Arsmirandite

Chemical formula: Na₁₈Cu₂+₁₂Fe₃+O₈(As₅+O₄)₈Cl₅

Arsmirandite is a very rare copper, sodium, and iron arsenate, forming characteristic, dark-green, acicular crystals.

## Characteristics Arsmirandite is a complex arsenate from the alluaudite group, distinguished by the presence of copper, sodium, iron, and chlorine in its structure. It forms aggregates of fine, acicular or hair-like crystals of an intense, dark-green color. Individual crystals are usually very small, not exceeding 1 mm in length and several tens of micrometers in thickness, which gives the aggregates the appearance of felt-like or radial crusts on the host rock. ## Physical properties Arsmirandite crystals exhibit a vitreous luster. They are flexible and can be bent. Due to the small size of the crystals, hardness and density have not been precisely measured. ## Colors and varieties This mineral occurs in a uniform, dark-green color. No color or commercial varieties are known. ## History and name Arsmirandite was discovered in volcanic exhalation products (fumaroles) of the Tolbachik volcano in Kamchatka, Russia. It was officially recognized as a new mineral by the International Mineralogical Association (IMA) in 2015 (IMA2015-078). Its name comes from the Latin words *arsenum* (arsenic) and *mirandus* (wonderful, extraordinary), referring to its complex and unique chemical composition. It was described by a team of mineralogists led by Igor V. Pekov. ## Applications Arsmirandite has no industrial application. Its significance is purely scientific and collectible, as a unique and rare representative of the mineral world.

Properties

Luster: Vitreous
Streak: Green
Cleavage: None
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Arsmirandite can be identified by its characteristic mode of occurrence – as crusts composed of very fine, acicular or hair-like crystals of an intensely dark-green color. Key to identification is its origin (fumaroles of the Tolbachik volcano) and co-occurrence with other rare exhalative minerals. ## Distinguishing from similar minerals It can be confused with other copper-bearing secondary minerals of green color and acicular form, such as atacamite, brochantite, or other rare arsenates from Tolbachik. Definitive differentiation requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS). ## Crystal forms This mineral forms elongated, acicular or hair-like crystals, often gathered into radial, star-shaped, or haphazardly entangled aggregates. These aggregates form thin crusts and coatings on the rock surface.

Geological environment

## Genesis Arsmirandite is a mineral of volcanic origin, crystallizing directly from hot gases (exhalations) in fumaroles. It forms in an environment rich in arsenic, copper, sodium, iron, and chlorine, at temperatures above 450°C. It is a product of sublimation on the surface of volcanic rocks. ## Mineral associations This mineral co-occurs with other rare fumarolic minerals of the Tolbachik volcano, such as hematite, tenorite, halite, sylvite, aphthitalite, langbeinite, sanidine, as well as arsenates: johillerite, erikssonite, kozyrevskite, svabite, and others. ## Localities The only confirmed locality of arsmirandite in the world is its type locality – the "Arsenatnaya" fumarole on the second cinder cone of the northern breach of the Great Tolbachik Fissure Eruption (1975-1976) in Kamchatka, Russia.

Rarity

Extremely rare

For collectors

## Quality criteria The most prized arsmirandite specimens are those that feature rich, dense coatings of intensely green, well-formed acicular crystals on a small fragment of host rock. The color contrast with the substrate and the presence of rare associated minerals increase the specimen's value. Due to the microscopic size of the crystals, specimens are primarily evaluated under magnification. ## Popular localities The only source of arsmirandite specimens is its discovery site – the Tolbachik volcano in Kamchatka. Material from this locality is extremely limited and available almost exclusively on the specialized collector's market.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Contact with water and any chemical agents should be avoided. Due to the extreme delicacy of the acicular crystals, mechanical cleaning (e.g., with a brush) is out of the question. ## What to avoid Contact with water, acids, detergents, and other liquids must be strictly avoided. The crystals are very brittle and fragile, so the specimen should be protected from shocks, impacts, and abrasion. It should not be heated or exposed to changes in humidity. ## Storage Arsmirandite specimens should be stored under stable conditions, preferably in a closed, airtight "micromount" box, which protects them from dust, moisture, and mechanical damage. Touching the fragile crystals should be avoided.

Sources

IMA Mineral List (CNMNC)