Baumhauerite (Pb3As4S9) is a rare lead sulfosalt mineral. It crystallizes in the triclinic system, is gray-black to blue-gray and its luster is metallic to dull. It is a rare sulfide mineral from the famous quarry at Lengenbach, Binnental, Valais, Switzerland. It is not radioactive. It has a hardness of 3.
Baumhauerite is usually a bright dark gray mineral with a nice luster and striated prismatic crystals embedded in a dolomitic marble. It is a rare mineral and is only found in a few localities. Lengenbach is the only locality where specimens can be found with regularity.
- Category: Sulfosalt mineral
- Formula: Pb3As4S9
- Crystal system: Triclinic
- Crystal class: Pedial (1) (same H-M symbol).
The physical and optical properties of baumhauerite 2a and baumhauerite differ only subtly, and the two minerals are difficult to distinguish without recourse to powder X-ray diffraction methods. In hand specimen, both minerals are opaque and steel gray with a metallic luster. The streak is dark reddish brown. The minerals are brittle with a conchoidal fracture, and baumhaueite-2a has one perfect cleavage, probably on (100), similar to that of baumhauerite.
- Color: Gray-black to blue-gray
- Twinning: Polysynthetic, on 
- Cleavage: Perfect on 
- Fracture: Conchoidal
- Mohs scale hardness: 3
- Luster: Metallic to dull
- Streak: chocolate brown
- Specific gravity: 5.33
Baumhauerite occurs as small crystals embedded in dolomitic marble. It is found primarily in the Lengenbach Quarry, Binnental, in the Valais region of Switzerland, the mineral is named after German mineralogist Heinrich Adolph Baumhauer (1848–1926), who discovered it at Lengenbach, famous among mineralogists for its array of rare minerals, in 1902. Baumhauerite has also been reported at Sterling Hill, New Jersey, United States, typically in association with molybdenite, and in aggregates at Hemlo, Thunder Bay, Ontario, Canada.