Petewilliamsite

Chemical formula: (Ni₂+,Co₂+)₃₀(As₅+₂O₇)₁₅

Petewilliamsite is a very rare nickel-cobalt arsenate, forming microscopic, hexagonal crystals of an intense brown color.

## Characteristics Petewilliamsite is an extremely rare mineral from the arsenate group, chemically classified as a nickel-cobalt arsenate. It occurs as very small, hexagonal, tabular or prismatic crystals, rarely exceeding 0.2 mm in length. It typically forms radial aggregates or rosettes. Its characteristic feature is an intense, dark brown color. ## Physical Properties Petewilliamsite crystals exhibit a strong, almost metallic or metallic luster. They are brittle, and due to the small size of the crystals, precise determination of hardness and density is difficult. The mineral is opaque. ## Colors and Varieties This mineral occurs in a uniform, dark brown color. No varieties have been distinguished. ## History and Name Petewilliamsite was discovered in the dumps of the Tsumeb mine in Namibia and described in 1999. Its name honors Peter (Pete) A. Williams (born 1950), Professor of Inorganic Chemistry at the University of Western Sydney, Australia, for his contributions to the mineralogy of secondary oxidation zones. ## Uses Due to its extreme rarity and microscopic size, petewilliamsite has no industrial application. It is of purely scientific interest and is an object of interest for specialized collectors of rare minerals.

Properties

Mohs hardness: 4
Luster: Metallic to submetallic
Streak: Brown
Density: 5.19
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Hexagonal

Diagnostic features

## Identification Identification of petewilliamsite is primarily based on analysis of its occurrence (Tsumeb mine), characteristic crystal morphology, and mineral paragenesis. Field identification is practically impossible. Definitive identification requires advanced laboratory methods, such as X-ray diffraction (XRD) and chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other rare, brown arsenates from Tsumeb, such as o'danielite or ludlockite. Visual differentiation is unreliable; chemical analysis, which will show the dominance of nickel and cobalt, and crystallographic analysis confirming the hexagonal structure, are key. ## Crystal Forms It forms small, hexagonal, tabular or short-prismatic crystals. Most often, they occur as radial or rosette-like aggregates on the surface of the host rock.

Geological environment

## Genesis Petewilliamsite is a secondary mineral that formed in the oxidation zone of the polymetallic (Cu-Pb-Zn-As-Ge) deposit at Tsumeb. It formed as a result of the weathering of primary sulfides and arsenides, especially tennantite, in a nickel- and cobalt-rich environment. It occurs in vugs and fissures within heavily corroded tennantite ore. ## Mineral Associations This mineral coexists with other rare secondary minerals. In its type locality (Tsumeb), it was found in association with ludlockite, leiteite-2O, zinc, o'danielite, tennantite, quartz, and carbonates (kaleyite). ## Localities The only confirmed locality of petewilliamsite in the world is the Tsumeb mine in the Oshikoto Region, Namibia. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of petewilliamsite specimens is primarily assessed based on the richness and development of crystals within the aggregate. The most prized specimens are those with distinct, radial aggregates or rosettes of intense brown color, contrasting well with the matrix. The size of individual crystals, although always microscopic, is also important. The presence of rare associated minerals is also significant. ## Popular Localities The only source of specimens is the historic Tsumeb mine in Namibia. All specimens available on the collector's market come from this single location and are part of old finds.

Care and storage

## Cleaning Specimens containing petewilliamsite should be handled with utmost care. Due to the microscopic size and brittleness of the crystals, mechanical cleaning is not recommended. If absolutely necessary, compressed air (from a safe distance) can be used to remove dust. Avoid contact with water and any chemicals. ## What to Avoid Ultrasonic cleaners, chemicals, acids, and sudden temperature changes should be strictly avoided. The mineral is an arsenate, so situations leading to its dusting and inhalation of dust should be avoided. ## Storage Specimens should be stored in stable conditions, in a closed, transparent "micromount" box, which protects them from mechanical damage, dust, and moisture. Store away from direct sunlight and heat sources.

Sources

IMA Mineral List (CNMNC)