Demicheleite-(Cl)

Chemical formula: Bi³⁺S²⁻Cl

Demicheleite-(Cl) is a rare bismuth sulfide chloride, forming tiny, dark red to black crystals in volcanic fumaroles.

## Characteristics Demicheleite-(Cl) is a very rare mineral from the sulfide group, chemically classified as a bismuth sulfide chloride. It occurs as very small, tabular or lamellar crystals, rarely exceeding a few tens of micrometers in length. It typically forms small aggregates or appears as single, scattered crystals on volcanic rocks. Its color ranges from dark red to black. ## Physical Properties The crystals are translucent and exhibit a metallic or submetallic luster. The mineral's density is high, at 5.93 g/cm³. Due to the microscopic size of the crystals, characteristics such as hardness or cleavage have not been precisely determined. ## History and Name The mineral was approved by the International Mineralogical Association (IMA) in 2008 under the number IMA2008-049. Its name honors Vincenzo de Michele (born 1920), former curator of the Natural History Museum in Milan, Italy. The suffix "-(Cl)" refers to the dominance of chlorine in its chemical composition.

Properties

Luster
Metallic
Streak
Brown
Density
5.934
Fracture
None observed
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of Demicheleite-(Cl) is possible only through advanced analytical methods, such as energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD), due to its microscopic size and occurrence. Visually, it is characterized by a dark red to black color and tabular crystal habit. ## Distinguishing from Similar Minerals It may be confused with other small, dark bismuth minerals found in fumaroles, such as bismuthinite. Definitive differentiation requires chemical analysis confirming the presence of chlorine, sulfur, and bismuth in appropriate proportions. ## Crystal Forms It forms very small, thin, tabular crystals with a lamellar habit, often grouped into small aggregates.

Geological environment

## Genesis Demicheleite-(Cl) is a mineral of volcanic origin. It forms through the sublimation process from volcanic gases (exhalations) in high-temperature fumaroles. It crystallizes directly from the gas phase on the surface of volcanic rocks. ## Mineral Associations It co-occurs with other volcanic fumarole minerals, such as cannizzarite, bismuthinite, and other metal sulfides. ## Localities The only confirmed locality for this mineral worldwide (type locality) is the La Fossa crater on Vulcano Island, part of the Aeolian Islands archipelago in Italy.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, the collector's value of Demicheleite-(Cl) primarily depends on the confirmation of its identity and the richness of its occurrence on the rock matrix. Specimens with numerous, well-formed microcrystals, studied and described by specialized laboratories, are most highly valued. ## Popular Localities All known specimens come from a single location worldwide – the La Fossa crater on Vulcano Island, Italy. Material from this locality is extremely difficult to acquire.

Care and storage

## Cleaning Due to the extreme rarity and microscopic size of the crystals, mechanical cleaning is not recommended. Specimens should be protected from dust in specialized containers. ## What to Avoid Avoid contact with chemicals, ultrasound, and extreme temperature changes. The mineral is a product of volcanic sublimation, so it may be sensitive to moisture and changes in atmospheric conditions. ## Storage Demicheleite-(Cl) specimens should be stored under stable conditions, preferably in sealed, transparent "micromount" boxes, which protect them from physical damage and contamination.

External references

Sources

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