Aubertite
Chemical formula: Cu<sup>2+</sup>Al(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>Cl·14H<sub>2</sub>O
Aubertite is a very rare, water-soluble copper and aluminum sulfate, forming blue, fibrous coatings and crusts.
Properties
- Mohs hardness
- 2
- Luster
- Vitreous
- Streak
- White
- Density
- 1.82
- Cleavage
- Perfect on {100}
- Fracture
- Uneven
- Transparency
- Transparent to Translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Key diagnostic features of aubertite are its blue color, occurrence in delicate, fibrous or acicular aggregates, and extreme instability. Water solubility is its characteristic feature. It occurs in a specific environment – as a product of volcanic fumarole activity. ## Distinguishing from similar minerals It can be confused with other secondary, blue copper sulfates, such as chalcanthite or linarite. However, chalcanthite usually forms better-developed, larger crystals and is less fibrous. Linarite is significantly harder (2.5) and denser. Crucial for differentiation, however, is the environment of occurrence (fumaroles) and confirmation of chemical composition using specialized analyses. ## Crystal Forms Aubertite crystallizes in the triclinic system. It forms elongated, acicular or hair-like crystals that combine into delicate, tangled aggregates, crusts, and fluffy coatings.
Geological environment
## Genesis Aubertite is a secondary mineral, forming under very specific conditions. It forms as a product of sublimation (resublimation) of volcanic gases in active fumaroles. It crystallizes directly from hot vapors rich in sulfur, copper, aluminum, and chlorine, depositing on cooler surfaces of volcanic rocks. ## Mineral Associations It co-occurs with other minerals formed in a similar environment, such as native sulfur, alunogen, metavoltine, cyanotrichite, chalcanthite, brochantite, and anglesite. ## Localities The most important and typical locality for aubertite is the fumaroles of Mount Vesuvius in Italy. It is also known from the Quetena volcano in the Antofagasta region of Chile. These are the only confirmed and well-documented localities for this mineral worldwide.
Rarity
Very rare
For collectors
## Quality Criteria For collectors, the abundance and quality of delicate, fibrous aggregates on the rock matrix are most important. Specimens with clearly visible, undamaged clusters of acicular crystals with an intense blue color are highly valued. The size of the specimen and the contrast with the matrix also affect its attractiveness. ## Popular Localities By far the most valued and sought-after specimens by collectors come from the classic locality – the fumaroles of Vesuvius in Italy. Specimens from Chile are much rarer on the market.
Care and storage
## Cleaning This mineral **must not be cleaned wet**, as it is water-soluble. To remove dust, a very soft brush or a photographic air blower (contactless) can be used with extreme caution. ## What to avoid Contact with water, water vapor, and all liquids must be strictly avoided. The mineral is hygroscopic, meaning it absorbs moisture from the air, which can lead to its disintegration. It should be protected from high temperatures, which can cause the loss of water from its crystal structure. Store away from heat sources and direct sunlight. ## Storage Aubertite specimens require storage in conditions with controlled, low humidity. The best solution is a sealed container (e.g., a "perky box") with a desiccant (e.g., silica gel). Display should only take place in enclosed display cases, away from moisture and temperature fluctuations.