Arsentsumebite

Chemical formula: Pb₂+₂Cu₂+(As₅+O₄)(S₆+O₄)(OH)

Arsentsumebite is a rare lead and copper arsenate and sulfate, forming characteristic green crystals with an adamantine luster.

## Characteristics Arsentsumebite is a secondary mineral of lead, copper, arsenic, and sulfur, occurring in the oxidation zones of polymetallic deposits. It forms small but well-formed, tabular or prismatic crystals, often aggregated into druses and crusts. Its most distinctive features are its intense, emerald-green or apple-green color and strong, almost adamantine luster, making it an attractive micromount mineral. ## Physical Properties Arsentsumebite crystals are relatively soft, with a hardness of approximately 4 on the Mohs scale. The mineral is quite brittle and heavy due to its high lead content, with a density of about 6.4 g/cm³. The luster is strongly vitreous to adamantine. It is transparent to translucent. ## Colors and Varieties Arsentsumebite occurs in various shades of green, from light green, apple-green, through emerald-green, to greenish-blue. No distinct color varieties or commercial names are recognized. ## History and Name The mineral's name refers to its chemical composition (arsenic content) and its discovery locality – the Tsumeb mine in Namibia. It was described as a new mineral in 1982 by Herbert Keller, Wolfgang Bartelke, and Paul Keller. The Tsumeb mine is its type locality. ## Uses Arsentsumebite has no industrial applications. However, it is a highly valued and sought-after collector's mineral, especially among systematic mineral collectors and micromount enthusiasts.

Properties

Mohs hardness: 4
Luster: Vitreous to adamantine
Streak: Light green
Density: 6.4
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Arsentsumebite can be identified by its intense green color, strong, almost adamantine luster, and characteristic small, tabular crystals. High density is also a helpful indicator. Its occurrence in association with other secondary minerals from Tsumeb is crucial for identification. ## Distinguishing from Similar Minerals It can be confused with other green arsenates, such as duftite, olivenite, or bayldonite. However, its higher luster, specific crystal habit, and mineral associations distinguish it. It often forms pseudomorphs after mimetite, which is a diagnostic feature. From tsumebite (its sulfate analogue), it is practically impossible to distinguish without advanced chemical analyses (EDS/XRD). ## Crystal Forms Crystals are usually small, with a tabular or short-prismatic habit. They often form druses, crusts, and fan-shaped aggregates. Pseudomorphs after other minerals, mainly after spheroidal mimetite aggregates, are also found.

Geological environment

## Genesis Arsentsumebite is a secondary mineral, formed in the oxidation zone (gossan) of hydrothermal polymetallic deposits rich in lead, copper, and arsenic. It crystallizes as a result of the weathering of primary sulfides, such as galena and tennantite. ## Mineral Associations It most commonly co-occurs with other rare secondary minerals, especially from the Tsumeb mine. Typical associated minerals include: tsumebite, anglesite, azurite, malachite, cerussite, smithsonite, wulfenite, olivenite, duftite, bayldonite, and mimetite. ## Localities The most important and classic locality for arsentsumebite is the Tsumeb mine in the Oshikoto Region, Namibia, from which the best specimens originate. It is also known from several other localities worldwide, including the Lavrion mines in Attica (Greece) and small occurrences in Arizona (USA).

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized arsentsumebite specimens are characterized by sharp, well-formed crystals with an intense, emerald-green color and strong luster. Contrast with the matrix (e.g., with white cerussite) and the presence of associated minerals are important. Crystal size rarely exceeds a few millimeters, so even small but aesthetic druses are highly valued. Specimens from the classic locality – Tsumeb – are particularly sought after. ## Popular Localities By far the most desirable and valuable specimens come from the Tsumeb mine in Namibia. They set the standard against which specimens from other, rare localities are compared.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Ultrasonic cleaners must not be used. ## What to Avoid Contact with acids and other chemicals that can damage the crystal surfaces should be avoided. The mineral is brittle, so it must be protected from impacts and scratching. It should not be heated or exposed to sudden temperature changes. ## Storage It is best to store arsentsumebite in a separate, padded "micromount" box to prevent abrasion and mechanical damage. It should be protected from dust and moisture. Due to its arsenic and lead content, it is recommended to wash hands after handling the mineral.

Sources

IMA Mineral List (CNMNC)