Arsenoústalečite
Chemical formula: Cu<sup>1+</sup><sub>6</sub>Cu<sup>1+</sup><sub>6</sub>(As<sup>3+</sup><sub>2</sub>Te<sup>4+</sup><sub>2</sub>)Se<sup>2-</sup><sub>13</sub>
Arsenoústalečite is a rare selenide of copper, arsenic, and tellurium, forming metallic, black, anhedral grains.
Properties
- Mohs hardness
- 2.5-3
- Luster
- Metallic
- Streak
- Black
- Density
- 6.84
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Cubic
Diagnostic features
## Identification Identification of arsenoústalečite is only possible using advanced laboratory methods, such as energy-dispersive spectroscopy (EDS) in conjunction with a scanning electron microscope (SEM) or X-ray diffraction (XRD). Visual identification in the field or even in a collection is impossible due to its appearance and small grain size, which are indistinguishable from many other black, metallic minerals. ## Distinguishing from similar minerals It can be confused with many other selenides, sulfides, or tellurides, such as clausthalite, umangite, berzelianite, or ústalečite. Differentiation requires chemical analysis to confirm the simultaneous presence of copper, arsenic, tellurium, and selenium in appropriate proportions. ## Crystal forms Arsenoústalečite occurs as anhedral, i.e., irregular grains, which do not form visible crystal faces. These grains are most often intergrown with other minerals.
Geological environment
## Genesis Arsenoústalečite forms through low-temperature hydrothermal processes. It occurs in calcite-dolomite veins that cut amphibolite rocks. Its formation is associated with selenide mineralization. ## Mineral associations This mineral coexists with other selenides and sulfides. In its type locality, it was found in association with clausthalite, umangite, berzelianite, eskebornite, haquite-(Zn), permingeatite, ústalečite, pyrite, chalcopyrite, and uraninite. These minerals form complex aggregates within the calcite-dolomite matrix. ## Localities The only confirmed occurrence of arsenoústalečite in the world is its type locality: the dumps of the former Předbořice uranium mine, near the village of Křižanovice in the Central Bohemian Region of the Czech Republic.
Rarity
Extremely rare
For collectors
## Quality criteria The quality of an arsenoústalečite specimen is assessed solely in the context of micromineralogy. The most important criterion is the size and abundance of grains on the smallest possible area of the host rock. Since the mineral is only analytically identifiable, the reliability and documentation of the specimen's origin from a reputable source or research institution are crucial. The presence of well-documented associated minerals also increases its scientific and collector value.
Care and storage
## Cleaning Due to its extreme rarity and small grain size, arsenoústalečite specimens are practically not subject to cleaning. Any attempts at mechanical or chemical cleaning may destroy the specimen. It should be left in its natural state as it occurs in the host rock. ## What to avoid Avoid contact with chemicals, ultrasound, and any tools. The mineral is very soft and brittle. Temperature and humidity changes should be kept stable to avoid damaging the host rock. ## Storage Specimens should be stored exclusively in specialized collector containers (so-called micromount boxes), protecting them from dust, shocks, and humidity changes. Display is only possible under a microscope.