Smithsonite

Smithsonite


Composition: ZnCO3
Crystal System : Trigonal
Colours: Colourless, white, grey, apple-green, blue-green, turquoise-blue, rose-pink, salmon-pink, orange-red, yellow
Lustre : Vitreous, oily, pearly
Hardness (H) : 4.0-4.5
Specific Gravity (S.G) : 4.2
References: Cairncross (2010)

Distribution

Abundant. Occurs in all three oxidation zones.

Occurrence

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

Notable Finds

From the first oxidation zone white, beige-coloured, and green smithsonites were particularly abundant. Large plates of drusy green smithsonites and the epimorphic casts, or "zinkschalen" are particularly notable, and the rare parageneses in which smithsonite is associated with otavite or with tsumebite are especially desireable. In the second oxidation zone, on 28 Level for example, very aesthetic groups of modified pink rhombohedra - the classic cobaltoan smithsonites - were found (Clive King, personal communication to M.Southwood). Perhaps the best-known named pocket - the so-called "Blue Pocket" was discovered in the mid 1970s and is believed to have come from the second oxidation zone (?). In the third oxidation zone, salmon-pink and pinkish-red rhombs of "manganoan" smithsonite were discovered between 45 and 47 levels, on a matrix of massive tennantite, pyrite and germanite, and associated with crystallised chalcocite (Gebhard, 1999); apple-green smithsonites were again encountered in the third oxidation zone.

Paragenetic and General Notes

Smithsonite was comfortably the most abundant secondary zinc mineral at Tsumeb and subordinate, in volume terms, only to cerussite in the supergene assemblage (Keller, 1977).

It occurs in all three oxidation zones, and with a wide lateral footprint, occurring in cavities in the country rock well away from the locus of primary mineralisation, as well as in immediate contact with oxidising sulphides.

Smithsonite occurs in a very wide range of colours and habits, and has been recorded in association with more than 50 other secondary minerals. Individual crystals are most commonly modified rhombohedrons, but combinations of prism and rhombohedron, and scalenohedrons, are also noted. Composite crystals and aggregates are typical, and highly varied.

Varietal names, based on colour are:

(1) Cuprian smithsonite (greens and blues);

(2) Cobaltoan smithsonite (pink);

(3) Manganoan smithsonite (salmon-pink to pinkish red);

(4) Cadmian smithsonite (yellow).

These terms, strictly speaking, should be used with caution unless backed by analysis; the distinction between "cobaltoan" and "manganoan" based on the perceived shade of pink seems particularly prone to error!

Although a very common mineral, and an important component of Keller's (1977) "Type I" parageneses (i.e. mineral sequences forming at higher pH ranges), smithsonite occurs in a smaller number of assemblages than cerussite. Typically, smithsonite is found alone, or with just one or two secondary associates. Keller (op.cit.) notes the following specific parageneses:

(1) (dundasite + smithsonite(i)) >> cerussite >> smithsonite(ii).

(2) (willemite(i) + smithsonite(i)) >> (rosasite + duftite) or mimetite >> willemite(ii) >> cerussite  >> smithsonite(ii).

Smithsonite is reported to form pseudomorphs after the following minerals:  aragonite (rare); azurite (rare); calcite (rare); cerussite (rare); enargite (rare); wulfenite (rare, as epimorphs).