Mimetite-M

Chemical formula: Pb₅(AsO₄)₃Cl

Mimetite-M is a lead arsenate chloride, valued for its vividly colored, barrel-shaped or acicular crystals.

## Characteristics Mimetite-M is a mineral from the apatite group, and its name refers to its monoclinic symmetry, in contrast to hexagonal mimetite. It forms prismatic, barrel-shaped, acicular, or tabular crystals. It often occurs as radial, reniform, botryoidal aggregates, or as crusts and coatings. Its crystals can be rounded or exhibit characteristic barrel-shaped forms. It is one of the end members of an isomorphic series with pyromorphite (Pb₅(PO₄)₃Cl) and vanadinite (Pb₅(VO₄)₃Cl), with which it can form solid solutions. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3.5-4. It is quite brittle and heavy due to its high lead content, with a density of approximately 7.24 g/cm³. Mimetite's luster is most often resinous, and on crystal faces, it can be almost adamantine. It is transparent to translucent. ## Colors and Varieties Mimetite's color is highly varied – from colorless, through yellow, orange, red, brown, to greenish. The intensity and hue depend on impurities and crystallization conditions. Specimens with vivid, saturated orange and yellow colors are particularly prized. Varieties rich in calcium (campylite) are known, forming characteristic barrel-shaped crystals. ## History and Name The name mimetite comes from the Greek word "mimetes" (μιμητής), meaning "imitator," which refers to its strong resemblance to pyromorphite. The mineral was first described by the French mineralogist François Sulpice Beudant in 1832. The suffix "-M" in Mimetite-M was added by the International Mineralogical Association (IMA) in 2010 to distinguish the monoclinic polymorph from the hexagonal one (Mimetite-H). ## Uses Due to its lead and arsenic content, mimetite was historically a minor ore of these metals. Currently, it has no industrial significance and is valued solely as a collector's stone.

Properties

Mohs hardness: 3.5-4
Luster: Resinous to subadamantine
Streak: White
Density: 7.24
Cleavage: Poor/Indistinct on {1010}
Fracture: Uneven to subconchoidal
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features of mimetite include its high density, resinous luster, and characteristic crystal forms (barrel-shaped, acicular). It often occurs in vivid, warm colors (yellow, orange, brown). It reacts with nitric acid. ## Distinguishing from Similar Minerals It is most often confused with pyromorphite and vanadinite. Distinguishing it from pyromorphite, which often has a greenish color, can be impossible without chemical analysis, especially for yellow and brown specimens. Vanadinite typically forms hexagonal, tabular crystals of an intense red color, although prismatic forms also occur. Definitive differentiation within the series requires chemical analysis (EDS, XRD). ## Crystal Forms Mimetite-M crystallizes in the monoclinic system, but its crystals often exhibit pseudohexagonal symmetry. Typical forms are hexagonal prisms, often terminated by pyramids or lacking terminations. Crystals can be strongly rounded, taking on barrel-shaped forms (campylite). It also occurs as radial, spherical, botryoidal, and reniform aggregates, as well as dense crusts.

Geological environment

## Genesis Mimetite is a secondary mineral, forming in the oxidation (weathering) zones of lead ore deposits that also contain arsenic minerals (e.g., arsenopyrite). It crystallizes from aqueous solutions in the presence of chloride ions. It forms as a result of the oxidation of galena and other lead sulfides in an arsenic-rich environment. ## Mineral Associations It most commonly co-occurs with other minerals from the oxidation zone, such as pyromorphite, vanadinite, cerussite, anglesite, smithsonite, hemimorphite, wulfenite, as well as with calcite, quartz, and iron oxides (limonite, goethite). ## Localities Outstanding mimetite specimens come from many places around the world. Important historical and contemporary localities include: Tsumeb in Namibia (famous, colorless and yellow crystals), San Pedro Corralitos in Mexico (yellow, prismatic "Campylos" crystals), Pingtouling Mine in China (intensely orange, silky aggregates), Johanngeorgenstadt in Germany (historical type locality), Dry Gill in England (yellow-brown campylites), and numerous sites in Arizona (USA) and Australia (e.g., Broken Hill).

Rarity

Not very common

For collectors

## Quality Criteria The most highly prized mimetite specimens are characterized by intense, saturated colors – particularly sought after are vivid shades of orange, red, and canary yellow. The crystal form is also important; well-formed, sharp, undamaged prisms or aesthetic, barrel-shaped forms (campylite) command higher prices. Transparency, luster, and crystal size are also significant. The attractiveness of a specimen is enhanced by a contrasting rock matrix and association with other minerals. ## Popular Localities Collectors particularly value specimens from classic localities. Orange, botryoidal aggregates from the Pingtouling Mine in China, yellow prisms from San Pedro Corralitos in Mexico, and historical specimens from Tsumeb in Namibia are among the most desirable on the market. English campylites from Caldbeck Fells are also a sought-after classic.

Care and storage

## Cleaning To clean mimetite specimens, use only distilled water and a very soft brush or paintbrush. Proceed gently to avoid damaging the brittle crystals. Ultrasonic cleaners are not recommended. ## What to Avoid The mineral is sensitive to acids and other strong chemicals, which can damage or dull it. Avoid sudden temperature changes and prolonged exposure to direct sunlight, which can cause some specimens to fade. Due to its lead and arsenic content, avoid inhaling dust and wash hands after contact with the mineral. ## Storage Mimetite specimens are best stored in separate, padded boxes or display cases to protect them from mechanical damage and dust. They should be kept away from sources of moisture and at a stable temperature.

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