Villyaellenite

Chemical formula: (Mn²⁺,Ca)Mn²⁺₂Ca₂(As⁵⁺O₄)₂(As⁵⁺O₃OH)₂·₄H₂O

Villyaellenite is a rare arsenate from the hureaulite group, forming colorless, orange, or pink crystals with a vitreous luster.

## Characteristics Villyaellenite is a hydrated manganese calcium arsenate, belonging to the hureaulite group. This mineral typically forms small, well-developed crystals with a tabular or short-prismatic habit. It often occurs as radial or fan-shaped aggregates, as well as coatings and crusts. It is transparent to translucent and characterized by a vitreous luster. ## Physical Properties Its Mohs hardness is 4, classifying it as a relatively soft mineral. The density is 3.20 g/cm³. It possesses good cleavage in one direction. ## Colors and Varieties The most common specimens are colorless, pale pink, or orange. The intensity of the color depends on impurities and crystallization conditions. No named varieties of this mineral are distinguished. ## History and Name The mineral was discovered and described in 1983. Its name honors Villy Aellen (born 1926), a Swiss zoologist and director of the Natural History Museum in Geneva, in recognition of his contributions to the study of bats inhabiting caves and mines, where rare minerals are often found. ## Uses Villyaellenite has no industrial application. It is solely a mineral of scientific and collector's interest, valued for its rarity and aesthetic crystals.

Properties

Mohs hardness: 4
Luster: Vitreous
Streak: White
Density: 3.20
Cleavage: {100} good
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Characteristic features of villyaellenite include its color (orange, pink, or colorless), vitreous luster, and typical forms of occurrence – radial aggregates of small, tabular crystals. It is relatively soft (hardness 4). ## Distinguishing from similar minerals It can be confused with other arsenates from the hureaulite group, such as hureaulite or sainfeldite. It is usually distinguished from hureaulite by its lighter color and the presence of calcium in its composition. Precise identification requires advanced analytical methods, such as spectroscopy or X-ray diffraction. ## Crystal Forms It crystallizes in the monoclinic system. It forms tabular or short-prismatic crystals, often gathered in fan-shaped, radial, or spherulitic aggregates. It also occurs as druses and crusts in rock fissures.

Geological environment

## Genesis Villyaellenite is a secondary mineral, formed in the oxidation (weathering) zones of polymetallic ore deposits rich in arsenic. It forms as a result of the alteration of primary arsenic, manganese, and calcium minerals under conditions of low temperatures and pressures. ## Mineral Associations It co-occurs with other secondary arsenate minerals. In the type locality (Sainte-Marie-aux-Mines), it was found in association with pharmacolite, picropharmacolite, sainfeldite, hureaulite, fluckite, and manganese oxides. ## Localities The most important and type locality is the former silver mines in the Sainte-Marie-aux-Mines region in Grand Est, France. It is a very rare mineral, reported from only a few places worldwide, including the Sterling Hill mine in New Jersey (USA) and Jáchymov in the Czech Republic.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, transparent crystals of intense, orange or pink color. Rich druses covering the rock matrix and aesthetic, radial aggregates are also highly valued. Crystal size rarely exceeds a few millimeters, so even small but well-formed specimens are sought after. ## Popular Localities By far the most desirable specimens come from the type locality – Sainte-Marie-aux-Mines in France. These are considered classic and the best for this mineral species.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided. ## What to Avoid The mineral is sensitive to acids and strong chemicals. As a relatively soft mineral, it is susceptible to scratches. Avoid sudden temperature changes and prolonged exposure to direct sunlight, which can affect its color. ## Storage It is recommended to store specimens in separate, padded boxes to protect them from mechanical damage and contact with harder minerals. It is best displayed in enclosed showcases, protecting against dust and humidity.

External references

https://www.mindat.org/min-4182.html