Downeyite

Chemical formula: Se<sup>4+</sup>O<sub>2</sub>

Downeyite is a naturally occurring, crystalline form of selenium dioxide, forming colorless, acicular crystals in volcanic environments.

## Characteristics Downeyite is a natural, crystalline form of selenium dioxide (SeO₂). It occurs as very small, colorless, acicular or hair-like crystals, forming coatings and aggregates. Due to its nature, it is extremely delicate and hygroscopic – it readily absorbs water from the environment, which can lead to its dissolution. It is a secondary mineral, formed under specific volcanic conditions. ## Physical Properties Downeyite crystals are very small and brittle. The mineral is characterized by a vitreous luster and is transparent. Its density is significantly higher than most common minerals, at approximately 3.95 g/cm³. It is very soft. ## Colors and Varieties Downeyite is colorless. No colored or commercial varieties are distinguished. ## History and Name The mineral was discovered in the 1970s near Frackville in Schuylkill County, Pennsylvania, USA, on burning anthracite coal mine dumps. It was described and named in 1979 in honor of Wayne F. Downey Jr. (born 1943), a mineral collector from Pennsylvania, who first found and drew attention to this new mineral. ## Uses Downeyite has no industrial application. Its significance is purely scientific and as a collector's item, as an extremely rare and unusual mineral.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
White
Density
3.95
Cleavage
Perfect on {110}
Fracture
Uneven
Transparency
Transparent
Crystal system
Tetragonal

Diagnostic features

## Identification Downeyite identification is based primarily on its appearance – it forms characteristic, colorless, acicular or hair-like crystals. The key is its occurrence: it is found almost exclusively as a sublimation product in coal mine fires. Its extreme rarity and specific formation environment are the most important diagnostic clues. ## Distinguishing from similar minerals It can be confused with other secondary sulfates or selenites forming similar coatings, such as sassolite (boric acid), which also forms in volcanic environments and fumaroles. Final differentiation requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal forms Downeyite crystallizes in the tetragonal system, forming very fine, elongated crystals with an acicular or hair-like habit. These crystals often arrange into tangled aggregates, coatings, or delicate, fluffy clusters on the host rock surface.

Geological environment

## Genesis Downeyite is a secondary mineral of anthropogenic origin. It forms as a result of the sublimation of volcanic gases or, as in the case of the type locality, from the oxidation of selenium present in coal during fires in mine dumps. It crystallizes directly from the gas phase at temperatures below 315°C. ## Mineral associations At the Pennsylvania locality, downeyite coexists with native sulfur and selenium. ## Localities The most important and type locality is the burning anthracite mine dumps near Frackville and St. Clair in Pennsylvania, USA. It has also been reported from the Campbell mine in Bisbee, Arizona (USA), and in connection with underground fires in Russia and the Czech Republic.

Rarity

Extremely rare

For collectors

## Quality criteria The quality of a downeyite specimen depends primarily on the richness and abundance of crystals on the rock matrix. Specimens with dense, well-formed clusters of acicular crystals are most valued. The size of individual crystals is always microscopic, so the overall aesthetics and concentration of the mineral are important. Due to its instability, proper preservation of the specimen is also crucial. ## Popular localities The only source of specimens available on the collector's market is its type locality – the area around Frackville in Pennsylvania, USA.

Care and storage

## Cleaning The mineral must absolutely not be wet cleaned. It is water-soluble and will be destroyed upon contact with water or even humid air. Mechanical cleaning is impossible due to the extreme delicacy of the crystals. ## What to avoid Avoid contact with water, steam, moisture, chemicals, and any tactile manipulation. The mineral must be protected from changes in humidity and direct sunlight, which could heat it and accelerate chemical reactions. ## Storage Downeyite specimens require storage in hermetically sealed containers, preferably with a desiccant (e.g., silica gel). They should be kept at a stable temperature, away from light and vibrations. Due to its fragility, it is best stored in its original, small "micromount" box.

Sources

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