Willyamite

Chemical formula: Co<sup>3+</sup>(SbS)<sup>3-</sup>

Willyamite is a rare, metallic cobalt and antimony mineral, forming isometric crystals of a silvery-white color, found in ore deposits.

## Characteristics Willyamite is a rare mineral from the cobaltite group, classified as a cobalt antimony sulfide. It occurs in the form of well-developed, isometric crystals, most often cubes and octahedra, which can reach up to 1 cm in size. Its appearance is typically metallic, with a bright, silvery-white color that can dull or become covered with a pinkish erythrite coating on weathered surfaces. ## Physical Properties This mineral is characterized by high hardness, 5.5 on the Mohs scale, and significant density, approximately 6.85 g/cm³. It has a metallic luster and is completely opaque. It exhibits perfect cleavage in three directions, which is typical for its regular crystal structure. The fracture is uneven. ## Colors and Varieties Willyamite has a characteristic silvery-white or tin-white color. No color varieties or commercial varieties are known. Over time, its surface can oxidize, taking on a dull, grayish hue or a pink coating of secondary erythrite. ## History and Name The mineral's name comes from its discovery location – the Willyama mining complex in the Broken Hill region of New South Wales, Australia. It was first described in 1893 by the Australian mineralogist Edward F. Pittman. ## Uses Due to its extreme rarity, willyamite has no industrial application. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of systematic minerals and rare ores.

Properties

Mohs hardness
5.5
Luster
Metallic
Streak
Black
Density
6.85
Cleavage
Perfect on {001}, {100}, and {010}
Fracture
Uneven
Transparency
Opaque
Crystal system
Cubic

Diagnostic features

## Identification Key diagnostic features of willyamite include its silvery-white color, metallic luster, regular, cubic or octahedral crystals, and high density. Its excellent, cubic cleavage is also characteristic. It often occurs in association with other sulfides and sulfosalts of silver, lead, and zinc. ## Distinguishing from Similar Minerals Willyamite can be confused with cobaltite, ullmannite, or gersdorffite. It is distinguished from cobaltite by the absence of a pinkish tint on fresh surfaces and by chemical tests for antimony. Ullmannite (NiSbS) and gersdorffite (NiAsS) have very similar physical properties, and definitive differentiation requires advanced chemical analyses (EDS/XRD) to determine the dominant elements (Co vs. Ni, Sb vs. As). ## Crystal Forms Willyamite crystallizes in the isometric system, most often forming well-developed cubes, octahedra, and their combinations. Crystals are usually embedded in the host rock; less commonly, they occur as loose granular aggregates.

Geological environment

## Genesis Willyamite is a mineral of hydrothermal origin. It forms in ore veins that cut through metamorphosed sedimentary rocks. Its crystallization occurs under medium to high temperature conditions, in an environment rich in cobalt, antimony, and sulfur. ## Mineral Associations This mineral most often co-occurs with other sulfides and sulfosalts. Typical associated minerals include siderite, sphalerite, galena, chalcopyrite, ullmannite, cobaltite, as well as rarer silver minerals such as dyscrasite and proustite. In the oxidation zone, it may be covered with an erythrite coating. ## Localities The most important and historical occurrence of willyamite is its type locality – the Broken Hill deposit in New South Wales, Australia, from which the best-known specimens originate. Its presence has also been confirmed in mines in the Cobalt region of Ontario (Canada) and in small quantities in the Bou Azzer deposit in Morocco.

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized willyamite specimens are those with sharp, well-formed crystals exhibiting a mirror-like, metallic luster and a pure, silvery-white color. Crystals larger than a few millimeters, clearly exposed on a contrasting matrix (e.g., on siderite), are particularly sought after. Specimens without mechanical damage and signs of weathering achieve the highest value. ## Popular Localities By far the most classic and valued specimens come from the historical type locality – Broken Hill, Australia. Material from this location is considered exemplary for the species and serves as a reference point for collectors. Specimens from Canada and Morocco are much rarer on the market and generally have less collector significance.

Care and storage

## Cleaning Willyamite specimens should only be cleaned mechanically, using a soft, dry brush or compressed air to remove dust. All contact with water and chemical agents that could cause reactions on its surface should be avoided. ## What to Avoid The mineral is sensitive to moisture and oxidation. Contact with water, acids, and other chemicals must be strictly avoided. It should not be heated or exposed to prolonged periods of humid air, which accelerates weathering processes. ## Storage It is recommended to store willyamite specimens in dry, airtight containers (e.g., "membrane boxes" or boxes with desiccant). Display should be in closed showcases, away from sources of moisture and sudden temperature changes.

Sources

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