Weilite

Chemical formula: Ca(As⁵⁺O₃OH)

Weilite is a rare calcium hydrogen arsenate, forming colorless, tabular crystals with a vitreous or pearly luster.

## Characteristics Weilite is a rare mineral from the arsenate group, chemically classified as a hydrated calcium hydrogen arsenate. It occurs as small, colorless or white crystals with a tabular habit, often flattened. Crystals are sometimes aggregated into radial clusters, rosettes, or form thin crusts and coatings. Due to its rarity and inconspicuous appearance, it is a mineral known mainly to specialists and advanced collectors. ## Physical Properties Weilite crystals are characterized by a hardness of 3.5-4 on the Mohs scale. They have a vitreous luster, and a pearly luster on cleavage surfaces. They are transparent to translucent. The mineral's density is approximately 3.32 g/cm³. ## Colors and Varieties Weilite is typically colorless or white. No colored varieties or trade names are distinguished for it. ## History and Name The mineral was named in honor of René Weil (1901-1972), a professor of mineralogy at the University of Strasbourg, France, who first described this mineral in 1963. It was discovered in the Gabe Gottes mine in Haut-Rhin, France. ## Uses Weilite has no industrial applications. Its significance is purely scientific and for collecting.

Properties

Mohs hardness: 3.5-4
Color: White
Luster: Vitreous, Pearly
Streak: White
Density: 3.48
Cleavage: Perfect on {010}
Fracture: Irregular/Uneven
Transparency: Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Weilite can be identified by its tabular crystals, often forming radial aggregates or rosettes. The pearly luster on cleavage planes and its occurrence in paragenesis with other arsenates in ore oxidation zones are also characteristic. ## Distinguishing from Similar Minerals Weilite is sometimes confused with other colorless or white arsenates, such as pharmacolite or haidingerite. However, pharmacolite forms acicular crystals and fibrous aggregates. Haidingerite has a similar composition but crystallizes in the orthorhombic system and often forms fan-shaped aggregates. Accurate distinction of these minerals usually requires advanced studies, e.g., by X-ray diffraction (XRD). ## Crystal Forms Weilite crystals are tabular, flattened parallel to the (010) plane. They often occur as radial or stellate aggregates, rosettes, and also as thin crusts and coatings on other minerals.

Geological environment

## Genesis Weilite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, especially in the presence of calcium-rich rocks. It forms as a result of reactions between arsenic acid-containing waters and calcium minerals. ## Mineral Associations This mineral co-occurs with other secondary arsenates, such as pharmacolite, picropharmacolite, haidingerite, sainfeldite, as well as with gypsum, pharmacosiderite, and iron oxides. ## Localities The most important weilite occurrences worldwide include: - France: Gabe Gottes mine, Sainte-Marie-aux-Mines, Haut-Rhin (type locality). - Germany: Anton mine in Heubach Valley, Schiltach, Black Forest, and in the Wittichen area. - Czech Republic: Jáchymov (St. Joachimsthal). - USA: Sterling Hill, Ogdensburg, New Jersey.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals forming aesthetic rosettes or radial aggregates are most valued by collectors. The clarity and transparency of the crystals, as well as their size, are also important. A contrasting matrix with another mineral, e.g., dark rock, significantly enhances the attractiveness of the specimen. ## Popular Localities Classic and most sought-after specimens come from historical localities in Germany (Black Forest) and France (Sainte-Marie-aux-Mines). Specimens from these locations are considered exemplary for this species.

Care and storage

## Cleaning Weilite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its reactivity, contact with water and any chemical agents should be avoided. If it is necessary to remove loose contaminants, compressed air can be used from a safe distance. ## What to Avoid Contact with water, acids, and other chemicals that could damage or dissolve the mineral must be strictly avoided. Weilite is sensitive to moisture and temperature changes. It should not be heated or exposed to direct sunlight. ## Storage It is recommended to store weilite in a dry place, preferably in a tightly sealed container or a display box with a desiccant. It should be protected from dust and mechanical damage, isolated from harder minerals.