> willemite

Willemite

Formula: Zn2SiO4
Crystal system: Hexagonal Space group: R3

Overview

Willemite (green) with minute rod-like inclusions of franklinite. Franklin, NJ. Width 1 cm.

Willemite is an abundant component of the Zn-Mn-Fe orebodies at both Franklin and Sterling Hill, and was mined in great quantities for its zinc content.

Willemite fluoresces a brilliant yellow-green under short-wave ultraviolet light, and this property was exploited by New Jersey Zinc Company workers monitoring ore concentrates, and involved in spot-assaying or surveying of underground workings.

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Willemite. Massive, gemmy green specimen. Franklin, NJ. Width 8 cm.

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Willemite fluorescence. Left, natural light; right, fluorescing under short-wave UV. Franklin, NJ. Width 16 cm.

Willemite is a silicate of zinc with the same structure as phenakite, a rare beryllium mineral. 

Zinc and silicon tetrahedra share corners to form hollow 'tubes' parallel to [0001]. The mineral crystallizes in the hexagonal system with rhombohedral symmetry. 

Most crystals are elongate on [0001]; some microscopic crystals exceptionally so, forming fine hair-like growths. 

         
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Willemite crystal structure. Coordination polyhedra (and a few atoms) shown. Left, views normal to c-axis. Right, viewed down c-axis. Zinc filled tetrahedra are green; silicon filled tetrahedra are blue.

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Willemite. Typical forms present on microcrystals. Crystals are almost always elongate on [0001]. See figure 118 in Palache (1935).

Willemite is found in grains, massive, and in crystals. Crude, opaque crystals many centimeters in length have been found.

Smaller crystals formed in open late-stage vugs are often exquisitely formed, are well described in Palache (1935), and make superb SEM study material.

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Crystal drawings of willemite from Palache (1935).

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SEM view of a willemite crystal. Franklin, NJ. Width 0.1 mm.

Willemite crystals in vug. Franklin, NJ. Width 0.6 mm.

Willemite is most commonly green, but it has an extremely diverse color range: white, yellow, red, orange, brown, black and blue varieties are known from the area.

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Willemite. Small green and yellow gemmy samples. Franklin, NJ. Width 5 cm.

Red willemite. Granular ore sample, with minor franklinite (black). Sterling Hill, NJ. Width 10 cm.

Some of this color variety is due to the presence of microscopic inclusions; nearly all Franklin-Sterling willemite makes fascinating material for study under the microscope.

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Willemite. Close-up view of a polished slab of massive, gemmy green willemite. Franklin, NJ. Width 4 cm.

Be sure to also visit the fluorescence pages for willemite. For further information on willemite, consult Dunn (1995).

'Radiating' willemite. Classic sample of white willemite in radial aggregates of thin needles. Franklin, NJ. Width 12 cm. Fluorescing (green) view under shortwave UV light.

 

 

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