Gaudefroyite
Chemical formula: Ca₄Mn³⁺₃(BO₃)₃(CO₃)O₃
A black, hexagonal mineral from the borate group, known for its pyramidal crystals found in Morocco.
Properties
- Mohs hardness
- 6
- Luster
- Vitreous to submetallic
- Streak
- Brown
- Density
- 3.35
- Cleavage
- Prismatic {11.0)
- Fracture
- Conchoidal
- Transparency
- Opaque
- Crystal system
- Hexagonal
Diagnostic features
## Identification Key diagnostic features of gaudefroyite include its hexagonal crystal habit (six-sided prisms, often terminated by a pyramid), black color, vitreous to submetallic luster, and brown streak. A hardness of 6 on the Mohs scale is also an important clue. ## Distinguishing from Similar Minerals Gaudefroyite is sometimes confused with black tourmaline (schorl) or ilvaite. It is primarily distinguished by its characteristic, sharply terminated hexagonal crystal shape, which is rare in schorl. Compared to ilvaite, gaudefroyite usually has more regular and better-defined crystal faces. The brown streak is also a distinguishing feature from many other black minerals. ## Crystal Forms This mineral forms classic, elongated prismatic crystals with a hexagonal base. The crystals are often terminated by steep, six-sided pyramids, giving them a characteristic, sharp appearance. It occurs as single crystals or groups of parallel intergrown individuals.
Geological environment
## Genesis Gaudefroyite is a hydrothermal mineral, forming at low temperatures. It forms in veins and fissures cutting through manganese deposits that have undergone contact metamorphism. ## Mineral Associations It most commonly co-occurs with other manganese minerals, such as braunite, hausmannite, marokite, as well as calcite and quartz. Specimens often grow on a matrix composed of these minerals. ## Localities The most important and historical locality for gaudefroyite, from which the world's best specimens originate, is the Tachgagalt mine in the Zagora Province, Morocco. It is also known from Kuruman in South Africa, where it occurs in the Kalahari manganese deposits.
Rarity
Rare
For collectors
## Quality Criteria The most highly prized gaudefroyite specimens are those with sharp, well-terminated crystals with strong luster and a deep black color. Single, large crystals (above 2-3 cm) or aesthetic groups of crystals on a contrasting matrix, for example with white calcite, are particularly sought after. Damage, such as chipped terminations or scratches, significantly reduces collector value. ## Popular Localities Undoubtedly, the most valued and known specimens come from the type locality – the Tachgagalt mine in Morocco. Crystals from there are considered exemplary for this species. Specimens from the Kalahari manganese fields in South Africa, while also important from a mineralogical perspective, less frequently achieve the same aesthetic quality as those from Morocco.
Care and storage
## Cleaning Gaudefroyite specimens can be cleaned with a soft, dry brush to remove dust. For heavier soiling, distilled water and a gentle brush may be used. After wet cleaning, the specimen should be thoroughly dried. ## What to Avoid Avoid contact with acids and strong chemicals, which can damage the crystal surfaces. Despite its hardness of 6, the mineral is brittle and should be protected from impacts and falls that could cause cracks or chips. ## Storage It is recommended to store specimens in individual boxes or on a soft surface to prevent scratches and mechanical damage. It can be safely displayed in a closed cabinet, protected from dust and direct, prolonged sunlight.