Chromium-dravite

Chemical formula: NaMg₃Cr³⁺₆(Si₆O₁₈)(BO₃)₃(OH)₃OH

Chrom-dravite is a rare mineral from the tourmaline group, distinguished by its intense green color, which it owes to its high chromium content.

## Characteristics Chrom-dravite is a mineral belonging to the tourmaline group, specifically it is the chromium analogue of dravite. It forms well-developed, prismatic crystals with the characteristic triangular cross-section and rounded faces typical of tourmalines. Its most striking feature is its intense, emerald-green to dark-green, and even almost black color. Crystals often exhibit strong pleochroism, changing hue from yellowish-green to dark-green depending on the observation direction. ## Physical Properties Chrom-dravite is characterized by a hardness of 7.5 on the Mohs scale, making it a durable and scratch-resistant mineral. It has a vitreous luster and is usually opaque, although thin fragments or smaller crystals can be translucent. Its density is approximately 3.25 g/cm³. ## Colors and Varieties The dominant and defining color is green in various shades, from vivid, emerald to very dark, almost black. The intensity of the color is directly related to the concentration of chromium in the crystal structure. No distinct commercial or color varieties are distinguished beyond its basic form. ## History and Name The mineral's name directly refers to its chemical composition – it is dravite in which aluminum has been largely replaced by chromium. It was formally described and recognized as a new mineral species by the International Mineralogical Association (IMA) in 1977. The first described specimens originated from Karelia, Russia. ## Applications Due to its rarity and small crystal sizes, chrom-dravite has no industrial application. However, it is a highly valued and sought-after collector's stone, and exceptionally clean and well-colored specimens can be faceted for specialized gemstone collectors.

Properties

Mohs hardness
7-7.5
Color
Dark emerald-green to greenish-black.
Luster
Vitreous to oily.
Streak
Greyish-green.
Density
0
Cleavage
None.
Fracture
Uneven, Conchoidal
Transparency
Translucent to Opaque
Crystal system
Trigonal

Diagnostic features

## Identification The key diagnostic feature of chrom-dravite is its intense, emerald-green color combined with the typical tourmaline form of prismatic crystals with a triangular cross-section. Strong pleochroism (color change depending on the crystal axis) is also very characteristic. The streak is grayish-green. ## Distinguishing from Similar Minerals Chrom-dravite can be confused with other green tourmalines, such as elbaite (verdelite) or dravite. Differentiation requires advanced chemical analysis (EDS, WDS) to confirm the dominance of chromium over aluminum. It is distinguished from emerald by its higher hardness, different crystal form (triangular cross-section instead of hexagonal), and lack of perfect cleavage. It is distinguished from green diopside (chromian diopside) by its significantly higher hardness (7.5 vs 5.5-6.5). ## Crystal Forms Chrom-dravite forms elongated, prismatic crystals, often terminated by a trigonal pyramid. A characteristic feature is the triangular cross-section with convex, rounded faces. Crystals often exhibit vertical striations on the prism faces. It occurs as single crystals or in radial aggregates.

Geological environment

## Genesis Chrom-dravite is a metamorphic mineral, forming under conditions of high pressure and temperature. Its formation is associated with metasomatic processes in ultramafic rocks (e.g., serpentinites, peridotites) or in chromium-rich sedimentary rocks that have undergone metamorphism, such as quartzites or mica schists. ## Mineral Associations This mineral often co-occurs with other metamorphic and chromium-rich minerals. Typical associated minerals include quartz, chromite, rutile, pyrite, micas (fuchsite, muscovite), talc, magnesite, dolomite, beryl (emerald), and other tourmalines. ## Localities The most important and classic localities for chrom-dravite are in Russia, particularly in Karelia (around Lake Onega) and the Urals. Significant specimens also come from Kajiado County in Kenya and the Nanyuki area in Tanzania. Occurrences have also been reported in Spain (Galicia), Finland, and Greenland.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized chrom-dravite specimens are those that combine several features: an intense, but not too dark, emerald-green color, good transparency, sharply terminated and undamaged crystals, and high luster. Large, single crystals with a distinct form are much more valuable than aggregates of small needles. Specimens on a quartz matrix or with other contrasting minerals are also highly sought after. ## Popular Localities Specimens from Karelia, Russia, which are historically the most important, are considered the best in the world. In recent years, high-quality crystals from Kenya and Tanzania have also appeared on the market, often characterized by a more vivid color and better transparency, making them attractive to collectors.

Care and storage

## Cleaning Chrom-dravite specimens are best cleaned with lukewarm water and a mild soap, using a soft brush, such as a toothbrush. Ultrasonic and steam cleaners should be avoided, as they can cause fractures in the crystal, especially if it contains internal inclusions or cracks. ## What to Avoid The mineral is relatively hard but sensitive to sudden temperature changes, which can lead to fractures. It should be protected from contact with strong acids and chemicals. Prolonged exposure to intense sunlight is not recommended, although there is no evidence of it fading. ## Storage Collector's specimens of chrom-dravite should be stored in separate boxes or on stands to prevent scratches from harder minerals (e.g., diamond, corundum) and damage to softer minerals. Due to its value, it is recommended to store it in stable room conditions, away from direct sunlight and heat sources.

External references

Sources

Read more