Nickelaustinite

Chemical formula: CaNi²⁺As⁵⁺O₄(OH)

Nickelaustinite is a rare calcium nickel arsenate, forming tiny, green crystals, valued by collectors of rare minerals.

## Characteristics Nickelaustinite is an arsenate belonging to the adelite-descloizite group. It occurs as very small, radial aggregates or spherical clusters, which rarely exceed 1 mm in diameter. Individual crystals are usually too small to be seen with the naked eye. It forms thin coatings and crusts on host rocks. ## Physical Properties Nickelaustinite crystals are characterized by a hardness of approximately 4.5 on the Mohs scale. They have a vitreous to earthy luster, depending on the form of occurrence. They are translucent to opaque. The mineral's density is about 4.23 g/cm³. ## Colors and Varieties This mineral ranges in color from light green, through greenish-yellow, to almost white. The intensity of the color depends on the nickel content. No named varieties are distinguished. ## History and Name Nickelaustinite was first described in 1986 by P.J. Dunn, D.R. Peacor, W.B. Simmons, and R.A. Ramik. Its name refers to its chemical composition – the dominant nickel – and its structural similarity to austinite, in which zinc is replaced by nickel. The type mineral (holotype) is a specimen from the Tsumeb deposit in Namibia. ## Applications Due to its rarity and occurrence as microscopic crystals, nickelaustinite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and micromounts.

Properties

Mohs hardness: 4
Color: Grass green, yellow-green
Luster: Vitreous to earthy
Streak: Light green to white
Density: 0
Cleavage: {110}
Fracture: Uneven
Transparency: Transparent
Crystal system: Orthorhombic

Diagnostic features

## Identification Nickelaustinite is identified by its characteristic light green color, its occurrence as small, radial or spherical aggregates, and its geological environment. Identification is confirmed by its co-occurrence with other nickel and calcium arsenates. Final determination requires advanced analytical methods, such as EDS spectroscopy or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals This mineral can be confused with other secondary nickel arsenates, such as annabergite, which, however, crystallizes in the monoclinic system and forms different crystal habits. Visually similar may also be gaspeite (nickel carbonate), which, however, effervesces with hydrochloric acid. It is distinguished from austinite by its green color (austinite is usually colorless, white, or greenish from inclusions) and chemical composition. ## Crystal Habits Nickelaustinite crystallizes in the orthorhombic system. It most often forms spherulitic or radial aggregates composed of very fine, acicular or bladed crystals. Single, well-formed crystals are extremely rare.

Geological environment

## Genesis Nickelaustinite is a secondary mineral, formed in the oxidation (weathering) zones of polymetallic ore deposits rich in arsenic and nickel. It forms as a result of reactions between arsenic-bearing solutions and nickel minerals, such as niccolite or gersdorffite, in the presence of calcium derived from carbonate rocks (e.g., dolomites). ## Mineral Associations This mineral often co-occurs with other arsenates, such as annabergite, gersdorffite, niccolite, tennantite, barite, quartz, and also with its zinc analog – austinite. ## Localities The most important and well-known occurrences of nickelaustinite worldwide include: - Tsumeb Mine, Namibia (type locality). - Km 3 Mine in Laurion, Attica, Greece. - Johanngeorgenstadt and Schneeberg in Saxony, Germany. - Bou Azzer, Morocco.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, spherical aggregates of an intense green color, contrasting with the rock matrix, are most valued by collectors. Due to the microscopic size of the crystals, the quality of the specimen is often assessed under magnification. The abundance of clusters on the rock surface and the absence of damage to delicate aggregates are also important. ## Popular Localities Specimens from the type locality, the Tsumeb mine in Namibia, are historically the most significant. High-quality micromounts also come from Greek Laurion and German Saxony.

Care and storage

## Cleaning Nickelaustinite specimens should be cleaned very carefully, preferably using compressed air to remove dust. Due to its fragility and often small crystal size, mechanical cleaning, including brushes, should be avoided. Contact with water should be minimized; if necessary, use distilled water and dry the specimen immediately. ## What to Avoid Nickelaustinite is an arsenate, so inhaling dust should be avoided, and hands should be washed after any contact with the mineral. It should be protected from acids and other chemicals that can damage it. It should not be heated or exposed to prolonged strong light. ## Storage It is recommended to store specimens in sealed, stable containers (so-called micromount boxes) to protect them from dust, mechanical damage, and moisture. The label should include a warning about toxicity due to arsenic content.

External references

https://www.mindat.org/min-2891.html