Willemite

Willemite


Composition: Zn2SiO4
Crystal System : Trigonal
Colours: Colourless, grey, yellow, green, red-brown, blue, black
Lustre : Vitreous, resinous
Hardness (H) : 5.5
Specific Gravity (S.G) : 4.05-4.19
References: Gebhard (1999)

Distribution

Common. Occurs in the first, second, and third oxidation zones.

Occurrence

  • Supergene
  • 1st Oxidation Zone
  • 2nd Oxidation Zone
  • 3rd Oxidation Zone

Notable Finds

Willemite was encountered quite early in the mine's history, high in the first oxidation zone; a specimen from c. 1915 comprises willemite epimorphs after mimetite, associated with smithsonite (Gebhard, 1999). A later find of gemmy blue willemite crystals to 10 mm on white calcite established Tsumeb as an important locality for willemite, although Gebhard (op. cit.) does not record the exact location in the mine for this discovery. In the third oxidation zone, between 43 and 46 levels, lemon-yellow willemite crystals, to 10 mm, were associated with pale blue rosasite or green adamite (Gebhard, op. cit.).

Paragenetic and General Notes

Willemite occurs most commonly as colourless or white bundles of elongated crystals; it is easily overlooked or mis-identified.

Radial aggregates of willemite crystals typically present gently curved surfaces or a botryoidal habit, and are commonly green or blue-green in colour.

The red-brown variety, informally known as troostite, is also quite common at Tsumeb.

Bright yellow, cadmium-rich willemite is relatively scarce.

Willemite is reported to form pseudomorphs after the following minerals: cerussite (rare); mimetite (rare).