Lengenbachite

Properties

Mohs hardness

1.5-2.5

Luster

Metallic

Streak

Black

Density

5.8-5.9

Cleavage

Perfect on {100}

Fracture

Fibrous

Transparency

Opaque

Crystal system

Triclinic

Diagnostic features

## Identification Lengenbachite can be identified by its unique form – very thin, flexible, bladed or hair-like crystals, often curled or tangled. Its strong metallic luster, black color, and very low hardness are also characteristic. Its occurrence in dolomitic marbles alongside other rare sulfosalts is a strong diagnostic indicator. ## Distinguishing from similar minerals Lengenbachite is sometimes confused with other acicular sulfosalts, such as jamesonite, boulangerite, or zinkenite. It differs from them primarily by the significantly greater flexibility and pliability of its crystals. Many of these minerals are rigid and brittle. However, definitive confirmation requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal forms Crystals are tabular, bladed, or hair-like, elongated in one direction. A characteristic feature is their tendency to curl into tubular or cylindrical forms. It forms radial aggregates, tangled aggregates resembling mineral wool (so-called "felt"), as well as crusts on other minerals.

Geological environment

## Genesis Lengenbachite is a mineral of hydrothermal origin. It forms as a result of metasomatic interaction of solutions rich in arsenic, sulfur, and metals (Pb, Ag, Cu) with carbonate rocks, such as dolomites and dolomitic marbles, under relatively low-temperature conditions. ## Mineral associations This mineral co-occurs with a rich assemblage of other, often rare, sulfosalts. At its type locality (Lengenbach), it is associated with, among others, jordanite, sartorite, dufrénoysite, baumhauerite, hutchinsonite, as well as realgar, orpiment, and pyrite. The host rock is usually white, sugary dolomite. ## Localities The most important and historical locality for lengenbachite, from which the world's best specimens originate, is the Lengenbach quarry in the Binn Valley (Binntal) in the canton of Valais, Switzerland. This is its type locality and practically the only place where it occurs in quantities and quality of collector significance. Occurrences are also known from several other places in the world, including the Uschkatyn-III mine in Kazakhstan, but these do not yield material of comparable quality.

Rarity

Very rare

For collectors

## Quality criteria The most highly prized lengenbachite specimens are those with rich, dense clusters of well-formed, lustrous, flexible crystals, contrasting with a white dolomitic matrix. Specimens with visible, partially curled crystals are particularly sought after. The size of the specimen and the richness of the aggregates are crucial for its value. The presence of other rare, well-identified associated minerals from Lengenbach also enhances its appeal. ## Popular localities The only source of valuable collector specimens is the historical locality – the Lengenbach quarry in Switzerland. Specimens from this location are considered mineralogical classics and serve as a benchmark for this mineral.

Care and storage

## Cleaning Lengenbachite specimens should generally not be wet cleaned. Dust can be removed very carefully with a soft brush or a photographic air blower. Any contact with water or chemicals is risky. ## What to avoid Contact with water, acids, and other chemicals that can damage the delicate crystals must be strictly avoided. The mineral is very soft and brittle, so it should be protected from impacts, scratches, and vibrations. Prolonged exposure to strong light or humidity can lead to its slow oxidation and degradation. ## Storage Lengenbachite specimens are best stored in closed, stable containers (e.g., "micromount" type), protecting them from dust, moisture, and mechanical damage. Due to the presence of arsenic and lead, hands should be washed after handling the mineral.

Sources

• IMA Mineral List (CNMNC)