Mimetite
| Mimetite | |
|---|---|
Mimetite, Bilbao mine, Zacatecas, Mexico. Size: 7.9 x 5.8 x 3.4 cm
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| 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(PO4)3Cl) and with vanadinite (Pb5(VO4)3Cl). Notable occurrences are Mapimi, Durango, Mexico and Tsumeb, 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.
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.
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