Demicheleite-(I)

Chemical formula: Bi³⁺S²⁻I¹⁻

Demicheleite-(I) is a rare bismuth sulfide iodide, forming tiny, acicular crystals of dark red to black color.

## Characteristics Demicheleite-(I) is a mineral from the sulfiodide group, chemically classified as a bismuth sulfide iodide. It occurs as very small, acicular or bladed crystals, rarely exceeding one millimeter in length. It typically forms radial or tangled aggregates. Its color ranges from dark red to black, which, combined with the small size of the crystals, makes it difficult to observe with the naked eye. ## Physical Properties This mineral is characterized by a high density of approximately 6.41 g/cm³. It is translucent, and its streak is brown. Hardness has not been precisely determined due to the small size and rarity of the crystals. The luster is described as metallic to submetallic. ## Colors and Varieties The dominant colors are dark red and black. Due to its rarity and recent discovery, no varieties of this mineral have been distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2008 under the number IMA2008-049. Its name honors Vincenzo de Michele (born 1936), former curator of the Natural History Museum in Milan, Italy, for his contributions to mineralogy. The suffix "-(I)" in the name refers to the dominance of iodine in its chemical composition. ## Uses Due to its extreme rarity and microscopic size, Demicheleite-(I) has no industrial applications. It is solely an object of scientific and collecting interest for specialized micromount mineral collectors.

Properties

Luster
Metallic
Streak
Brown
Density
6.411
Fracture
None observed
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Amateur identification of Demicheleite-(I) is practically impossible and requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical analysis (EDS). Preliminary identification in a collection relies on knowledge of the locality, the habit of acicular crystals, dark red to black color, and co-occurrence with other fumarolic minerals. ## Distinguishing from Similar Minerals This mineral may be confused with other rare bismuth sulfides and sulfosalts, such as bismuthinite. Definitive differentiation is possible only through chemical analysis, which will show the presence of iodine. ## Crystal Forms It forms elongated, acicular or bladed crystals with a rectangular cross-section, reaching up to 1 mm in length. These crystals often occur as radial or haphazardly tangled aggregates.

Geological environment

## Genesis This is a volcanic mineral, formed by sublimation from volcanic gases (exhalations) in a fumarolic environment. It crystallizes at high temperatures, directly from the gas phase on the surface of volcanic rocks. ## Mineral Associations This mineral occurs in association with other products of volcanic exhalations. In its type locality (La Fossa crater), it was found in association with sulfur, sassolite, realgar, and cannizzarite. ## Localities The only confirmed occurrence of Demicheleite-(I) in the world is its type locality – the active La Fossa crater on Vulcano Island, part of the Aeolian Islands archipelago in Italy.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of Demicheleite-(I) specimens is primarily assessed based on the richness and development of crystals within the aggregate. Specimens with clearly visible, well-formed, acicular crystals forming visually attractive groups are most prized. Due to the microscopic nature of the mineral, its association with a well-defined rock matrix is also crucial. ## Popular Localities The only source of specimens of this mineral is the La Fossa crater on Vulcano Island, Italy. Collecting material in this location is strictly regulated and potentially dangerous due to volcanic activity.

Care and storage

## Cleaning Specimens of this mineral are extremely small and fragile. Cleaning is not recommended and should be limited to an absolute minimum. If necessary, compressed air can be used from a safe distance to remove loose dust particles. Any mechanical contact should be avoided. ## What to Avoid Contact with water, chemicals, and especially acids, must be strictly avoided. The mineral is sensitive to changes in temperature and humidity. It should not be exposed to direct sunlight, which can cause its degradation. ## Storage Demicheleite-(I) specimens should be stored under stable conditions, in tightly sealed "micromount" containers that protect them from dust, humidity, and mechanical damage. It is best to keep them in a dark and dry place.

External references

Sources

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