Forêtite
Chemical formula: Cu²⁺₂Al₂(As⁵⁺O₄)(OH,O,H₂O)₆
Forêtite is a very rare copper and aluminum arsenate, forming microscopic, blue crystals with a bladed or acicular habit.
Properties
- Mohs hardness
- 3-4
- Color
- Pale blue
- Luster
- Vitreous
- Streak
- Very pale blue
- Density
- 3.286
- Cleavage
- Parallel to the flattening of the platelets
- Fracture
- Brittle
- Transparency
- Transparent,Translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Identification of forêtite is possible almost exclusively using advanced analytical methods, such as Raman spectroscopy or chemical analysis with an electron microprobe (EDS/WDS). Visually, at the occurrence site, it can be suspected based on its light blue color and the form of small, radial aggregates. ## Distinguishing from Similar Minerals It can be confused with other secondary, blue copper arsenates, such as cyanotrichite, geminite, or liroconite. It differs from cyanotrichite by the absence of silky luster and crystal habit (blades instead of needles). Definitive distinction from other rare arsenates requires specialized studies. ## Crystal Forms It forms very small, thin, bladed crystals, elongated along the [100] axis and flattened parallel to {001}. These crystals combine to form radial, spherical or hemispherical aggregates and rosettes.
Geological environment
## Genesis Forêtite is a secondary mineral, forming in the oxidation zones (gossans) of polymetallic ore deposits. It forms as a result of the weathering of primary minerals containing arsenic, copper, and aluminum, in a sulfate-rich environment. At the type locality, it formed as a result of post-mining processes on the dumps of the former mine. ## Mineral Associations This mineral co-occurs with other rare secondary minerals. At the Cap Garonne mine, it has been found in association with kankite, geminite, arseniosiderite, olivenite, as well as gypsum and quartz. ## Localities The only confirmed occurrence of forêtite in the world is its type locality – the Cap Garonne mine, Le Pradet, near Toulon in the Provence-Alpes-Côte d'Azur region of France.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of a specimen primarily depends on the abundance of its occurrence and the size of crystal aggregates on the rock matrix. The most valued specimens are those with clearly formed, spherical or rosette-like clusters of intense blue color, contrasting well with the substrate. Association with other rare minerals is also important. ## Popular Localities The only source of specimens is the Cap Garonne mine in France. Material from this locality is extremely rare and sought after by collectors specializing in rare minerals and "micromounts."
Care and storage
## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to extreme delicacy and reactivity, contact with water, even distilled, and any mechanical tools, including soft brushes, should be avoided. ## What to Avoid Contact with all chemicals, acids, and detergents must be strictly avoided. The mineral is likely sensitive to changes in temperature and humidity. It should not be exposed to direct sunlight, which may cause fading or degradation. ## Storage Forêtite specimens should be stored under stable conditions, preferably in closed, airtight "micromount" containers away from dust, light, and humidity fluctuations. They should be protected from vibrations and shocks.