Mimetite, a lead arsenate chloride mineral forms as a secondary mineral in lead deposits, typically by oxidation of galena and arsenopyrite. The name is a pointer to its semblance to mineral Pyromorphite. This similarity is not unintentional, since Mimetite forms a mineral series with vanadinite (Pb5 (VO4) 3Cl), and Pyromorphite (Pb5 (PO4) 3Cl).
Pb5 (AsO4) 3Cl
Derivation of Name
Mimetite derives its name from the Greek Μιμητής, or mimetes, meaning "imitator".
It is widespread, occurring as a secondary mineral in the oxidation zone of deposits of lead rich ores. Crystal size varies from millimetric to centimetrix and is generally solid and hexagonal in shape with pinacoids, but regularly with pyramidal terminations and frequently barrel-shaped to globular. Its resemblance to pyromorphite (q.v.) is significant, wherein phosphorus replaces arsenic in the crystal structure; Mimetite has similar physical characteristics and manner of occurrence (barrel-shaped crystals or rounded masses) but is less frequent. Mimetite also forms a continuous solid-solution series with vanadinite wherein vanadium replaces mimetite's arsenic in the crystal structure.
Though available in the form of prismatic crystal forms, it is not used as a gemstone because of its softness.
Prominent occurrences are in Durango, Mapimi, Mexico, Tsumeb and Namibia. Excellent prismatic forms have been realized in Johanngeorgenstadt in Saxony and Wheal Unity at Gwennap in Cornwall, England.
Crystal structure of Mimetite is similar to that of Apatite. Occasionally the shape of two crystals will be similar. It also forms a chemical series with two other minerals: Vanadinite (Pb5 (VO4) 3Cl) and Pyromorphite (Pb5 (PO4) 3Cl). This series is slightly different than most chemical series that involve replacement of cations such as Calcium for Magnesium. Alternatively, this series replaces its basic chemical units the anion groups: Arsenate (AsO4), Phosphate (PO4) and Vanadate (VO4).
Pale to bright yellow, yellow-orange, yellowish brown, white, could be colorless.