Melanarsite

Chemical formula: K₃Cu₂+₇Fe₃+O₄(As₅+O₄)₄

A black potassium, copper, and iron arsenate, forming small, prismatic crystals in volcanic exhalations.

## Characteristics Melanarsite is a very rare mineral from the arsenate group, chemically classified as a potassium, copper, and iron oxyarsenate. It forms small, elongated, prismatic or acicular crystals, rarely exceeding 0.5 mm in length. They typically occur as radial aggregates, crusts, or small, tangled masses on the rock substrate. The mineral is completely opaque and characterized by a metallic or submetallic luster. ## Physical Properties Its Mohs hardness is approximately 3, making it a relatively soft mineral. The density calculated from the chemical formula and unit cell parameters is 4.65 g/cm³. It is brittle. Due to its arsenic content, the mineral is toxic. ## Colors and Varieties Melanarsite is uniformly black, and its streak is brownish-black. No color varieties or commercial forms are known. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2018. Its name comes from the Greek word *μέλας* (*mélas*), meaning "black," in reference to its color, and from the presence of arsenic in its chemical composition. It was discovered by a team of mineralogists, including Igor V. Pekov and Fedor D. Sandalov, in the products of fumarolic activity of the Tolbachik volcano. ## Applications Due to its extreme rarity and microscopic crystal sizes, melanarsite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals.

Properties

Mohs hardness: 3
Luster: Metallic
Streak: Brownish-black
Density: 4.65
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Melanarsite can be identified by its black color, metallic luster, and characteristic form of prismatic or acicular crystals, often forming radial aggregates. A key diagnostic feature is its unique occurrence environment – as a product of crystallization from volcanic gases in fumaroles. Definitive identification is only possible using advanced analytical methods, such as electron microprobe (EDS/WDS) and X-ray diffraction (XRD). ## Distinguishing from Similar Minerals At first glance, it may be confused with other black, metallic minerals found in the same environment, such as tenorite, hematite, or other complex arsenates from the lammerite group. Distinguishing it based on visual characteristics is practically impossible and requires specialized chemical and structural analysis. ## Crystal Forms Crystals are prismatic, often elongated along one axis, up to acicular forms. They typically form radial or chaotically tangled aggregates, as well as thin crusts on the lava surface.

Geological environment

## Genesis Melanarsite is a mineral of volcanic origin, formed by sublimation from volcanic gases under high-temperature (450-600°C), oxidizing conditions. It crystallizes directly on the walls of fumarole channels and in cavities within basaltic lava. ## Mineral Associations This mineral co-occurs with a rich assemblage of other fumarolic minerals. The most common associations include: tenorite, hematite, sanidine, lammerite, klyuchevskite, alarsite, arsmirandite, urusovite, and johillerite. ## Localities The only confirmed locality of melanarsite in the world is its type locality – the Arsenatnaya fumarole on the Second Scoria Cone of the Northern Breakthrough of the Great Fissure Eruption of Tolbachik volcano in Kamchatka, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of melanarsite specimens, being a typical micromineral, is primarily assessed based on the richness and aesthetics of the crystal aggregates. Samples with well-formed, sharp, lustrous crystals forming visually attractive, radial groups on a small rock matrix are most valued. The presence of rare associated minerals, which increase the scientific and collectible value of the specimen, is also important. ## Popular Localities All known melanarsite specimens come from a single location in the world – the fumarole of Tolbachik volcano in Kamchatka. This is a classic locality for many rare arsenates and sulfates, and material from there is highly prized by collectors specializing in systematic minerals and micromounts.

Care and storage

## Cleaning Melanarsite specimens are extremely delicate and usually microscopic in size. Cleaning should be kept to an absolute minimum. If necessary, compressed air (from a safe distance) can be used to remove loose dust particles. Avoid all contact with water and chemical agents. ## What to Avoid The mineral is toxic due to its arsenic content – avoid inhaling dust and direct skin contact; wash hands after any contact. It must not be washed in water or cleaned with ultrasound. It is soft and brittle, making it very susceptible to mechanical damage. Protect it from acids and other chemicals. ## Storage It is recommended to store specimens in closed, transparent "micromount" containers that protect against dust, mechanical damage, and accidental contact. The container should be clearly labeled as toxic material. Store in a dry place, away from sources of moisture.

Sources

IMA Mineral List (CNMNC)