Ondrušite

Chemical formula: CaCu₂+₄(As₅+O₄)₂(As₅+O₃OH)₂·₁₀H₂O

Ondrušite is a rare, hydrated calcium and copper arsenate, forming characteristic blue-green aggregates of acicular crystals.

## Characteristics Ondrušite is a mineral from the arsenate group, chemically classified as a hydrated calcium and copper arsenate. It forms very small, acicular or thin-tabular crystals, most often occurring as radial aggregates, rosettes, and crusts. Individual crystals rarely exceed a few millimeters in length, but their vivid color makes them clearly visible against the host rock. ## Physical properties It is a relatively soft mineral, with a Mohs hardness of 2-3. It has a vitreous luster, and on fracture surfaces, it can be slightly silky. It is translucent. Its density, calculated based on the chemical formula and unit cell parameters, is approximately 3.36 g/cm³. ## Colors and varieties Ondrušite occurs in characteristic shades of blue and green. The most common color is sky-blue, blue-green to pale green. No commercial or color varieties have been distinguished. ## History and name The mineral was approved by the International Mineralogical Association (IMA) in 2000. Its name honors Petr Ondruš (1962–2009), a Czech mineralogist from the Czech Geological Survey, who was a specialist in hydrothermal mineralogy, especially the Jáchymov deposit in the Czech Republic, where this mineral was first found. ## Applications Ondrušite has no industrial applications. It is solely an object of interest for collectors of rare minerals and scientists studying secondary oxidation zones of ore deposits.

Properties

Mohs hardness: 2-3
Luster: Vitreous
Streak: Pale blue
Density: 3.36
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Key diagnostic features of ondrušite are its characteristic sky-blue or blue-green color, acicular crystal habit forming radial aggregates, and occurrence in paragenesis with other rare copper arsenates. It is a secondary mineral, found in oxidation zones. ## Distinguishing from similar minerals Ondrušite can be confused with other blue and green copper arsenates, such as geminite, lindackerite, lavendulan, or picropharmacolite, with which it often co-occurs. Geminite forms more prismatic crystals, and lindackerite forms bladed crystals. Certain distinction of these minerals under amateur conditions is very difficult and most often requires advanced studies, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal forms It forms elongated, acicular or flattened, bladed crystals. They almost always occur as radial or fan-shaped aggregates, as well as thin crusts on rock surfaces.

Geological environment

## Genesis Ondrušite is a secondary mineral, forming in oxidation zones (so-called gossans) of hydrothermal ore deposits rich in arsenic and copper. It is often a product of post-mining processes, crystallizing on the walls of old workings as a result of weathering of primary sulfides and arsenides. ## Mineral associations This mineral co-occurs with a whole range of other rare, secondary arsenates. In the type locality (Jáchymov), it was found in association with geminite, lindackerite, erythrite, annabergite, pharmacosiderite, scorodite, and native arsenic. ## Localities The most important and classic occurrence of ondrušite is its type locality – Jáchymov (Svornost mine) in the Ore Mountains, Czech Republic. Its presence has also been confirmed in the Maria Josefa mine in Rodalquilar, Spain, and in the Km 3 mine in Laurion, Greece. These are the only confirmed localities worldwide.

Rarity

Very rare

For collectors

## Quality criteria The most valued by collectors are specimens with well-formed, radial aggregates of intense, sky-blue color, set on a small rock matrix. The contrast of the mineral's color with the substrate significantly enhances its attractiveness. Due to its rarity, even small micro-aggregates are sought after. The value of a specimen increases if it is accompanied by other rare arsenates. ## Popular localities By far the most desirable specimens come from the type locality – Czech Jáchymov. Material from this historic deposit is considered classic and serves as a benchmark for this mineral.

Care and storage

## Cleaning It should only be cleaned dry, using a soft brush or a gentle stream of compressed air to remove dust. The mineral is soft and brittle, and contact with water or chemicals can lead to its damage or dissolution. ## What to avoid As an arsenate, ondrušite is toxic. Avoid inhaling dust and always wash hands thoroughly after any contact with a specimen. It should not be heated, as it may lose crystallization water and undergo permanent changes. It should be protected from moisture, extreme temperatures, and chemicals. ## Storage Ondrušite specimens are best stored in closed, stable containers ("micromount" boxes) that protect them from mechanical damage, dust, and sudden changes in humidity. Keep out of reach of children and pets.

Sources

IMA Mineral List (CNMNC)