Zálesíite

Chemical formula: Cu₂+₆Ca(As₅+O₄)₂(As₅+O₃OH)(OH)₆·₃H₂O

Zálesíite is a rare secondary mineral from the mixite group, forming characteristic acicular crystals of an intense green or blue-green color.

## Characteristics Zálesíite is a hydrated calcium copper arsenate belonging to the mixite group. It occurs as very fine, acicular or fibrous crystals that form radial aggregates, crusts, or spherical clusters. Individual crystals are usually too small to be seen with the naked eye, and the clusters appear as felt-like or velvety coatings on the host rock. ## Physical Properties Zálesíite crystals exhibit a silky or vitreous luster. Due to their microcrystalline nature, hardness and density are difficult to measure precisely for the entire aggregate. The mineral is translucent, and in the case of very thin needles, it can be almost transparent. ## Colors and Varieties This mineral is characterized by a vibrant palette of colors, from yellowish-green, through emerald-green, to blue-green. No named varieties are distinguished, and color differences result from slight variations in chemical composition or crystallization conditions. ## History and Name The mineral's name comes from its discovery locality in 1997 – the historic uranium deposit of Zálesí near Javorník in Czech Silesia. It was described and approved as a new mineral species by a team of Czech mineralogists: P. Ondruš, F. Veselovský, J. Hloušek, J. Sejkora, R. Skála, P. Berlepsch, and T. Armbruster.

Properties

Mohs hardness: 2-3
Luster: Silky
Streak: Light green
Density: 3.29
Cleavage: None
Fracture: Uneven
Transparency: Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Zálesíite is identified by its characteristic mode of occurrence – as fine, acicular crystals forming radial or spherical aggregates. Key features also include its intense green or blue-green color and its presence in the oxidation zones of ore deposits rich in arsenic and copper. ## Distinguishing from Similar Minerals It can be confused with other minerals from the mixite group, such as mixite itself (which does not contain calcium) or agardite (which contains rare earth elements). Definitive distinction requires advanced chemical (EDS) or X-ray (XRD) analyses. Visually, it is almost identical to agardite and mixite. ## Crystal Forms Crystals are very small, elongated in the form of needles or fibers. They form characteristic radial, rosette-like, spherical (spherulitic) aggregates, or dense, felt-like crusts on rock surfaces.

Geological environment

## Genesis Zálesíite is a secondary mineral, forming in the oxidation (weathering) zones of polymetallic deposits, especially those rich in arsenopyrite and chalcopyrite. It crystallizes from aqueous solutions containing copper, calcium, and arsenate ions under conditions of low temperatures and pressures. ## Mineral Associations It often co-occurs with other secondary minerals of the oxidation zone, such as malachite, azurite, olivenite, tyrolite, brochantite, chalcophyllite, as well as goethite, quartz, and uranyl minerals. ## Localities The most important and classic locality is Zálesí near Javorník in the Czech Republic. Other known localities include the Clara mine in the Black Forest (Germany), Cap Garonne (France), Lavrion (Greece), and several places in the American states of Utah and Nevada.

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens are those with rich, dense coverings of fine, silky needles of intense, vivid color. Specimens with well-formed, spherical or radial aggregates contrasting with the color of the host rock are also highly regarded. Due to the microscopic size of the crystals, purity is not a key criterion – the aesthetics of the entire aggregate are what matters. ## Popular Localities Specimens from the type locality (Zálesí, Czech Republic) are of particular value to systematic collectors. Very aesthetic, rich specimens also come from the Clara mine in Germany and from Lavrion, Greece.

Care and storage

## Cleaning Zálesíite specimens are extremely delicate. They should be cleaned only with compressed air to remove dust. Any contact with water, especially chemicals, is highly inadvisable and can lead to damage or dissolution of the fragile crystals. ## What to Avoid Avoid ultrasonics, steam cleaners, acids, detergents, and solvents. The mineral is sensitive to high temperatures and direct, prolonged exposure to sunlight, which can cause fading. It should be protected from moisture and sudden temperature changes. ## Storage It is recommended to store specimens in closed, stable containers (so-called micromount boxes) to protect them from dust, mechanical damage, and humidity fluctuations. Display should be away from heat sources and direct light.

Sources

IMA Mineral List (CNMNC)