Wallkilldellite

Chemical formula: Ca<sub>4</sub>Mn<sup>2+</sup><sub>6</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>4</sub>(OH)<sub>8</sub>·18H<sub>2</sub>O

Wallkilldellite is a very rare calcium manganese arsenate, forming microscopic, hexagonal, reddish-brown crystals.

## Characteristics Wallkilldellite is a hydrated calcium manganese arsenate, occurring as extremely small, hexagonal crystals. Typical specimens consist of radial aggregates or rosettes composed of tabular crystals, rarely exceeding 0.5 mm in size. The mineral is characterized by a reddish-brown to brown color and a vitreous luster. ## Physical Properties Wallkilldellite crystals are very soft, with a Mohs hardness of 2. They exhibit a vitreous luster and are transparent to translucent. The mineral's density has been calculated at 2.96 g/cm³. ## Colors and Varieties This mineral occurs in uniform shades from reddish-brown to brown. No varieties have been distinguished. ## History and Name Wallkilldellite was discovered at the Sterling Hill Mine in Ogdensburg, New Jersey (USA). Its name, approved in 1982, comes from the Wallkill Valley, where the discovery site is located. It was described by Pete J. Dunn, Donald R. Peacor, and William B. Simmons. ## Uses Due to its extreme rarity and microscopic size, Wallkilldellite has no commercial or industrial applications. It is solely an object of interest for collectors specializing in rare minerals or minerals from the Franklin-Sterling Hill type locality.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Pale red
Density
2.96
Cleavage
Perfect on {0001}
Fracture
Micaceous
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of Wallkilldellite is primarily based on its characteristic appearance under a microscope – radial aggregates of small, hexagonal, tabular crystals of a reddish-brown color. Association with the locality (Sterling Hill) and co-occurring minerals is also crucial. Due to its size, definitive confirmation requires advanced analytical methods such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Differentiation from Similar Minerals It can be confused with other microscopic manganese arsenates. Differentiation under amateur conditions is practically impossible. It is distinguished from similar minerals of the brandholzite group by its chemical composition and crystallographic parameters. ## Crystal Forms Wallkilldellite forms spherical or radial aggregates composed of very small, thin, tabular crystals with a hexagonal outline.

Geological environment

## Genesis Wallkilldellite is a secondary mineral, formed in zinc and manganese ore deposits that have undergone contact metamorphism. It forms in rock fissures and vugs as a result of hydrothermal alteration of primary ore minerals. ## Mineral Associations This mineral occurs in association with other rare minerals such as franklinite, willemite, zincite, andradite, rhodochrosite, barite, and especially kraisslite, to which it is structurally closely related. ## Localities The only confirmed occurrence of Wallkilldellite in the world is its type locality – the Sterling Hill Mine in Ogdensburg, Sussex County, New Jersey, USA.

Rarity

Very rare

For collectors

## Quality Criteria The quality of Wallkilldellite specimens is primarily assessed based on the richness and definition of the crystal aggregates. The most prized specimens are those with clearly formed, radial rosettes of intense, reddish-brown color, contrasting well with the rock matrix. Due to the microscopic nature of the mineral, the quality of the specimen as seen under magnification is crucial. ## Popular Localities The only source of specimens is the Sterling Hill Mine in the USA. Material from this historic locality is highly sought after by collectors specializing in systematic minerals and minerals from the Franklin-Sterling Hill region.

Care and storage

## Cleaning Specimens should be handled with utmost care. Cleaning is not recommended due to the extreme fragility and small size of the crystals. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust. ## What to Avoid Avoid contact with water, chemicals, ultrasound, and all forms of mechanical cleaning (e.g., brushes). The mineral is very soft and sensitive to shocks and temperature changes. Protect it from direct sunlight, which can cause dehydration and damage. ## Storage It is recommended to store specimens exclusively in specialized, sealed "micromount" boxes, protecting them from dust, shocks, and humidity changes. Display should occur under stable environmental conditions.

Sources

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