Otwayite

Chemical formula: Ni²⁺₂CO₃(OH)₂·H₂O

Otwayite is a rare, basic hydrated nickel carbonate, forming microcrystalline aggregates of an intense green color.

## Characteristics Otwayite is a rare mineral from the carbonate group, chemically classified as a basic hydrated nickel carbonate. It occurs as very fine, acicular or platy crystals, which typically form compact, chalky or earthy aggregates and coatings. Individual crystals are usually too small to be seen with the naked eye. Its most characteristic feature is its intense, bright green or emerald green color, associated with its high nickel content. ## Physical properties Due to its occurrence in the form of microcrystalline aggregates, determining the hardness of a single crystal is difficult; the hardness of aggregates is very low, about 1 on the Mohs scale. The mineral is characterized by a silky or dull luster. It is opaque. The calculated density is approximately 3.41 g/cm³. ## Colors and varieties Otwayite occurs in uniform, bright green or emerald green shades. No color or commercial varieties are distinguished. ## History and name The mineral was discovered in the Otway nickel mine in the Nullagine district of Western Australia. It was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 1977. Its name comes directly from its discovery location (the Otway mine). ## Applications Otwayite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or nickel minerals.

Properties

Mohs hardness: 4
Color: Green, bright green
Luster: Silky
Streak: Light green
Density: 3.41
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Translucent,Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Otwayite can be recognized by its characteristic bright green color, very low hardness, and occurrence in the form of earthy or chalky coatings and aggregates. It reacts vigorously with hydrochloric acid, which is a diagnostic feature for carbonates. ## Distinguishing from similar minerals It can be confused with other secondary nickel minerals, such as bright green gaspeite, reevesite, or zaratite. Distinguishing it from these often requires advanced analytical methods, such as X-ray diffraction (XRD). It differs from zaratite, which is amorphous, by its crystalline structure. It differs from gaspeite and reevesite in chemical composition and physical properties, although visually they can be very similar. ## Crystal forms It forms microscopic, acicular or platy crystals up to 20 micrometers long, which combine into rosette aggregates, spherulites, and most commonly into compact, earthy or chalk-like masses and coatings.

Geological environment

## Genesis Otwayite is a secondary mineral formed in the oxidation (weathering) zones of nickel sulfide deposits. It forms as a result of the weathering of primary nickel minerals, such as pentlandite, in the presence of carbonate rocks (e.g., dolomite), which provide carbonate ions. It forms under low temperature and pressure conditions, close to the Earth's surface. ## Mineral associations It most often co-occurs with other secondary nickel and iron minerals. In the type locality (Otway mine), it was found in association with gypsum, magnesite, goethite, as well as relics of primary pentlandite. It may also occur with related nickel carbonates, such as gaspeite and reevesite. ## Localities The most important and type locality is the Otway nickel mine in the Nullagine region of Western Australia. It is a very rare mineral, and its confirmed occurrences outside this locality are few and poorly documented.

Rarity

Very rare

For collectors

## Quality criteria The collector appeal of otwayite depends primarily on the intensity and purity of its bright green color. The most valued specimens are those where the mineral forms thick, uniform coatings on a small rock matrix. Due to the microscopic size of the crystals, their form is not a criterion for visual assessment. The contrast with darker host rock can enhance the aesthetic value of the specimen. ## Popular localities The only source of valuable collector specimens is its type locality - the Otway mine in Western Australia. Specimens from this location are the standard for this mineral.

Care and storage

## Cleaning Otwayite specimens are extremely delicate and fragile. They should only be cleaned with compressed air to remove dust. Any contact with water, and especially with ultrasonics, is unacceptable and can destroy the specimen. ## What to avoid Avoid contact with water, chemicals (especially acids, in which it dissolves rapidly), high temperatures, and direct sunlight, which can cause fading. Due to its low hardness, it should be protected from any scratches and impacts. ## Storage It is recommended to store specimens in closed, padded collector boxes to protect them from mechanical damage, dust, and moisture. It is not suitable for display in open showcases.