Bandylite
Chemical formula: Cu²⁺B(OH)₄Cl
Bandylite is a rare copper borate, known for forming characteristic, square crystals of an intense blue color.
Properties
- Mohs hardness
- 2.5
- Color
- Deep blue with greenish portions; cendre blue to Italian blue, becoming greener with atacamite inclusions; blue in transmitted light.
- Luster
- Vitreous
- Streak
- Pale blue
- Density
- 2.81
- Cleavage
- on {001}.
- Fracture
- Uneven
- Transparency
- Transparent
- Crystal system
- Tetragonal
Diagnostic features
## Identification The most important diagnostic feature of bandylite is its unique crystal habit – thin, square tablets. The intense blue color combined with this form is almost unambiguous. An additional clue is its low hardness and perfect cleavage in one direction. ## Distinguishing from Similar Minerals Bandylite is sometimes confused with other blue secondary copper minerals. It is distinguished from linarite and azurite by its characteristic square crystal shape and lower hardness. Unlike them, bandylite is also water-soluble. From boleite, which also forms regular crystals, it is distinguished by its tetragonal, not isometric, form and a much simpler chemical composition. ## Crystal Forms Bandylite crystallizes as thin, sharply outlined, square tablets. Crystals rarely occur individually, more often forming intersecting groups, aggregates, or growing on the surface of the host rock.
Geological environment
## Genesis Bandylite is a secondary mineral, forming in the oxidation zones of copper deposits in arid and desert climates. It forms as a result of the reaction of copper-rich solutions with boron-containing minerals, such as ulexite, in the presence of chloride ions. This process requires very specific chemical and climatic conditions. ## Mineral Associations This mineral most often occurs in association with other secondary copper and boron minerals. Typical co-occurring minerals include atacamite, eriocalcite, ulexite, as well as gypsum and halite. ## Localities The type locality and also the main source of the best bandylite specimens is the La Compañía mine in the Sierra Gorda district, Antofagasta region, Chile. This is the only place in the world where this mineral occurs in quantities and quality of collector significance.
Rarity
Very rare
For collectors
## Quality Criteria The most desirable bandylite specimens are characterized by sharp, well-formed crystals with a perfectly square outline. An intense, deep blue color without greenish tints is highly valued. Crystal size is key – specimens with tablets exceeding 5 mm are already considered exceptional. The aesthetic arrangement of crystals on the rock matrix, especially in association with contrasting minerals like white ulexite, increases its attractiveness. ## Popular Localities The only significant source of collector-grade bandylite specimens is the La Compañía mine in Sierra Gorda, Chile. Specimens from this locality are the global standard for this mineral.
Care and storage
## Cleaning Bandylite is extremely sensitive to water and moisture. It must not be cleaned with water or any liquids. The only safe method is to use a soft, dry brush or a very gentle stream of compressed air to remove dust. ## What to Avoid Contact with water, water vapor, and high air humidity must be strictly avoided, as the mineral is soluble and will be destroyed. It should be protected from high temperatures, which can cause the loss of water from its structure and its decomposition. Avoid contact with chemicals, acids, and detergents. Due to its softness, it is very susceptible to scratches. ## Storage Bandylite specimens must be stored in dry conditions, preferably in sealed, closed boxes (so-called "perky boxes") or display cases with a desiccant (e.g., silica gel). They should be kept away from heat sources and direct sunlight.