Picaite

Chemical formula: NaCa[As₅+O₃OH][As₅+O₂(OH)₂]

Picaite is a rare, colorless sodium calcium arsenate, discovered in the Torrecillas mine in Chile, forming transparent, tabular crystals.

## Characteristics Picaite is a hydrated sodium calcium arsenate. This mineral forms very small, colorless, and transparent crystals with a tabular habit, reaching sizes up to 0.2 mm. They usually occur as radial or randomly oriented aggregates and clusters. Due to its chemical composition, it is a potentially toxic mineral and requires careful handling. ## Physical Properties Picaite crystals exhibit a hardness of 3.5 on the Mohs scale. They are transparent and have a vitreous luster. The mineral's density is approximately 3.24 g/cm³. It is brittle, and its fracture is described as stepped. ## Colors and Varieties Picaite is a colorless mineral. No colored varieties have been distinguished. ## History and Name The mineral's name comes from the word "pica," which in Spanish means pickaxe, referring to the mining character of the region where it was discovered. Picaite was approved as a new mineral by the International Mineralogical Association (IMA) in 2013. Its discovery was made in the Torrecillas mine in the Iquique Province, Chile, which is its only known locality (type locality). ## Applications Picaite has no industrial applications. As a microscopic mineral of great rarity, it is of purely scientific importance and is an object of interest for specialized collectors of rare minerals (so-called micromounts).

Properties

Mohs hardness: 3.5
Luster: Vitreous
Streak: White
Density: 3.24
Cleavage: {010} and {001}
Fracture: Step-Like
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of picaite under amateur conditions is practically impossible due to its microscopic size and lack of characteristic features visible to the naked eye. Recognition requires advanced analytical techniques, such as Raman spectroscopy or X-ray diffraction (XRD), combined with chemical analysis (EDS). ## Distinguishing from Similar Minerals Picaite can be confused with other colorless, secondary arsenate minerals found in the oxidation zones of ore deposits, such as pharmacolite, weilite, or haidingerite. Differentiation is possible only through detailed laboratory studies. ## Crystal Forms Picaite crystallizes in the form of very small, thin, tabular crystals, often flattened parallel to the {010} plane. These crystals combine into radial or disordered aggregates on the surface of the host rock.

Geological environment

## Genesis Picaite is a secondary mineral that forms as a result of the oxidation of primary arsenic ore minerals (like arsenopyrite) in desert conditions and a dry climate. Its formation is associated with the activity of sodium- and calcium-rich solutions. ## Mineral Associations At the type locality, picaite coexists with other rare arsenates, such as torrecillasite, canutite, lavendulan, erythrite, annabergite, as well as gypsum and quartz. ## Localities The only confirmed occurrence of picaite in the world is its type locality – the Torrecillas mine, located north of Salar Grande, in the Iquique Province, Tarapacá Region in northern Chile.

Rarity

Very rare

For collectors

## Quality Criteria As a microscopic mineral, the collector's value of picaite primarily depends on the richness and aesthetics of the crystalline aggregates visible under magnification. The most prized specimens are those with numerous, well-formed, radial clusters of transparent crystals, clearly contrasting with the host rock. The presence of rare associated minerals is also important. ## Popular Localities All collector specimens come from a single location in the world – the Torrecillas mine in Chile. This is the only source of this mineral.

Care and storage

## Cleaning Due to the microscopic size of the crystals, mechanical cleaning is not recommended and can easily damage the specimen. If necessary, compressed air can be used to remove dust. Avoid contact with water and other chemicals. ## What to Avoid As an arsenate, picaite is a toxic mineral. Avoid inhaling dust and direct skin contact. Always wash hands after handling the specimen. It should not be heated or exposed to acids, which can decompose it and release toxic arsenic compounds. Store away from moisture. ## Storage Picaite specimens should be stored in tightly sealed, clearly labeled containers or "micromount" boxes to prevent accidental contact and protect delicate crystals. They should be kept away from children and pets.

External references

https://www.mindat.org/min-52988.html