Bernalite

Chemical formula: Fe<sup>3+</sup>(OH)<sub>3</sub>

Bernalite is a rare iron(III) hydroxide, forming small, octahedral crystals of dark green to black color with a vitreous luster.

## Characteristics Bernalite is an iron(III) hydroxide, occurring as very small but well-formed crystals. They typically adopt an octahedral habit, less commonly cubic, with a size not exceeding 0.5 mm. This mineral is most often found as single, scattered crystals or small aggregates on the surface of other minerals, especially in cavities of volcanic rocks. ## Physical Properties Bernalite crystals are characterized by a vitreous, and on some surfaces almost metallic, luster. They are brittle and exhibit an uneven fracture. The Mohs hardness is approximately 4, and the density is quite high, measured at 3.33 g/cm³. The mineral is usually opaque, only translucent on its thinnest edges. ## Colors and Varieties Bernalite has a characteristic dark green to almost black color. Under a microscope in transmitted light, it shows an intense green color. No varieties have been distinguished. ## History and Name The mineral was first described in 1993 by a team of researchers (Walenta, Dunn, Birch, Tiekink) based on specimens found in the Smart iron mine in South Australia. The name "bernalite" honors John Desmond Bernal (1901-1971), an Irish crystallographer and scientist, a pioneer in protein crystallography and one of the founders of modern molecular biology. His work on the structure of iron hydroxides was fundamental to understanding this group of compounds. ## Uses Bernalite has no industrial application due to its rarity and microscopic crystal sizes. It is of purely scientific and collecting interest, being a coveted object for collectors specializing in rare minerals and micromounts.

Properties

Mohs hardness
4
Luster
Vitreous
Streak
Greenish-brown
Density
3.33
Cleavage
None
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Cubic

Diagnostic features

## Identification Bernalite is difficult to identify without specialized equipment. Its characteristic features include: occurrence as small, sharp, octahedral crystals, dark green to black color, vitreous luster, and a specific environment of occurrence (oxidized zones of deposits). Final identification requires chemical analysis (EDS) and crystallographic analysis (XRD). ## Distinguishing from Similar Minerals Due to its appearance and small size, bernalite can be confused with other fine-grained, dark minerals such as magnetite, goethite, or some spinels. It is distinguished by its regular, octahedral crystal shape, lack of magnetism (unlike magnetite), and characteristic green color in transmitted light. Goethite rarely forms such well-developed, isometric crystals. ## Crystal Forms Bernalite crystallizes in the isometric system, forming almost exclusively crystals with an octahedral habit {111}. Less commonly, cubic forms {100} or combinations thereof are observed. Crystals are usually single and grown on the surface of other minerals.

Geological environment

## Genesis Bernalite is a secondary mineral, formed in the oxidation (weathering) zones of ore deposits. It forms under low-temperature conditions as a result of hydrothermal alteration or weathering of primary iron-rich minerals such as siderite or pyrite. It often occurs in cavities and fractures of volcanic or sedimentary rocks. ## Mineral Associations This mineral co-occurs with other secondary minerals of the oxidation zone. At the type locality (Smart mine, Australia), it was found in association with kankite, schneiderhöhnite, arsenosiderite, scorodite, strengite, and quartz. In other localities, it is accompanied by goethite, hematite, and siderite. ## Localities The most important and confirmed bernalite occurrences worldwide include: - **Australia** - Smart iron mine in Wirrabara, Flinders Ranges, South Australia (type locality). - **Germany** - Clara mine in Oberwolfach, Black Forest, and several other localities in the Harz Mountains and Saarland. - **Italy** - in volcanic exhalations of Vesuvius. - **USA** - in Utah County.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a bernalite specimen is primarily assessed based on the development and size of its crystals. The most prized specimens are those with sharp, lustrous, well-defined octahedra, clearly contrasting with the matrix. The aesthetics of the entire aggregate and the absence of mechanical damage are also important. Due to the microscopic nature of the mineral, even crystals a fraction of a millimeter in size are considered valuable if they are well-formed. ## Popular Localities The most classic and sought-after specimens by collectors come from the type locality in South Australia and from the Clara mine in Germany. Specimens from these locations are considered exemplary for this mineral species.

Care and storage

## Cleaning Bernalite specimens, due to their microscopic size and crystal fragility, should be cleaned with the utmost care. The safest method is to use compressed air (from a safe distance) to remove loose dust. For heavier soiling, careful rinsing in distilled water can be applied, followed by thorough drying. Any mechanical contact, including the use of brushes, should be avoided. ## What to Avoid Bernalite is sensitive to acids, which can damage or completely dissolve it. Contact with all household and laboratory chemicals should be avoided. It should also not be heated or subjected to ultrasound in cleaners. ## Storage The best way to store and display bernalite is in specialized micromount boxes. These protect delicate crystals from mechanical damage, dust, and sudden changes in humidity, while allowing observation under a microscope.

Sources

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