Oxy-chromium-dravite

Chemical formula: NaCr₃+₃(Cr₃+₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O

Oxy-chrom-dravite is a very rare, dark green to black mineral from the tourmaline group, distinguished by the dominance of chromium and oxygen in its structure.

## Characteristics Oxy-chrom-dravite is an extremely rare mineral belonging to the tourmaline supergroup. It forms columnar, hexagonal crystals, usually small, not exceeding 1 mm in length. Its appearance is typical for tourmalines, with characteristic vertical striations on the prism faces. It usually occurs as well-formed crystals embedded in the host rock. ## Physical Properties This mineral is characterized by a hardness of 7.5 on the Mohs scale, which is typical for tourmalines. It has a vitreous luster and is opaque. Its density, calculated based on the chemical formula and unit cell parameters, is approximately 3.25 g/cm³. ## Colors and Varieties Oxy-chrom-dravite ranges in color from dark green to black. No color varieties or commercial names are known. ## History and Name The mineral's name refers to its chemical composition – the dominance of oxygen (Oxy) and chromium (chromium) – and its structural relationship to dravite, another mineral from the tourmaline group. It was officially approved as a new mineral species by the International Mineralogical Association (IMA) in 2011 (IMA2011-097). It was described based on material from its type locality in Russia.

Properties

Mohs hardness: 7.5
Luster: Vitreous
Streak: Gray
Density: 3.25
Cleavage: None
Fracture: Uneven, Conchoidal
Transparency: Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Identification of oxy-chrom-dravite is impossible without advanced analytical methods, such as electron spectroscopy (EDS/WDS) to confirm its unique chemical composition with the dominance of chromium and oxygen. Visually, it is indistinguishable from other dark, chromium-rich tourmalines, such as chrom-dravite. ## Distinguishing from Similar Minerals It can be confused with chrom-dravite, dravite, schorl, and uvite. Key to differentiation is the determination of oxygen dominance over hydroxyl (OH) groups in a specific structural position, which requires specialized laboratory analysis. ## Crystal Forms It forms well-developed, prismatic (columnar) crystals with a hexagonal cross-section, terminated by pyramids. The crystals are small, usually less than 1 mm in length and up to 0.3 mm in width.

Geological environment

## Genesis Oxy-chrom-dravite forms under specific metamorphic conditions. In its type locality, it was found in chromitite rocks (composed mainly of chromite) within metamorphosed ultramafic rocks (serpentinites). ## Mineral Associations This mineral co-occurs with chromite, chrom-dravite, uvite, chromian diopside, chromian tremolite, titanite, rutile, pyrite, calcite, and quartz. ## Localities The only confirmed occurrence of oxy-chrom-dravite in the world is its type locality: the Sludyanka chromite deposit in the Saranovskiy district, Central Urals, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its extreme rarity, every analytically confirmed specimen is extremely valuable to specialized collectors of systematic minerals and "micromounts." Specimens with well-formed, sharp crystals, clearly visible against the host rock, are most highly prized. Crystal size is a key factor influencing value. ## Popular Localities The only source of specimens is the Sludyanka deposit in the Urals, Russia. Material from this locality is extremely difficult to acquire.

Care and storage

## Cleaning Due to its extreme rarity and typically small size of crystals embedded in the rock matrix, mechanical cleaning is not recommended. If dust removal is necessary, compressed air or a very soft brush can be used. Contact with water should be limited. ## What to Avoid Avoid contact with strong acids and cleaning chemicals, which could damage not only the mineral itself but, more importantly, the co-occurring minerals. It should not be subjected to sudden temperature changes or ultrasonic cleaning. ## Storage Specimens should be stored in stable conditions, in sealed "micromount" boxes, to protect them from dust and mechanical damage. Direct, prolonged exposure to sunlight should be avoided, although there is no evidence of fading.

Sources

IMA Mineral List (CNMNC)