Widgiemoolthalite

Chemical formula: Ni²⁺₅(CO₃)₄(OH)₂·₄-₅H₂O

Widgiemoolthalite is a rare, hydrated nickel carbonate, forming characteristic blue-green, spherical aggregates and rosettes.

## Characteristics Widgiemoolthalite is a rare mineral from the carbonate group, chemically classified as a hydrated basic nickel carbonate. It occurs as small, spherical or hemispherical aggregates with a radial internal structure. It often forms rosette-like clusters and crusts. Its crystals are usually very small, acicular or bladed, which gives the aggregates a delicate, fibrous texture. It is a secondary mineral, forming in the oxidation zones of nickel sulfide deposits. ## Physical Properties This mineral is characterized by relatively low hardness, approximately 3.5 on the Mohs scale, making it susceptible to scratching. Its density is about 3.13 g/cm³. The crystals are transparent and exhibit a vitreous luster. Due to the fine-grained structure of the aggregates, the luster on their surface may appear more dull or silky. ## Colors and Varieties Widgiemoolthalite ranges in color from green to blue-green, which is typical for hydrated nickel minerals. The intensity and hue depend on specific crystallization conditions and possible impurities. No named varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – Widgiemooltha in Western Australia. It was first described in 1992 based on specimens found in the 132 North nickel mine. Thus, it is a mineral relatively recently known to science. ## Uses Due to its extreme rarity and small occurrence sizes, widgiemoolthalite has no industrial application. It is solely an object of interest for collectors of rare minerals and scientists.

Properties

Mohs hardness: 3.5
Luster: Vitreous
Streak: Light green-blue
Density: 3.13
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Characteristic features of widgiemoolthalite include its blue-green color, occurrence in spherical or rosette-like aggregates with a radial structure, and genesis related to weathering zones of nickel deposits. It reacts vigorously with acids, releasing carbon dioxide, which is typical for carbonates. A light greenish-blue streak is also helpful in identification. ## Distinguishing from Similar Minerals Widgiemoolthalite can be confused with other secondary nickel minerals, such as gaspeite, reevesite, or zaratite. From gaspeite and reevesite, which have a similar form of occurrence, it is distinguished by a slightly different color and chemical composition. Zaratite, an amorphous nickel carbonate, most often forms vitreous crusts and lacks the crystalline internal structure visible in widgiemoolthalite aggregates. Precise differentiation of these minerals often requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms This mineral forms tiny, acicular or bladed crystals, which almost always occur as radial clusters. These aggregates take the form of spherulites (spheres), hemispheres, fans, and rosettes. Individual, well-formed crystals are extremely rare and microscopic in size.

Geological environment

## Genesis Widgiemoolthalite is a secondary mineral. It forms in the oxidation (weathering) zones of hydrothermal nickel sulfide deposits, especially in dry or semi-arid climates. It forms as a result of the reaction of carbon dioxide-rich waters with primary or secondary nickel minerals, such as pentlandite, violarite, or millerite. ## Mineral Associations It most often co-occurs with other secondary nickel minerals and surrounding rock minerals. Typical associations include gaspeite, reevesite, takovite, goethite, magnesite, as well as relics of primary nickel sulfides (pentlandite, millerite) and serpentine and quartz. ## Localities The most important and type locality, from which the best specimens originate, is the 132 North mine in the Widgiemooltha area of Western Australia. This mineral has also been identified in several other places around the world with similar geology, including nickel mines in the Kambalda region (also Western Australia) and the Pafuri deposit in South Africa.

Rarity

Very rare

For collectors

## Quality Criteria The most prized widgiemoolthalite specimens are those consisting of well-formed, sharply defined spherulites or rosettes with an intense, blue-green color. Contrast with the host rock matrix is important – most often it is brownish limonite or light magnesite. Specimens with numerous, undamaged aggregates, at least several millimeters in diameter, fetch higher prices. Purity (absence of coatings and contaminants) also increases collectible value. ## Popular Localities By far the most desirable and classic locality is the type locality – 132 North mine, Widgiemooltha, Western Australia. Specimens from there are considered exemplary for this species. Other Australian localities, such as the Otter-Juan mine in Kambalda, also provide good quality specimens, although they are less commonly found on the market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using only a soft brush to remove dust. Due to its reactivity with acids and potential solubility, contact with water, and especially chemical solutions, should be avoided. If it is necessary to remove loose contaminants, compressed air can be used from a safe distance. ## What to Avoid Contact with acids, which rapidly decompose it, must be absolutely avoided. The mineral is soft and brittle, so it must be protected from impacts, scratching, and vibrations. It should not be exposed to ultrasound or steam cleaners. Prolonged exposure to moisture can lead to its slow degradation. ## Storage Widgiemoolthalite is best stored in a closed, stable environment, such as a display box or cabinet, away from dust and chemical contaminants. To avoid mechanical damage, it should be isolated from harder minerals. Storage in a dry place is recommended.