Bracewellite
Chemical formula: Cr³⁺O(OH)
Bracewellite is a rare chromium oxyhydroxide, forming microscopic, orthorhombic crystals with a metallic luster.
Properties
- Mohs hardness
- 5.5-6.5
- Color
- Deep red to black
- Luster
- Metallic
- Streak
- Dark brown
- Density
- 4.45
- Cleavage
- Perfect on {010}, good on {100}
- Fracture
- Irregular/Uneven
- Transparency
- Translucent,Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Identification of bracewellite is possible only through advanced laboratory methods, such as X-ray diffraction (XRD) and chemical analysis (e.g., electron microprobe). It is impossible to identify with the naked eye in field conditions or in a collection. Its characteristic feature under the microscope is a greenish tint in reflected light. ## Distinguishing from Similar Minerals Bracewellite is structurally related to diaspore and goethite. It can be confused with other fine-grained, dark oxides and hydroxides, such as chromite, eskolaite, or guyanaite, with which it often co-occurs. Certain differentiation requires specialized instrumental analysis. ## Crystal Forms It forms microscopic, bladed or tabular crystals with an orthorhombic habit, elongated along the [001] axis. These crystals often combine into radial or rosette-like aggregates. It also occurs as granular masses and earthy accumulations.
Geological environment
## Genesis Bracewellite is a secondary mineral, forming in the weathering zones of chromium deposits. It forms in chromium-rich laterites and weathered rocks, often in the presence of other chromium and aluminum minerals. ## Mineral Associations This mineral most commonly co-occurs with eskolaite, guyanaite, chromite, as well as quartz, kaolinite, and gibbsite. At its discovery site (Guyana), it was found in oxide concretions in river alluvium. ## Localities The most important and confirmed bracewellite localities worldwide are very few. They include the type locality in the Merume River region of Guyana and the Sarany chromite deposit in the Middle Urals, Russia.
Rarity
Very rare
For collectors
## Quality Criteria The quality of bracewellite specimens is primarily assessed based on the abundance and development of microscopic crystals on the host rock. The most valued samples are those where well-formed, though still very small, crystalline aggregates are visible. Due to the nature of the mineral, the purity and size of individual crystals are not key criteria. ## Popular Localities Specimens from the type locality in Guyana and from the Sarany deposit in the Urals, Russia, are the only ones available on the collector's market and are of greatest importance to collectors specializing in rare or systematic minerals.
Care and storage
## Cleaning Due to its extreme rarity and microscopic crystal size, bracewellite specimens practically do not require cleaning and should not be subjected to any mechanical or chemical treatments. Any contaminants should be left undisturbed. ## What to Avoid Avoid contact with any chemicals, acids, and bases. Specimens should not be exposed to ultrasonic cleaners or steam cleaners. Protect from high temperatures and sudden temperature changes. ## Storage Bracewellite specimens, usually in the form of fragments of host rock with microscopic crystals, should be stored under stable conditions, in sealed "micromount" boxes, to protect them from dust and mechanical damage. Avoid exposure to moisture.