Calcioandyrobertsite

Chemical formula: KCaCu₂+₅(As₅+O₄)₄[As₅+(OH)₂O₂]·₂H₂O

Calcioandyrobertsite is a very rare, secondary copper arsenate of intense blue color, forming characteristic, tabular crystals.

## Characteristics Calcioandyrobertsite is a hydrated potassium, calcium, and copper arsenate. It occurs as small, tabular or bladed crystals, often grouped into rosette-like or radial aggregates. Its most characteristic feature is its intense, azure-blue to sky-blue color. Crystals are usually very small, rarely exceeding 1 mm, and form thin coatings or fillings in rock vugs. ## Physical Properties This mineral is relatively soft, with a hardness of approximately 3 on the Mohs scale. The crystals exhibit a vitreous luster. They are transparent to translucent. Due to the small size of the crystals, precise determination of density is difficult, but the calculated value is approximately 4.25 g/cm³. ## Colors and Varieties Calcioandyrobertsite occurs in shades from azure-blue to light blue. No varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of calcium (Latin: *calcius*) – and its structural relationship to andyrobertsite. It was approved as a new mineral by the International Mineralogical Association (IMA) in 2012 (IMA number 2012-025). It was described by A.R. Kampf and other researchers in 2013 based on specimens found in the Tsumeb mine in Namibia. ## Uses Calcioandyrobertsite has no industrial application. Its significance is purely scientific and as a collector's item, being a valued micromineral due to its rarity and attractive appearance.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Light blue
Density: 4.25
Cleavage: Perfect on {010}
Fracture: Brittle
Transparency: Transparent to translucent
Crystal system: Triclinic

Diagnostic features

## Identification Key diagnostic features include its intense blue color, characteristic habit of tabular crystals forming rosette-like aggregates, and occurrence in the oxidation zones of ore deposits. Final identification requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Calcioandyrobertsite can be confused with other secondary blue copper arsenates, such as cyanotrichite, as well as other minerals from the andyrobertsite group. Distinguishing it from andyrobertsite or nickelandyrobertsite with the naked eye is impossible and requires chemical analysis to determine the dominant cation (calcium, cadmium, nickel). ## Crystal Forms It forms thin, tabular crystals with a hexagonal or octagonal outline, elongated in one direction. These crystals often combine into radial or rosette-like aggregates, and also form spherical clusters.

Geological environment

## Genesis Calcioandyrobertsite is a secondary mineral, forming in the oxidation zones (gossans) of polymetallic, hydrothermal ore deposits rich in arsenic. It forms as a result of the weathering of primary sulfides and arsenides, especially tennantite. ## Mineral Associations This mineral co-occurs with other secondary arsenates and sulfates. In its type locality (Tsumeb), it was found in association with kottigite, tennantite, gypsum, anglesite, and other minerals from the andyrobertsite group. ## Localities The only confirmed and described locality for calcioandyrobertsite in the world is its discovery site – the famous Tsumeb mine in the Oshikoto region of Namibia. It is a type mineral for this single locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The most highly prized specimens are those with well-formed, sharp crystals creating aesthetic, rosette-like aggregates. The intensity and uniformity of the blue color are crucial. Contrast with the rock matrix and the presence of rare associated minerals are also important. Due to its microscopic nature, the quality of the specimen is assessed under magnification. ## Popular Localities The only source of specimens is the Tsumeb mine in Namibia, which is currently closed. All specimens available on the collector's market come from old collections, which significantly increases their value and rarity.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Any contact with water or other liquids is inadvisable due to the risk of damaging the delicate crystals. ## What to Avoid Avoid contact with chemicals, acids, and detergents. The mineral is sensitive to high temperatures and mechanical shocks. It should not be cleaned with ultrasound. Prolonged exposure to strong sunlight may potentially cause fading. ## Storage Calcioandyrobertsite specimens, as typical microminerals, should be stored in specialized, sealed boxes (so-called "micromount boxes") that protect them from dust, moisture, and mechanical damage. Avoid storage in humid environments.

Sources

IMA Mineral List (CNMNC)