Dervillite

Chemical formula: Ag₂AsS₂

Dervillite is a very rare silver sulfarsenide, forming small, platy crystals with a metallic luster and gray color.

## Characteristics Dervillite is a very rare mineral from the sulfosalt group, chemically classified as a silver sulfarsenide. It occurs as very small, platy or bladed crystals, rarely exceeding a fraction of a millimeter. It typically forms granular aggregates or occurs as inclusions in other minerals. It is opaque and has a gray color and metallic luster. ## Physical Properties This mineral is extremely soft, with a Mohs hardness of only 1-1.5, meaning it can be scratched with a fingernail. It has a metallic luster. Its density has not been precisely measured, but the calculated value is approximately 6.22 g/cm³. ## Colors and Varieties Dervillite is a mineral of constant color – it occurs exclusively in shades of gray. It does not form color varieties or commercial types. ## History and Name The mineral was first described in 1941 by J.H. Strunz. Its name commemorates Henri Derville (1888-1959), a French professor of mineralogy at the University of Strasbourg. ## Uses Due to its extreme rarity and microscopic crystal sizes, dervillite has no industrial application. It is solely an object of scientific and collecting interest for specialized collectors of rare minerals.

Properties

Mohs hardness
1-1.5
Luster
Metallic
Density
0
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identifying dervillite is extremely difficult without advanced analytical methods. Visual identification is practically impossible due to the microscopic size of the crystals. Helpful characteristics may include: very low hardness, gray color, metallic luster, and co-occurrence with other silver sulfosalts in a specific geological environment. Final confirmation requires chemical (EDS) and X-ray (XRD) analysis. ## Distinguishing from Similar Minerals Dervillite can be confused with many other silver sulfosalts, such as proustite, pyrargyrite, stephanite, or acanthite. Distinguishing them based on visual characteristics is impossible – all are gray, metallic minerals. However, proustite and pyrargyrite can sometimes be translucent red at the edges, unlike opaque dervillite. ## Crystal Forms It forms very small, thin, platy or bladed crystals, often arranged in irregular aggregates. Crystals show a tendency to striation.

Geological environment

## Genesis Dervillite is a hydrothermal mineral. It forms in the final stages of crystallization in ore veins rich in silver and arsenic, forming at low to medium temperatures. ## Mineral Associations In its type locality (Sainte-Marie-aux-Mines), it co-occurs with minerals such as native arsenic, proustite, xanthoconite, pyrargyrite, stephanite, acanthite, native silver, and galena. ## Localities It is an extremely rare mineral, known from only a few localities worldwide. The most important and type locality is the historic silver mines in the Sainte-Marie-aux-Mines region in the Vosges (Grand Est, France). Outside of France, its occurrence has also been reported in the Uchucchacua mine in Peru and the Příbram mine in the Czech Republic.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a dervillite specimen is primarily assessed based on the confirmation of its identity through analysis. Since the crystals are microscopic, any specimen where the mineral is visible under magnification and forms even minimally developed shapes is considered valuable. Rich associations with other rare silver sulfosalts from the type locality are particularly prized. ## Popular Localities The only source of collectible specimens of historical and reference importance is the Sainte-Marie-aux-Mines region in France. Specimens from other localities are practically unavailable on the market.

Care and storage

## Cleaning Specimens of dervillite should generally not be cleaned mechanically or chemically due to their extreme rarity and delicacy. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water. ## What to Avoid The mineral is very soft and susceptible to mechanical damage. Avoid contact with harder minerals, acids, detergents, and sudden temperature changes. As a sulfide, it may be sensitive to moisture and oxidation in the long term. ## Storage Specimens should be stored in stable conditions, in a closed, dry container or display case, away from direct sunlight and heat sources. It is best to store it in its original "micromount" box to protect the delicate crystals from damage.

External references

Sources

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