Mimetite

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Mimetite
Mimetite-139756.jpg
General
CategoryArsenate minerals
Apatite group
Formula
(repeating unit)
Pb5(AsO4)3Cl
Strunz classification8.BN.05
Crystal systemHexagonal
Crystal classDipyramidal (6/m)
(same H-M symbol)
Space groupP63/m
Unit cella = 10.250(2) Å,
c = 7.454(1) Å; Z = 2
Identification
ColorPale to bright yellow, yellowish brown, yellow-orange, white, may be colorless
Crystal habitPrismatic to acicular crystals; reniform, botryoidal, globular,
TwinningRare on {1122}
Cleavage[1011] Imperfect
FractureBrittle, conchoidal
TenacityBrittle
Mohs scale hardness3.5 - 4
LusterResinous, subadamantine
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity7.1 - 7.24
Optical propertiesUniaxial (-), anomalously biaxial
Refractive indexnω = 2.147 nε = 2.128
Birefringence0.019
PleochroismWeak
References[1][2][3]

Mimetite, whose name derives from the Greek Μιμητής mimetes, meaning "imitator", is a lead arsenate chloride mineral (Pb5(AsO4)3Cl) which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. The name is a reference to mimetite's resemblance to the mineral pyromorphite. This resemblance is not coincidental, as mimetite forms a mineral series with pyromorphite (Pb5(PO4)3Cl) and with vanadinite (Pb5(VO4)3Cl). Notable occurrences are Mapimi, Durango, Mexico and Tsumeb, Namibia.

Uses of mimetite[edit]

Mimetite from Namibia

Industrially, mimetite is a minor ore of lead. The chief use of mimetite is as a collector's specimen, often creating attractive botryoidal crusts on the surface of the specimen. Though mimetite is also found in prismatic crystal forms, it is not used as a gemstone due to its softness. Quality prismatic forms have been found in Johanngeorgenstadt in Saxony and Wheal Unity at Gwennap in Cornwall, England.

Associated minerals[edit]

Mimetite is found in association with lead and arsenic minerals, including those minerals with which it forms a series. Some associated minerals include: calcite, galena, pyromorphite, smithsonite, vanadinite, and wulfenite.

Alternative names[edit]

Alternative names of mimetite include arsenopyromorphite, mimetesite, and prixite. Campylite is the name for a variety with barrel shaped crystals of a brownish-red or orange-yellow color and containing a considerable proportion of phosphoric acid.

Notes for identification[edit]

Useful information pertaining to the field identification of mimetite include its habit, color, and high density. However, this mineral's similarity to pyromorphite can be problematic, especially since these minerals are known to share colors. Pyromorphite is typically green, and mimetite is typically yellow, but specimens of each are known in the other's colors. As a result, some identification may require lab analysis.

Mimetite specimen gallery[edit]

References[edit]

External links[edit]

  • "Mimetite" . Encyclopædia Britannica. 18 (11th ed.). 1911.
  • Mineral Galleries