on a fissure in a calcite vein. Also in solid masses and in calcite in rounded club-shaped masses with distinct partings.

Physical properties

Orange-yellow to deep red. Luster subadamantine; hardness 4; specific gravity 5.4-5.7; streak orange-yellow; fracture conchoidal; cleavage prismatic (plus a basal parting). Brittle; translucent to transparent.

Composition

Zinc oxide (80.3% Zn, 19.7% O). Some Mn present, except in synthetic crystals, which are usually greenish or golden.

Tests

Infusible on charcoal, but on heating the assay turns black--and with cooling regains the original color. Eventually acquires coating of yellow (hot) or white (cold) zinc oxide, which turns green in oxidizing flame after touch of cobalt nitrate solution. This test is improved by crushing and mixing with sodium carbonate. Soluble in acids.

Distinguishing characteristics

The infusibility, plus the zinc test on charcoal and solubility in nitric or hydrochloric acid, will give a positive identification. The crystals, when present, help; but the most important aspect of all is its invariable association (at Franklin, New Jersey) as a reddish orange mineral with black magnetic franklinite, greenish ultraviolet-fluorescing willemite, and white calcite.

Occurrence

This mineral would not be included here were it not once of considerable economic importance as an ore of zinc at Sterling Hill and Franklin, New Jersey, the only important locality. It occurs there in red grains and masses in a white, highly fluorescent calcite, associated with willemite and franklinite. It is the most abundant red Franklin mineral and so is always easily identified in the ore mixtures that are typical of the Franklin specimens. Minor amounts have been noted in Poland; Tuscany, Italy; Spain; and Tasmania.

Remarks

Manganese is believed to cause its red color. Synthetic crystals are light yellow to colorless to pale blue. Some form accidentally from smelter fumes, and examples were marketed as "willemite from Algeria" and still larger gemmy ones from Poland. They can be grown by hydrothermal methods and from a vapor.

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