Baumhauerite
Chemical formula: Pb<sup>2+</sup><sub>12</sub>As<sup>3+</sup><sub>16</sub>S<sup>2-</sup><sub>36</sub>
A very rare lead-arsenic sulfosalt with a metallic luster, known for its perfectly formed crystals from the Lengenbach quarry in Switzerland.
Properties
- Mohs hardness
- 3
- Luster
- Metallic
- Streak
- Red-brown
- Density
- 5.26-5.33
- Cleavage
- Perfect on {100}
- Fracture
- Conchoidal
- Transparency
- Opaque
- Crystal system
- Triclinic
Diagnostic features
## Identification The key diagnostic feature of baumhauerite is its characteristic reddish-brown or cherry-red streak, which clearly distinguishes it from many similar sulfosalts. Other important features include its lead-gray color, strong metallic luster, perfect cleavage, and the specific form of tabular, striated crystals. Knowledge of the origin (Binn Valley) is a strong indicator. ## Distinguishing from Similar Minerals Baumhauerite can be confused with other rare sulfosalts from Lengenbach, such as sartorite, rathite, or dufrenoysite. However, most of them have a different streak color (e.g., sartorite's streak is reddish-brown, but usually a different shade). Reliable distinction of these minerals often requires advanced analytical methods, such as chemical composition analysis (EDS) or X-ray diffraction (XRD). ## Crystal Forms Crystals are usually flattened (tabular) parallel to the cleavage plane, often slightly elongated. Their surfaces are rich in details and covered with deep, parallel striations. Twinning is common.
Geological environment
## Genesis Baumhauerite forms as a result of low-temperature hydrothermal processes. It occurs in veins cutting Triassic dolomites. The classic and most important environment for its occurrence is the deposits in the Binn Valley in Switzerland. ## Mineral Associations This mineral almost always occurs in association with other, often rare, arsenic sulfides and sulfosalts. Its most common associations include realgar, orpiment, jordanite, sartorite, dufrenoysite, rathite, and pyrite. ## Localities The most important and historical locality, from which the world's best baumhauerite specimens originate, is the Lengenbach quarry in the Binn Valley (Binnatal) in the canton of Valais, Switzerland. The mineral has been reported in several other locations worldwide, e.g., in the Sterling Hill mine (New Jersey, USA), but specimens from these localities are not of collector's significance.
Rarity
Very rare
For collectors
## Quality Criteria The most highly prized baumhauerite specimens are those with sharp, well-formed, and undamaged crystals with a strong luster. The size of the crystals is of great importance – those exceeding a centimeter are exceptionally rare. The value of a specimen is also enhanced by aesthetic associations with contrasting minerals, such as red realgar, yellow orpiment, or other rare sulfosalts. ## Popular Localities The only locality that provides specimens of collector's significance is the Lengenbach quarry in Switzerland. This is a classic, world-renowned locality for this and many other rare minerals.
Care and storage
## Cleaning Baumhauerite specimens should only be dry-cleaned, using a soft brush to remove dust. Any contact with water and chemical agents should be avoided, as they can lead to oxidation and damage to the mineral's surface. ## What to Avoid The mineral is soft and brittle, requiring careful handling to avoid scratches and mechanical damage. It should be protected from moisture, chemicals, and ultrasound. Some associated minerals, such as realgar, are light-sensitive, so the entire specimen should be protected from prolonged exposure to sunlight. ## Storage It is recommended to store specimens in stable, dry conditions, preferably in sealed boxes or display cases, to limit access to dust and moisture and protect against damage.