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.