Baumhauerite II
Chemical formula: Pb²⁺₃As³⁺₄S²⁻₉
Baumhauerite-II is a rare lead arsenic sulfosalt, forming metallic-lustered, acicular crystals, found in hydrothermal deposits.
Properties
- Mohs hardness
- 3
- Luster
- Metallic
- Streak
- Reddish-brown
- Density
- 5.3
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Triclinic
Diagnostic features
## Identification Baumhauerite-II is recognized by its characteristic crystal habit – long, prismatic or acicular, often with longitudinal striations. Key features also include its steel-gray color, metallic luster, and high density. However, definitive identification requires advanced analytical methods such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from Similar Minerals This mineral is difficult to distinguish from other rare lead arsenic sulfosalts, such as baumhauerite, rathite, dufrénoysite, or sartorite, which often occur in the same localities and have a similar appearance. Certain differentiation is possible almost exclusively based on crystal structure analysis. ## Crystal Forms Crystals are elongated along the b-axis, forming prismatic, acicular, or bladed habits. They often exhibit longitudinal striations parallel to the elongation. Crystals can occur as single needles or form tangled, woolly aggregates and radial clusters.
Geological environment
## Genesis Baumhauerite-II is a mineral of hydrothermal origin. It forms in ore veins that developed as a result of the metamorphism of carbonate rocks, primarily dolomites. It is a product of crystallization from solutions rich in lead, arsenic, and sulfur under relatively low-temperature conditions. ## Mineral Associations It most commonly co-occurs with other lead arsenic sulfosalts, such as sartorite, rathite, dufrénoysite, as well as with pyrite, sphalerite, galena, and realgar. These minerals form complex parageneses within dolomitic host rocks. ## Localities The most important and classic locality for baumhauerite-II is the Lengenbach quarry in the Binn Valley (Binnatal) in Valais Canton, Switzerland. This is the type locality for this mineral, known for the occurrence of many rare sulfosalts. Beyond Switzerland, its presence has also been reported in the Hemlo mine in Ontario, Canada.
Rarity
Very rare
For collectors
## Quality Criteria Most valued by collectors are specimens with well-formed, sharp, and undamaged crystals, forming aesthetic, radial or tangled aggregates on a contrasting rock matrix (most often on white dolomite). Crystal size is a key factor – specimens with needles a few millimeters long are already considered valuable. Luster and absence of oxidation traces also enhance the specimen's appeal. ## Popular Localities By far the most sought-after and classic baumhauerite-II specimens come from the Lengenbach quarry in Switzerland. Material from this locality is the benchmark for the species and constitutes the majority of specimens available on the collector's market.
Care and storage
## Cleaning Baumhauerite-II specimens are very delicate and should be cleaned with the utmost care. The safest method is to use compressed air to remove dust. Any wet cleaning is discouraged due to the risk of chemical degradation. ## What to Avoid Avoid contact with water, acids, and other chemicals that may react with sulfides. The mineral is soft and brittle, so it should be protected from impacts, scratching, and vibrations. Prolonged exposure to humid air can lead to its oxidation and the formation of secondary coatings. ## Storage It is recommended to store specimens in dry, stable conditions, preferably in closed collector's boxes away from dust and humidity. Due to the presence of lead and arsenic, hands should be washed after handling the mineral.