Mimetite
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 | |
|---|---|
Golden yellow botryoidal mimetite from Congreso-León mines, Mexico | |
| General | |
| Category | Arsenate minerals Apatite group |
| Formula (repeating unit) | Pb5(AsO4)3Cl |
| IMA symbol | Mim |
| 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 | |
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