Melanothallite

Chemical formula: Cu²⁺₂OCl₂

Melanothallite is a rare copper chloride, forming dark, almost black coatings and crusts in volcanic exhalations.

## Characteristics Melanothallite is a secondary copper mineral, occurring as very fine crystals that form coatings, crusts, and aggregates. Individual crystals are usually too small to be seen with the naked eye. Observed under a microscope, they have the form of thin, flattened tablets. The mineral's name comes from its appearance – aggregates are typically dark green to almost black, giving the impression of a "black coating." ## Physical Properties Melanothallite crystals exhibit a luster ranging from vitreous to greasy. They are transparent to translucent, but in dense aggregates, they appear opaque. It is a relatively soft and brittle mineral. ## Colors and Varieties The dominant color is dark green, which transitions to black in dense aggregates. Some specimens may have a greenish-brown hue. No varieties of this mineral are distinguished. ## History and Name Melanothallite was first described in 1870 by Arcangelo Scacchi. The name comes from the Greek words μέλας (melas) meaning "black" and θαλλός (thallos) meaning "coating" or "twig," which accurately describes its typical form of occurrence as dark coatings on volcanic rocks. The type locality is Mount Vesuvius in Italy. ## Uses Melanothallite has no industrial application. It is of purely scientific and collector's interest, being an interesting example of a mineral formed under specific volcanic conditions.

Properties

Mohs hardness: 2.5
Color: Black, bluish black
Luster: Vitreous to Greasy
Streak: Green
Density: 0
Cleavage: One direction, perfect, probably prismatic
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification A characteristic feature is its occurrence as dark green to black, fine-grained coatings and crusts directly on lava or volcanic tuffs, near active or extinct fumaroles. Its instability and reaction to moisture are also diagnostic. ## Distinguishing from Similar Minerals It can be confused with other dark copper minerals found in similar environments, such as tenorite (CuO) or atacamite/paratacamite (Cu₂Cl(OH)₃). Tenorite is usually black and harder. Atacamite and paratacamite, although also green, form in the oxidation zones of copper deposits, not as direct volcanic sublimates, and have different crystal forms. ## Crystal Forms Melanothallite forms very small, thin, tabular crystals, often arranged in rosette-like or chaotic aggregates. It occurs mainly as porous crusts, coatings, and earthy aggregates.

Geological environment

## Genesis Melanothallite is a high-temperature mineral, formed by direct sublimation from volcanic gases (exhalations) at temperatures ranging from 150 to over 450°C. It crystallizes on the surfaces of volcanic rocks near fumarole vents, where hot, chlorine- and copper-rich volcanic gases react with atmospheric oxygen. ## Mineral Associations It most commonly co-occurs with minerals formed under similar conditions, such as sal ammoniac (NH₄Cl), cotunnite (PbCl₂), eriochalcite (CuCl₂·2H₂O), into which it often transforms under the influence of moisture, and less frequently with tenorite (CuO). ## Localities The most important and classic localities for this mineral are associated with volcanic activity. These include: - Vesuvius, Campania, Italy (type locality) - Tolbachik Volcano, Kamchatka, Russia, where some of the best specimens have been found - Fogo Volcano, Cape Verde Islands

Rarity

Rare

For collectors

## Quality Criteria The quality of a melanothallite specimen is primarily assessed based on the richness and preservation of the crystalline coatings. The most prized specimens are those with well-formed, though still microscopic, crystals forming dense, undamaged crusts on a small piece of matrix rock. The contrast of the mineral's dark green color with lighter lava also enhances visual appeal. Due to its fragility, the state of preservation is absolutely crucial. ## Popular Localities The most sought-after specimens by collectors come from two historically important localities: the classic discovery site, Mount Vesuvius in Italy, and the Great Fissure Eruption of Tolbachik Volcano in Kamchatka, Russia. Specimens from Tolbachik, originating from the 1975-76 and 2012-13 eruptions, are often considered the best in the world in terms of quality and abundance of occurrences.

Care and storage

## Cleaning Melanothallite specimens are extremely delicate and sensitive. They should absolutely not be cleaned mechanically or with liquids. Any attempt at cleaning risks irreversible damage to the fragile crystals. ## What to Avoid The mineral is very sensitive to changes in humidity and temperature. As a product of volcanic exhalations, it is unstable under normal atmospheric conditions. Contact with water, even atmospheric moisture, can lead to its decomposition or transformation into other minerals (e.g., eriochalcite). Avoid touching, vibrations, and exposure to chemicals and direct sunlight. ## Storage The only safe storage methods are hermetically sealed containers or specialized collector boxes with controlled, low humidity. Storage in a dry environment is crucial for preserving the specimen in its original form.

External references

https://www.mindat.org/min-2623.html