Mimetite

Mimetite
Mimetite
General
Category	Arsenate minerals ; Apatite group
Formula; (repeating unit)	Pb5(AsO4)3Cl
Strunz classification	8.BN.05
Crystal system	Hexagonal
Crystal class	Dipyramidal (6/m) ; (same H-M symbol)
Space group	P63/m
Unit cell	a = 10.250(2) Å, ; c = 7.454(1) Å; Z = 2
Identification
Color	Pale to bright yellow, yellowish brown, yellow-orange, white, may be colorless
Crystal habit	Prismatic to acicular crystals; reniform, botryoidal, globular,
Twinning	Rare on {1122}
Cleavage	[1011] Imperfect
Fracture	Brittle, conchoidal
Tenacity	Brittle
Mohs scale hardness	3.5 - 4
Luster	Resinous, subadamantine
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	7.1 - 7.24
Optical properties	Uniaxial (-), anomalously biaxial
Refractive index	nω = 2.147 nε = 2.128
Birefringence	δ = 0.019
Pleochroism	Weak
References

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(Template:Phosphorus Template:Oxygen₄)₃Template:Chlorine) and with vanadinite (Pb₅(Template:Vanadium Template:Oxygen₄)₃Template:Chlorine). Notable occurrences are Mapimi, Durango, Mexico and Tsumeb, Namibia.

Uses of mimetite

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

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

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

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.