Mimetite

Mimetite


Composition: Pb5(AsO4)3Cl
Crystal System : Hexagonal; Monoclinic
Colours: Colourless, pale-yellow, yellow-white, greenish-yellow, grey
Lustre : Resinous
Hardness (H) : 3.5-4.0
Specific Gravity (S.G) : 7.1-7.3
References: Cook (2001)

Distribution

Common. First, second, and third oxidation zones.

Occurrence

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

Notable Finds

Abundant in the first oxidation zone but most notable as pseudomorphs and epimorphs, partly or wholly replaced by arsentsumebite (and sometimes bayldonite or duftite). The famous "Gem Pocket" was found in 1971 in the second oxidation zone, and is believed to have been located on 28 Level (?). It yielded gemmy, transparent, very pale-yellow prisms of mimetite, to 65 mm in length.

Paragenetic and General Notes

Mimetite occurs at Tsumeb in a wide variety of colours, habits and associations. Crystals are always hexagonal and terminated with combinations of the pinacoid and / or various prisms, but they vary in habit from elongate (exceptionally to 120 mm) to tabular, the latter being rather rare.

Composite crystals are common, presenting as cauliflower- or wheatsheaf-like aggregates, or as mamilary crusts.

Mimetite occurs in association with a diverse list of other minerals and it is found in small amounts in a wide range of parageneses.  Keller (1977) included mimetite in three of his "Type I" parageneses (i.e. mineral sequences forming at relatively high pH ranges):

(1)  cerussite >> (duftite(I) + malachite) >> mimetite >> cerussite >> duftite(II)

(2)  goethite >> wulfenite >> mimetite >> malachite

(3)  (willemite(I) + smithsonite(I) )  >> (mimetite (or rosasite) + duftite) >> willemite(II) >> cerussite >> smithsonite(II)

The monoclinic polytype (formerly clinomimetite) has not been reported from Tsumeb.

Mimetite is reported to form pseudomorphs after the following minerals:  cerussite (rare).

The following minerals are reported to form pseudomorphs after mimetite:  arsendescloizite (rare); arsentsumebite (common); bayldonite (common);  calcite (rare); cerussite (rare); duftite (rare); hedyphane (rare); tsumebite (rare, dubious, probably arsentsumebite); willemite (rare).