Graeserite

Chemical formula: Fe³⁺₄Ti⁴⁺₃As³⁺O₁₃(OH)

Graeseryite is a rare iron, titanium, and arsenic oxide, forming black, acicular crystals with a metallic luster.

## Characteristics Graeseryite is a very rare mineral from the oxide group, with a complex chemical composition including iron, titanium, and arsenic. It occurs as very small, acicular or hair-like crystals, which often form tangled aggregates or radial clusters. Its color is black, and due to the small size of the crystals, detailed visual features are difficult to observe without magnification. ## Physical Properties Graeseryite crystals exhibit a strong, metallic luster. They are opaque. Due to the extreme rarity and small size of the crystals, many of its physical properties, such as hardness or density, have not been precisely measured. ## Colors and Varieties This mineral is uniform in color and occurs exclusively in black. No varieties have been distinguished. ## History and Name Graeseryite was discovered in the Lengenbach quarry in the Binn Valley (Binntal) in Switzerland, a location known for its unique arsenic minerals. It was approved as a new mineral by the International Mineralogical Association (IMA) in 1996. Its name honors Professor Stefan Graeser (born 1935), a Swiss mineralogist from the University of Basel, for his contributions to the study of the mineralogy of Alpine deposits, particularly the Binn Valley.

Properties

Mohs hardness: 5.5
Color: Black
Luster: Metallic
Streak: Black
Density: 0
Cleavage: Moderate on {100}.
Fracture: Conchoidal
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of graeseryite is primarily based on its characteristic form – black, acicular or hair-like crystals with a metallic luster. Key for certain identification is the locality (Lengenbach quarry) and its association with other rare sulfosalts and arsenic oxides. Final identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Distinguishing from Similar Minerals Graeseryite can be confused with other black, acicular minerals found in the same locality, such as sartorite or baumhauerite. Visual distinction is practically impossible and requires specialized laboratory tests. ## Crystal Forms Graeseryite forms elongated, acicular or hair-like crystals, reaching up to 2 mm in length. These crystals often occur as tangled clusters or form radial aggregates embedded in dolomitic marble.

Geological environment

## Genesis Graeseryite forms as a result of metamorphic processes in hydrothermal deposits. In its type locality (Lengenbach), it occurs in saccharoidal dolomites that have been subjected to solutions rich in arsenic, sulfur, and metals. ## Mineral Associations This mineral co-occurs with many other rare minerals, mainly sulfosalts and oxides. In its environment, one can find, among others, realgar, sartorite, baumhauerite, tennantite, sphalerite, piemontite, as well as hematite and quartz. ## Localities The only confirmed and described locality for graeseryite in the world is the Lengenbach quarry in the Binn Valley (Binntal), in the canton of Valais, Switzerland. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria Graeseryite is a mineral of interest exclusively to specialized collectors of rare minerals and micromounts. The value of a specimen is primarily determined by the abundance and quality of the crystals – their length, luster, and aesthetic arrangement on the rock matrix. Due to its microscopic nature, clarity is not assessed in the traditional way. The most prized specimens are those with well-formed, undamaged needles, forming visually attractive aggregates.

Care and storage

## Cleaning Graeseryite specimens are extremely delicate and small. Mechanical cleaning is not recommended. If necessary, compressed air can be used to remove dust, but this should be done from a distance and with the utmost caution. ## What to Avoid Avoid contact with chemicals, ultrasonics, and all forms of mechanical cleaning. The crystals are brittle and easily damaged. Protect them from moisture and sudden temperature changes. ## Storage It is recommended to store specimens exclusively in specialized, sealed "micromount" boxes, which protect them from dust, mechanical damage, and humidity changes. Display should be away from direct sunlight and heat sources.

External references

https://www.mindat.org/min-6977.html