Franksousaite
Chemical formula: Pb²⁺Cu²⁺(Se⁶⁺O₄)(OH)₂
Franksousaite is a very rare copper and lead selenite, forming microscopic, platy crystals of an intense green color.
Properties
- Mohs hardness
- 2-2.5
- Color
- Blue
- Luster
- Vitreous
- Streak
- Very pale blue
- Density
- 5.64
- Cleavage
- Cleavage could not be determined due to the small crystal size. By its structural analogue to linarite, it should have perfect cleavage on {100}.
- Transparency
- Transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Amateur identification of franksousaite is practically impossible. It requires advanced analytical techniques such as Raman spectroscopy or X-ray diffraction (XRD). Preliminary identification in a collection is based on the characteristic emerald-green color, the form of microscopic, platy crystals forming rosette-like aggregates, and its co-occurrence with other selenites from the El Dragón mine. ## Distinguishing from Similar Minerals Franksousaite can be confused with other green, secondary copper minerals, such as brochantite, malachite, or other rare selenites (e.g., olsacherite). Differentiation requires specialized chemical and structural analysis. A key diagnostic feature is the presence of selenium, lead, and copper in its chemical composition. ## Crystal Forms The mineral forms very small, thin, platy crystals, often elongated and bladed. These crystals combine into characteristic, rosette-like or radial aggregates. It also occurs as thin coatings on rock surfaces.
Geological environment
## Genesis Franksousaite is a secondary mineral, formed in the oxidation (weathering) zone of polymetallic ore deposits rich in selenium. It forms as a result of the reaction of selenite-ion-containing solutions with primary lead and copper minerals. Its formation is associated with a dry, desert climate, which favors the concentration of selenium. ## Mineral Associations At the type locality (El Dragón mine), franksousaite co-occurs with other rare selenites, such as ahlfeldite, penroseite, olsacherite, as well as with anglesite, barite, goethite, and quartz. ## Localities The only confirmed occurrence of franksousaite in the world is the El Dragón mine, located in the Antonio Quijarro Province, Potosí Department, Bolivia. This is the type locality for this mineral.
Rarity
Very rare
For collectors
## Quality Criteria The quality of franksousaite specimens is primarily assessed based on the richness and aesthetics of the crystal aggregates. Most valued are specimens with well-formed, undamaged, rosette-like clusters of intensely green crystals, contrasting with the color of the host rock. Due to the microscopic nature of the mineral, the quality of the specimen as seen under magnification is crucial. ## Popular Localities All known collector specimens come from a single location in the world – the El Dragón mine in Bolivia. This is the only source of this mineral, making every specimen from there unique.
Care and storage
## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to the extreme delicacy and reactivity of the mineral, the use of water, even distilled, and any chemical agents is discouraged. Mechanical cleaning (e.g., with a brush) is impossible and leads to the destruction of the crystals. ## What to Avoid Contact with water, acids, bases, and other chemicals 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 fading. Vibrations and shocks should be avoided. ## Storage Franksousaite specimens should be stored exclusively in stable conditions, in closed, padded "micromount" boxes that protect them from dust, mechanical damage, and changes in humidity. Storage in a dry place is crucial for preserving the integrity of the specimens.