Chromo-alumino-povondraite

Chemical formula: NaCr³⁺₃(Al₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O

Chromium-alumino-povondraite is a very rare, green mineral from the tourmaline group, distinguished by its complex chemical composition with chromium and sodium.

## Characteristics Chromium-alumino-povondraite is an extremely rare mineral belonging to the tourmaline supergroup. It forms transparent, well-formed crystals of an intense green color, which is its most characteristic visual feature. As a member of the complex tourmaline family, its identification requires advanced analytical methods. It occurs as small, acicular or prismatic crystals, often embedded in the host rock. ## Physical Properties This mineral is characterized by significant hardness, reaching 7.5 on the Mohs scale, which is typical for tourmalines. Its density, calculated from crystallographic data, is approximately 3.23 g/cm³. It has a vitreous luster and a conchoidal fracture. It is transparent, allowing observation of its pure, green color. ## Colors and Varieties The dominant and only recorded color of chromium-alumino-povondraite is green, which is directly related to the presence of chromium ions (Cr³⁺) in its chemical structure. No distinct color or commercial varieties are distinguished. ## History and Name The mineral's name reflects its chemical composition: the prefix "chromo-" indicates the dominance of chromium, "alumino-" refers to the presence of aluminum, and "povondraite" alludes to its kinship with another tourmaline, povondraite. It was formally described and approved as a new mineral species relatively recently, highlighting its rarity and scientific significance. ## Uses Due to its extreme rarity and microscopic crystal sizes, chromium-alumino-povondraite has no industrial applications. Its significance is purely scientific and collectible, being an object of desire for specialized collectors of rare and systematic minerals.

Properties

Mohs hardness: 7.5
Luster: Vitreous
Streak: Pale-green
Density: 3.227
Fracture: Conchoidal
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Macroscopic identification of chromium-alumino-povondraite is practically impossible and requires advanced analytical techniques, such as X-ray microanalysis (EDS/WDS) to confirm its unique chemical composition (presence of sodium, chromium, aluminum, and magnesium in specific proportions). A preliminary indication may be its green color, trigonal crystal habit, high hardness, and co-occurrence with other minerals in calcite marbles. ## Distinguishing from Similar Minerals It can be confused with other green tourmalines, such as chromo-dravite, dravite, elbaite (verdelite), or uvite. Distinguishing it from these relies solely on precise chemical analysis. For example, chromo-dravite has magnesium in a different structural position, and elbaite is a lithium tourmaline. Visually, they are often indistinguishable. ## Crystal Forms Crystals are typically well-formed, with a prismatic or acicular habit, characteristic of tourmalines. In cross-section, they often exhibit the rounded trigonal outline typical of this group.

Geological environment

## Genesis Chromium-alumino-povondraite forms under conditions of high-grade metamorphism. Its occurrence is associated with rocks rich in chromium and boron that have been subjected to high temperatures and pressures. At its type locality (Pereval quarry), it forms in calcite marbles. ## Mineral Associations This mineral co-occurs with other minerals typical of its formation environment. At the type locality, these include: calcite, quartz, chromo-dravite, chromophyllite, phlogopite, diopside, tremolite, pargasite, pyrite, titanite, and apatite. ## Localities The only confirmed and described locality for chromium-alumino-povondraite in the world is its type locality – the Pereval marble quarry near Sludyanka on Lake Baikal, in the Irkutsk Oblast, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria The most important criterion for collectors' value is the confirmation of the specimen's authenticity through chemical analysis. Specimens with well-formed, transparent crystals of intense green color, clearly contrasting with the host rock (matrix), are highly prized. The size of the crystals, though usually small, also significantly affects the value. ## Popular Localities The only source of specimens of this mineral is the Pereval quarry in Russia. Any specimen originating from this locality is by definition rare and valuable for specialized collectors of systematic minerals.

Care and storage

## Cleaning Specimens should be cleaned with great care, using a soft brush to remove dust. Due to its rarity and potential scientific value, the use of water or any chemical agents without consulting a specialist is discouraged. Ultrasonic cleaning is absolutely forbidden. ## What to Avoid Avoid contact with chemicals, acids, and bases. The mineral is sensitive to rapid temperature changes, which can lead to internal fracturing. Despite its good hardness, it should be protected from impacts and scratching by harder materials (e.g., diamond, corundum). ## Storage Specimens of chromium-alumino-povondraite should be stored in separate, padded collector's boxes to prevent abrasions and damage. They should be protected from dust and direct, prolonged exposure to sunlight. It is best to store them under stable humidity and temperature conditions.

External references

https://www.mindat.org/min-39891.html