Mimetite from Tsumeb, Namibia. Specimens and photo by – J de Jongh


Crystal system:            Hexagonal                                 Hardness:        3,5 - 4
Density:                        7,1 – 7,24                                  Cleavage:         Imperfect
Streak:                         White                                         Colour:                         White, yellow, orange, brown, red.
Occurrence:                  A secondary mineral found in the oxidized zones of lead deposits and in other settings   where lead and arsenic occur together.
Habitat:                        Crystals are usually found as small, slender prismatic crystals or larger, stubby hexagonal crystals, often plain but may be modified. Large crystals may be hollowed out at the ends.
Composition:               Lead arsenatePb5(AsO4)3Cl
 
The mineral name mimetite is derived from the Greek word mimethes, meaning imitator. The name is a reference to mimetite’s resemblance to the mineral pyromorphite. This resemblance is not coincidental, since mimetite forms a mineral series with pyromorphite and with vanadinite. 
 
It was first discovered in 1832 at the Treue Freundschaft mine, Johanngeorgestadt, Saxony, Germany and is described as a valid species since 1835. 
 
World-wide localities include Germany, Czech Republic, England, Australia, Mexico, U.S.A, China and of course the Tsumeb mine in Namibia.
 
It is not known when mimetite was first discovered at Tsumeb. The first reports on Tsumeb only mention the phosphate-analogue, pyromorphite. All specimens that were initially labeled as pyromorphite were later analysed and found to actually be mimetite. Up to now only one specimen of pyromorphite is known to exit from Tsumeb and this single unique specimen was from the third oxidation zone.
 
The initial discoveries of mimetite during the 1920s drew the attention of all the collectors around the world to this mineral from Tsumeb. Previously, the finest specimens were known from Johanngeorgenstadt, Germany. The quality of these crystals was, however, surpassed by the large yellow-green Tsumeb mimetite crystals measuring up to 6 cm. The finest crystals were found at a depth of 300 m. These early mimetite specimens from the first oxidation zone can be recognized by their flat terminations, often coated by a reddish mineral, probably goethite. Pyramidal terminations, however, have also occurred. These early specimens were altered into green-yellow bayldonite, olive-green duftite or emerald-green arsentsumebite. These pseudomorphs are the classic specimens from Tsumeb. By measuring fragments of these pseudomorphs it can be presumed that mimetite crystals measuring up to 30 cm in length have existed in Tsumeb.
 
In 1971 the world famous “Gem Pocket“, containing approximately 30 mimetite specimens, was discovered. Gemmy, pale lemon-yellow prisms up to 6 cm and some doubly terminated 5 cm crystals on matrix, were recovered from the second oxidation zone. Most of these crystals measured less than 2 cm but were of excellent quality and clarity, which set a new standard for the species. JDJ
References
Gebhard, Georg, (1999) – Tsumeb II.