Nyholmite

Chemical formula: Cd<sup>2+</sup><sub>3</sub>Zn<sup>2+</sup><sub>2</sub>(As<sup>5+</sup>O<sub>3</sub>OH)<sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·4H<sub>2</sub>O

Nyholmite is a very rare zinc and cadmium arsenate, forming colorless to pale green, bladed crystals, discovered in the Block 14 mine in Australia.

## Characteristics Nyholmite is a hydrated zinc and cadmium arsenate, belonging to the brandtite group. It occurs as aggregates composed of bladed, elongated crystals, which can reach up to 0.5 mm in length. Crystals are often grouped into radial or disordered clusters. It is a secondary mineral, forming in the oxidation zones of ore deposits. ## Physical Properties Nyholmite crystals exhibit a vitreous luster. The mineral is transparent to translucent. Its Mohs hardness is 3-4, and its density is high, approximately 4.36 g/cm³. ## Colors and Varieties Nyholmite is usually colorless, sometimes taking on pale green or pale yellowish-green hues. No varieties have been distinguished. ## History and Name The mineral was named in honor of Sir Ronald Sydney Nyholm (1917-1971), an Australian chemist and president of the British Chemical Society. It was approved by the IMA in 2009, and its discovery was described by P. Elliott, G. Giester, E. R. T. Tiekink, and P. A. Williams in 2011. The type locality is the Block 14 mine in Broken Hill, New South Wales, Australia. ## Uses Due to its extreme rarity, nyholmite has no industrial applications and is solely of interest to collectors specializing in rare minerals and scientific institutions.

Properties

Mohs hardness
3-4
Luster
Vitreous
Streak
White
Density
4.36
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Nyholmite can be identified by its characteristic bladed crystals forming radial aggregates. Its high density, vitreous luster, and occurrence in association with other arsenates in oxidation zones are key features. However, definitive identification requires advanced analytical methods, such as chemical analysis (EDS) and X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other secondary arsenates, such as adamite, brandtite, or talmessite. From brandtite, with which it is structurally related, it is distinguished by the presence of cadmium and zinc in its chemical composition. Visual differentiation from similar white or colorless secondary minerals is practically impossible without specialized studies. ## Crystal Forms Nyholmite crystals are bladed, flattened, and elongated. They form radial or randomly oriented aggregates and crusts on the host rock.

Geological environment

## Genesis Nyholmite is a secondary mineral that forms in the oxidation zones of polymetallic hydrothermal deposits rich in arsenic, zinc, and cadmium. It originates from the weathering of primary ore minerals, such as sphalerite and arsenopyrite. ## Mineral Associations At the type locality in Broken Hill, nyholmite co-occurs with minerals such as goethite, quartz, smithsonite, and other rare arsenates, including brandtite, parasymplesite, and pharmacosiderite. ## Localities The only confirmed and described occurrence of nyholmite in the world is its type locality – the Block 14 mine in Broken Hill, Yancowinna County, New South Wales, Australia.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a nyholmite specimen is assessed based on the richness and development of crystals within the aggregate. The most prized specimens are those with distinct, well-formed, radial clusters of bladed crystals, contrasting with the rock matrix. Purity (lack of damage) and the aesthetics of the entire cluster are crucial, given the microscopic nature of most specimens. ## Popular Localities The only source of nyholmite specimens is the Block 14 mine in Broken Hill, Australia. Material from this locality is extremely rare and sought after by specialized collectors of systematic minerals and "micromounts."

Care and storage

## Cleaning Nyholmite specimens should be cleaned with utmost care, using only a soft brush to remove dust. Due to its fragility and potential reactivity, contact with water and any chemical agents should be avoided. ## What to Avoid Nyholmite is an arsenate mineral and contains toxic cadmium, so inhalation of dust should be avoided, and hands should be washed after any contact. It should be protected from moisture, acids, detergents, and high temperatures. Ultrasonic cleaning should not be used. ## Storage Specimens should be stored under stable conditions, in a closed, dry container or display cabinet, away from direct sunlight and heat sources. It is recommended to store it away from other minerals that it could scratch or that could damage it.

Sources

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