Dufrénoysite

Chemical formula: Pb₂+₂As₃+₂S₂-₅

Dufrénoysite is a rare lead arsenic sulfosalt mineral, forming elongated, prismatic crystals with a metallic luster.

## Characteristics Dufrénoysite is a lead arsenic sulfosalt belonging to the sartorite group. It forms characteristic, elongated, prismatic or bladed crystals, often exhibiting longitudinal striations on their faces. It usually occurs as embedded or overgrown aggregates. It is an opaque mineral, with a steel-gray to lead-gray color and a metallic luster. Its surface may be covered with a dull, black tarnish or exhibit iridescence. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3. It is quite brittle, and its density is significant, around 5.56 g/cm³, which is typical for lead-bearing minerals. The luster is strongly metallic on fresh surfaces, dulling over time. ## Colors and Varieties Dufrénoysite has a consistent, characteristic color ranging from steel-gray to lead-gray. Sometimes, a colorful, iridescent tarnish or a dull, black coating, a product of its weathering, appears on its surface. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1845 by Augustin Alexis Damour, honors the French geologist and mineralogist Pierre-Armand Dufrénoy (1792-1857). It was first described based on specimens from the Lengenbach dolomite quarry in the Binn Valley (Binnental) in Switzerland, which is its type locality. ## Uses Due to its rarity, dufrénoysite has no industrial applications. It is solely an object of interest for collectors specializing in rare sulfosalts and minerals from alpine parageneses.

Properties

Mohs hardness: 3
Luster: Metallic
Streak: Reddish-brown
Density: 5.5-5.56
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Dufrénoysite is identified by its characteristic habit of elongated, prismatic crystals with longitudinal striations, steel-gray color, metallic luster, and high density. A reddish-brown streak is also an important diagnostic feature. It often occurs in association with other rare sulfosalts in dolomites. ## Distinguishing from Similar Minerals It can be confused with other lead sulfosalts, such as sartorite, baumhauerite, or jordanite, which occur in the same paragenesis. Differentiation often requires chemical analysis (EDS) or crystallographic analysis (XRD). From more common minerals, like antimonite, it differs in crystal habit and reddish streak (antimonite has a grayish-black streak). From boulangerite, it is distinguished by its habit – boulangerite more often forms fibrous aggregates. ## Crystal Forms Crystals are typically elongated along one of the crystallographic axes, forming prismatic, bladed, or acicular habits. Crystal faces are often deeply striated along the elongation axis. Crystals can be slightly flattened and occur as inclusions in calcite or dolomite, or form overgrown, radial aggregates.

Geological environment

## Genesis Dufrénoysite is a hydrothermal mineral. It forms under low to medium temperature conditions. It is most commonly found in ore veins cutting through sedimentary rocks, especially dolomites and limestones. It can also occur in polymetallic deposits. ## Mineral Associations This mineral often co-occurs with other lead and arsenic sulfosalts, such as sartorite, jordanite, baumhauerite, as well as realgar, orpiment, sphalerite, pyrite, and tennantite. In its type locality (Lengenbach), it is accompanied by a rich paragenesis of rare sulfosalt minerals in white dolomite. ## Localities The most important and classic locality for dufrénoysite is the Lengenbach dolomite quarry in the Binn Valley (Binnental) in the canton of Valais, Switzerland. The world's best, well-formed crystals originate from there. Other known localities include the Binntal mine in Peru (Huancavelica region), the Allchar deposit in North Macedonia, and the Uchinotai mine in Japan (Akita Prefecture).

Rarity

Rare

For collectors

## Quality Criteria The most prized dufrénoysite specimens are those with well-formed, sharp, undamaged crystals with a strong metallic luster. Specimens where dark, steel-gray crystals contrast with a light, white background of dolomite or calcite are highly valued. The size of the crystals, their abundance on the matrix, and aesthetic arrangement in the form of radial aggregates significantly increase their collector's value. The presence of rare associated minerals, especially from Lengenbach, is an additional asset. ## Popular Localities Undoubtedly, the most sought-after and classic locality for collectors is the Lengenbach quarry in Switzerland. Specimens from there are considered exemplary for this species. High-quality crystals, though usually smaller, also come from the Binntal mine in Peru.

Care and storage

## Cleaning Dufrénoysite specimens should be cleaned very carefully, preferably dry, using a soft brush to remove dust. Compressed air can be used from a safe distance. Due to the arsenic and lead content, avoid inhaling dust during cleaning and wash hands after contact with the mineral. ## What to Avoid Absolutely avoid contact with water and all chemicals, including acids and bases, which can damage the mineral's surface. Dufrénoysite is sensitive to moisture and oxidation, which causes it to dull and form black tarnishes. It should not be heated or exposed to prolonged sunlight. ## Storage It is recommended to store specimens in dry conditions, preferably in closed, airtight containers or display cases away from moisture. To prevent oxidation, desiccants can be used. Due to its brittleness, avoid vibrations and store it in a place where it will not be subjected to impacts.

Sources

IMA Mineral List (CNMNC)