Mimetite: Properties and Occurrences

Mimetite is a lead arsenate and chloride isomorphous with pyromorphite. It is a yellow or brown mineral consisting of a chloride and arsenate of lead, typically found as a crust or needle-like crystals in lead deposits. Its color ranges from brown to olive green, yellow, or orange. Its 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

Mimetite is a member of the Apatite group, a group of isomorphous hexagonal minerals. The name is a reference to mimetite’s resemblance to the mineral pyromorphite. It is very similar in structure and appearance to Pyromorphite, and maybe partially replaced by it.

General Information

  • Category: Arsenate minerals (Apatite group)
  • Formula: (repeating unit) [Pb5(AsO4)3Cl]
  • Crystal system: Hexagonal
  • Crystal class: Dipyramidal (6/m) (same H-M symbol)
  • Color: Pale to bright yellow, yellowish-brown, yellow-orange, white, maybe colorless.

Fig: Mimetite – a member of Apatite group

Properties

It greatly resembles pyromorphite (q.v.), in which phosphorus replaces arsenic in the crystal structure; mimetite is similar in physical characteristics and mode of occurrence but is less common. Transparent mimetite crystals are extremely rare, and very few have ever been faceted. It also forms a continuous solid-solution series with vanadinite in which vanadium replaces mimetite’s arsenic in the crystal structure.

  • 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

Occurrences: A common secondary mineral in the oxidized zone of arsenic-bearing lead deposits.

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. Very fine specimens of Mimetite have come out of Mexico and Namibia, although it can also be found in Australia, Greece, and the USA.

Uses

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.

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.