Mimetite

Mimetite
Golden yellow botryoidal mimetite from Congreso-León mines, Mexico
General
Category	Arsenate minerals Apatite group
Formula (repeating unit)	Pb5(AsO4)3Cl
IMA symbol	Mim[1]
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	0.019
Pleochroism	Weak
References	[2][3][4]

Mimetite 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 derives from the Greek Μιμητής mimetes, meaning "imitator" and refers 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.

Mimetite is a lead chloride arsenate mineral with the composition Pb₅(AsO₄)₃Cl. It is a secondary mineral, formed by oxidation of primary lead minerals in arsenic-bearing lead deposits.^[2] It typically forms short hexagonal crystals that are yellow to brown to orange in color, very brittle, moderately hard (Mohs hardness 3.5–4), and dense (specific gravity 7.24). It is distinctive for its lack of transparency, its resinous to adamantine luster, and its solubility in nitric acid.^[5]

Mimetite forms a complete solid solution series with pyromorphite, with phosphate (PO³⁻
₄) substituting for arsenate (AsO³⁻
₄). The two minerals are almost identical in their properties and can be difficult to distinguish except by laboratory tests. Pyromorphite is the more common mineral at most locales.^[5]

Campylite is a name applied to mimetite or pyromorphite that crystallizes as distinctive barrel-shaped crystals forming curved hemispherical aggregates.^[5] Bellite is a name formerly applied to a chromium-bearing mimetite, or possibly a mixture of crocoite, mimetite and quartz, which forms attractive orange red crystals, but has been discredited as a distinct mineral species.^[6]

Mimetite is found in association with lead and arsenic minerals, including pyromorphite, cerussite, hemimorphite, smithsonite, vanadinite, anglesite, pyrite, mottramite, willemite, and wulfenite.^[5][2] Good specimens have been reported from Gila County, Arizona, US; Ojocaliente, Zacatecas, Mexico; Cumberland, England; Johanngeorgenstadt, Saxony, Germany; Namibia; and Broken Hill, Australia.^[5]

• 
  Mimetite from Tsumeb Mine, Namibia
• 
  Cluster of translucent, golden mimetite crystals
• 
  Spherical aggregates of botryoidal mimetite
• 
  Mimetite, Pingtouling Mine, Guangdong Province, China. Size: 2.2 x 2.1 x 1.8 cm

Wikimedia Commons has media related to Mimetite.

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