Geuerite

Chemical formula: Ag¹⁺₂Tl¹⁺₄Pb²⁺₄As³⁺₂₂S²⁻₄₀

Geueryite is an extremely rare, lead-gray sulfide of silver, thallium, lead, and arsenic, forming microscopic, metallic grains.

## Characteristics Geueryite is a very rare mineral from the sulfosalt group, scientifically described only in 2023. It occurs as extremely small, anhedral (irregularly shaped) grains, not exceeding 150 micrometers in size, which are intergrown with other minerals. Observed in reflected light, it has a lead-gray color and metallic luster. Due to the microscopic size of its crystals, its macroscopic features are unknown. ## Physical Properties Due to the extremely small size of the grains, most physical properties of geueryite, such as hardness, cleavage, fracture, or density, could not be determined. The density was only calculated based on the chemical formula and unit cell parameters, and is 5.63 g/cm³. ## History and Name The mineral's name honors Dr. Marc Geuer, curator of the mineralogical collection at the Natural History Museum in Bern (Switzerland), in recognition of his contribution to research on minerals from the Lengenbach quarry. Geueryite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2023 (IMA2023-010).

Properties

Luster
Metallic
Streak
Black
Density
5.63
Cleavage
None
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of geueryite is possible only through advanced laboratory methods, such as chemical composition analysis by electron microprobe (EDS/WDS) and X-ray diffraction (XRD). In reflected light under a microscope, it is lead-gray and exhibits weak pleochroism. ## Distinguishing from Similar Minerals It is practically impossible to distinguish it with the naked eye from other rare sulfosalts from Lengenbach, such as hutchinsonite, imhofite, or wallisite, with which it co-occurs. Differentiation requires specialized chemical and structural analysis.

Geological environment

## Genesis Geueryite forms as a result of hydrothermal processes in sedimentary-metamorphic deposits. It occurs in Triassic dolomitic rocks, where it crystallizes in small voids and fractures. ## Mineral Associations This mineral occurs in paragenesis with other rare arsenic and thallium sulfosalts. It is most commonly associated with hutchinsonite, imhofite, wallisite, as well as realgar, orpiment, and pyrite. The entire assemblage crystallizes on a dolomite substrate. ## Localities The only confirmed locality of geueryite in the world is its type locality – the Lengenbach quarry in the Binn Valley (Binntal) in the canton of Valais, Switzerland. This site is known for the occurrence of many unique sulfosalts.

Rarity

Extremely rare

For collectors

## Quality Criteria Geueryite does not form macroscopic specimens. Its value is purely scientific and systematic. For specialized collectors of "microminerals" or systematic minerals, the only value is possessing an analytically documented rock fragment (most often dolomite) containing microscopic geueryite grains, with a precisely marked inclusion locality. ## Popular Localities The only source of specimens is the Lengenbach quarry in Switzerland. Research and collector material comes exclusively from this single location.

Care and storage

## Cleaning Due to its microscopic nature of occurrence (as inclusions in other minerals), individual geueryite grains are not subject to cleaning. The host specimen (most often dolomite) can be mechanically cleaned with a very soft brush. ## What to Avoid Avoid contact with acids and strong chemicals, which can react with sulfides. As a mineral containing arsenic and thallium, it requires careful handling – avoid inhaling dust and wash hands after contact with the specimen. ## Storage Specimens containing geueryite should be stored in stable conditions, away from moisture, in a closed box or display cabinet to limit oxidation and dusting. Clear labeling of the specimen is recommended due to the toxicity of its components.

External references

Sources

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