Oxy-dravite

Chemical formula: Na(Al₂Mg)(Al₅Mg)(Si₆O₁₈)(BO₃)₃(OH)₃O

Oxy-dravite is a rare mineral from the tourmaline group, distinguished by the dominance of oxygen in place of hydroxyl ions in its crystal structure.

## Characteristics Oxy-dravite is a member of the tourmaline supergroup, belonging to the alkali group. It is the oxygen analogue of dravite. It forms columnar, prismatic crystals, often terminated by a pyramid. It usually occurs as radial aggregates or embedded grains. The crystals are longitudinally striated. ## Physical Properties This mineral is characterized by a hardness of 7 on the Mohs scale and a vitreous luster. It is brittle, and its density is approximately 3.13 g/cm³. It does not exhibit fluorescence under ultraviolet light. ## Colors and Varieties Oxy-dravite most commonly ranges in color from dark brown to almost black. It exhibits strong pleochroism, changing color from dark brown to almost colorless or pale yellow depending on the observation direction. ## History and Name The name "oxy-dravite" refers to its chemical composition – it is a dravite in which the dominant anion at the W site is oxygen (O²⁻), rather than a hydroxyl group (OH⁻). It was described as a new mineral in 2000 by Joel D. Grice and T. Scott Ercit based on material from Osarara in the Narok region of Kenya.

Properties

Mohs hardness: 7
Luster: Vitreous
Streak: Light brown
Density: 3.13
Cleavage: None
Fracture: Uneven, Conchoidal
Transparency: Translucent to opaque
Crystal system: Trigonal

Diagnostic features

## Identification Key diagnostic features include crystal habit (longitudinally striated prisms), high hardness (7 on the Mohs scale), dark brown to black color, and strong pleochroism. However, definitive identification requires advanced analytical methods, such as chemical analysis (EDS/WDS) and X-ray diffraction (XRD), to distinguish it from other dark tourmalines. ## Distinguishing from Similar Minerals Oxy-dravite is visually indistinguishable from other dark tourmalines, such as schorl, dravite, or buergerite. Schorl is typically black and iron-rich, while dravite has dominant (OH) groups in its structure instead of oxygen. Certain distinction is only possible through specialized laboratory tests. ## Crystal Forms It forms elongated, columnar crystals with a trigonal cross-section, often with rounded faces. The crystals are characteristically striated along the vertical axis. It also occurs as radial aggregates and irregular grains in the host rock.

Geological environment

## Genesis Oxy-dravite forms in magnesium-rich metamorphic rocks, such as marbles and schists, and in some granitic pegmatites. Its formation is associated with metasomatic processes, where boron-rich fluids interact with aluminum and magnesium-rich rocks. ## Mineral Associations It often co-occurs with minerals such as quartz, calcite, dolomite, tremolite, phlogopite, scapolite, titanite, and other tourmalines. ## Localities The most important localities worldwide include the type locality in Osarara, Kenya, as well as Slyudyanka on Lake Baikal in Russia. Occurrences have also been reported in Gouverneur, New York (USA), and Tajikistan.

Rarity

Very rare

For collectors

## Quality Criteria The most prized oxy-dravite specimens are those with well-formed, sharp, and undamaged crystals with a distinct luster. Specimens exhibiting strong pleochroism are particularly sought after. The attractiveness is also enhanced by the contrasting placement of dark crystals on a light rock matrix, for example, on white marble.

Care and storage

## Cleaning Specimens should only be cleaned with lukewarm distilled water and a soft brush. Ultrasonic cleaners should be avoided, as they can cause fractures in brittle crystals. ## What to Avoid Avoid contact with strong acids and bases. The mineral is stable under normal conditions, but it should be protected from sudden temperature changes and strong, direct sunlight, which can affect its color over time. ## Storage Store in stable room conditions, away from dust and moisture. Due to its brittleness, specimens should be stored in separate boxes or display cases to prevent scratches and mechanical damage.

Sources

IMA Mineral List (CNMNC)