Dioptase

Dioptase


Composition: Cu6Si6O18.6H2O
Crystal System : Trigonal
Colours: Emerald-green, blue-green
Lustre : Vireous, sub-adamantine
Hardness (H) : 5
Specific Gravity (S.G) : 3.28-3.35
References: Cook (2002a)

Distribution

Common. However, very rare in the first oxidation zone (Cusanus, 1938; Strunz et al., 1958; Bartelke, 1976); abundant in the second oxidation zone (notably 29 through 32 levels); locally significant in the third oxidation zone (Gebhard, 1999).

Occurrence

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

Notable Finds

30 Level: ".... miners encountered numerous pockets, some very large, that contained thousands of specimens of breathtakingly beautiful, dark emerald-green dioptase." (Key, 1996). 32 Level, Zero Stope: Some of the best dioptase on contrasting white calcite matrix (Clive King, 2014, private communication to M.Southwood). 33 Level: Dioptase supplanted by gemmy crystals of cerussite, both on white calcite, discovered in 1975 (Key, 1977). In the third oxidation zone, on 44 Level, with malachite and conichalcite (Gebhard, 1999). Also in the third oxidation zone with wulfenite (Gebhard, op.cit.).

Paragenetic and General Notes

The absence of dioptase from early collections is notable, and demonstrates that dioptase was either absent or exceedingly rare in the first oxidation zone. Strunz et al. (1958) and Bartelke (1976) list dioptase as a first oxidation zone mineral, but the potential for early specimens attributed to Tsumeb to have originated, in fact, from the nearby Guchab mine, leaves room for doubt. Tsumeb mine captain Willem Klein (in Cusanus, 1938) notes that dioptase occurred "extremely rarely", and only down to 100 m from surface.

From the deeper levels (i.e. second and third oxidation zones), American dealer Charles Key estimates that the value of dioptase specimens recovered from Tsumeb exceeds the value of all of the other specimen minerals combined (Key, 1996).

Keller placed dioptase in one of the subdivisions of his “Type I” paragenetic sequences (i.e. sequences forming at relatively high pH values) as follows:

Dolomite >> duftite >> plancheite >> dioptase(i) >> calcite >> dioptase (ii)