Mcnearite

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

Mcnearyte is a rare, hydrated sodium-calcium arsenate, forming tiny, colorless or white crystals and coatings.

## Characteristics Mcnearyte is a very rare mineral from the arsenate group, chemically classified as a hydrated sodium and calcium arsenate. It occurs as very small, bladed or acicular crystals, rarely exceeding 1 mm in length. These crystals often form radiating aggregates, rosettes, or thin coatings and crusts on rock surfaces. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Mcnearyte crystals are characterized by a vitreous or pearly luster. They are transparent to translucent. The hardness of the mineral has not been precisely determined but is low, similar to other hydrated arsenates. Density is also unknown. It exhibits perfect cleavage in one direction. ## Colors and Varieties Mcnearyte is typically colorless or white. No colored or commercial varieties have been distinguished. ## History and Name The mineral was discovered in the Getchell Mine in Humboldt County, Nevada (USA). Its description was published in 1993. The name honors Elizabeth McNear (1926-1990), an American mineralogist and curator of mineral collections at the California Academy of Sciences, in recognition of her contributions to mineralogy. ## Uses Mcnearyte has no industrial applications. Its significance is purely scientific and as a collector's item, being a very rare and unique mineral species.

Properties

Luster
Vitreous to pearly
Streak
White
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Amateur identification of mcnearyte is practically impossible. This mineral is distinguished by its occurrence as very small, colorless or white, bladed crystals forming radiating aggregates. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). ## Distinguishing from similar minerals Mcnearyte can be confused with many other secondary arsenic minerals, such as pharmacolite, guerinite, or picropharmacolites, which form similar white, acicular or fibrous aggregates. Differentiation is almost exclusively possible based on chemical and crystallographic analysis. ## Crystal forms Mcnearyte crystallizes as fine, bladed or acicular crystals, elongated along one axis. These crystals often combine into radiating aggregates, rosettes, and also form thin crusts and coatings.

Geological environment

## Genesis Mcnearyte is a secondary arsenic mineral that forms in the oxidation zones (weathering) of ore deposits rich in arsenic, especially realgar and orpiment. It forms as a result of the reaction of arsenic-rich solutions with calcium-containing minerals, under conditions of low temperature and pressure. ## Mineral associations This mineral co-occurs with other secondary arsenates. In the type locality (Getchell Mine), it was found in association with realgar, orpiment, guerinite, pharmacolite, and picropharmacolites. ## Localities The only confirmed and well-documented occurrence of mcnearyte in the world is the Getchell Mine in the Potosi District, Humboldt County, Nevada, USA. This is its type locality.

Rarity

Very rare

For collectors

## Quality criteria The quality of a mcnearyte specimen is primarily assessed based on the abundance and development of its crystals. Most valued are specimens with clearly visible, well-formed radiating aggregates or rosettes on a contrasting matrix rock. Due to the microscopic size of the crystals, their abundance on the matrix is crucial. Purity and lack of damage to the delicate needles also increase the specimen's value. ## Popular localities All known collector specimens originate from a single location worldwide - the Getchell Mine in Nevada, USA. This is the sole source of this mineral.

Care and storage

## Cleaning Mcnearyte specimens are extremely delicate and sensitive. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove loose dust particles. Contact with water, which can damage or dissolve the mineral, should be avoided. ## What to avoid Contact with water, acids, detergents, and all chemical agents must be strictly avoided. The mineral is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight. Due to its low hardness, it is very susceptible to mechanical damage. ## Storage It is recommended to store specimens in sealed, stable containers (e.g., "micromount boxes") to protect them from dust, humidity, and physical damage. They should ideally be kept in a dry place with a constant temperature.

Sources

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