Guyanaite

Chemical formula: Cr<sup>3+</sup>O(OH)

Guyanaite is a rare chromium hydroxide, forming microscopic, dark gray crystals with a metallic luster.

## Characteristics Guyanaite is a rare mineral from the hydroxide group, chemically classified as chromium(III) oxyhydroxide. It occurs as very small, tabular or prismatic crystals, rarely exceeding 0.5 mm in length. It typically forms rosetted or radial aggregates. Its color is dark gray to black, and due to the small size of the crystals, its precise observation requires the use of a microscope. ## Physical Properties Guyanaite crystals exhibit a strong, metallic or submetallic luster. The mineral is opaque. Due to the microscopic size of the crystals, hardness has not been precisely determined, but it is estimated to be around 3.5 on the Mohs scale. The density calculated based on the formula and unit cell parameters is 4.45 g/cm³. ## History and Name Guyanaite was discovered in river sediments in the Merume River region of Guyana. Its name comes from the country where its type locality is situated. The mineral was described and approved by the International Mineralogical Association (IMA) in 1969. ## Applications Due to its extreme rarity and microscopic size, guyanite has no industrial applications. It is solely an object of scientific and collecting interest for specialized rare mineral collectors.

Properties

Mohs hardness
3.5
Luster
Metallic
Streak
Greyish-black
Density
4.45
Cleavage
Perfect on {010}, Good on {100}
Fracture
Uneven
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Identifying guyanite under amateur conditions is practically impossible. It requires advanced analytical techniques such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS) to confirm its composition (chromium dominance) and crystal structure. Visually, it is characterized by a black color, metallic luster, and occurrence in the form of fine, radial aggregates. ## Distinguishing from similar minerals Guyanaite can be confused with other rare chromium oxides and hydroxides, such as bracewellite or grimaldiite, with which it often co-occurs. Distinguishing them from each other is only possible with X-ray analysis. It may also resemble small crystals of hematite or goethite, but its genesis and paragenesis are different. ## Crystal forms Guyanaite forms very small, elongated, prismatic crystals, often flattened. These crystals combine into radial or rosetted aggregates.

Geological environment

## Genesis Guyanaite is a secondary mineral, forming in the weathering zones of chromium deposits. It forms under oxidizing conditions in alluvial river sediments (placers) and in weathered ultramafic rocks rich in chromium. ## Mineral associations This mineral co-occurs with other chromium oxides and hydroxides, such as bracewellite, grimaldiite, and mcconnellite. It is also accompanied by chromite, gibbsite, quartz, rutile, anatase, and in some localities, native gold. ## Localities The most important and well-documented occurrences of guyanite worldwide include its type locality in the Merume River region of Guyana. It is also known from the Bou Azzer ultramafic complex in Morocco and from several locations in Russia, including the Rai-Iz massif in the Polar Urals.

Rarity

Very rare

For collectors

## Quality criteria For a mineral as rare as guyanite, the main criterion for value is the mere confirmation of its presence on a specimen. Samples with well-formed, though still microscopic, crystals forming visible aggregates are most prized. A rich association with other rare chromium minerals, such as bracewellite or grimaldiite, is also important. Specimens from the type locality in Guyana are particularly sought after by specialized collectors. ## Popular localities The most classic and valued specimens come from the Merume River alluvial deposits in Guyana. Material from Bou Azzer in Morocco also provides specimens of collecting significance.

Care and storage

## Cleaning Guyanaite specimens are usually very small and embedded in a rock matrix. Cleaning should be limited to gently removing dust with compressed air or a very soft brush. Avoid contact with water and chemical agents. ## What to avoid Avoid ultrasonic cleaners, chemical agents, acids, and sudden temperature changes. The mineral is brittle and susceptible to mechanical damage. ## Storage Specimens should be stored in stable conditions, in a closed display box, to protect them from dust and mechanical damage. Due to the small size of the crystals, storage in "micromount" type boxes is recommended.

External references

Sources

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