Demicheleite-(Br)
Chemical formula: Bi<sup>3+</sup>S<sup>2-</sup>Br
A very rare bismuth sulfide and bromide, forming small, hexagonal crystals in volcanic fumaroles.
Properties
- Mohs hardness
- 2
- Luster
- Metallic to Sub-Metallic
- Streak
- Black
- Density
- 7.18
- Cleavage
- Perfect on {0001}
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification The characteristic features of Demicheleite-(Br) are its metallic luster, dark color, low hardness (2 on the Mohs scale), and the specific form of thin, hexagonal plates. Its occurrence environment – high-temperature volcanic fumaroles – is also crucial for identification. ## Distinguishing from Similar Minerals At first glance, it might be confused with other dark, platy minerals with a metallic luster, such as graphite or molybdenite. However, it is distinguished by its slightly higher hardness (graphite and molybdenite have a hardness of 1-1.5) and lack of a greasy feel. From native bismuth, it differs by its darker color (bismuth has a silvery-pink hue). Final confirmation of identity requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS). ## Crystal Forms It forms thin, platy crystals with a hexagonal outline. These crystals often intergrow into aggregates resembling rosettes or globular forms.
Geological environment
## Genesis Demicheleite-(Br) is a fumarolic mineral. It forms by sublimation from hot volcanic gases at temperatures of about 450-600°C. It crystallizes directly on the walls of fumarole channels as a product of volcanic exhalations. ## Mineral Associations It co-occurs with other fumarolic minerals, such as native bismuth, cannizzarite, kirkiite, and galenobismutite. ## Localities The only confirmed locality for this mineral in the world is its type locality – La Fossa crater on Vulcano Island (Aeolian Islands) in the province of Messina, Sicily, Italy.
Rarity
Very rare
For collectors
## Quality Criteria As a "micromount" type mineral, its collector value is assessed based on different criteria than for cabinet specimens. Samples with sharp, well-formed hexagonal crystals, forming aesthetic rosettes, are most highly prized. The size of the crystals, even if not exceeding tenths of a millimeter, is important. Rich clusters on a rock matrix and associations with other rare fumarolic minerals are also highly valued. ## Popular Localities All known collector specimens come from a single location in the world: La Fossa crater on Vulcano Island, Italy. This is the classic and only locality for this mineral.
Care and storage
## Cleaning This is a very soft and brittle mineral with perfect cleavage, thus requiring extreme caution. To remove dust, it is best to use a soft, dry brush or a photographic air blower. Wet cleaning should be avoided; if absolutely necessary, distilled water can be used, but the specimen must be dried immediately. Ultrasonic cleaners are strictly not recommended. ## What to Avoid Avoid any impacts and scratches due to its low hardness. The mineral may be sensitive to chemical reagents, especially acids. It should be protected from moisture and sudden temperature changes. ## Storage Demicheleite-(Br) specimens are typically micromounts, so the best way to store them is in a sealed, appropriately labeled "micromount" box. This protects the fragile crystals from mechanical damage, dust, and moisture.