Akaganeite

Chemical formula: (Fe<sup>3+</sup>,Ni<sup>2+</sup>)<sub>8</sub>(OH,O)<sub>16</sub>Cl<sub>1.25</sub>·nH<sub>2</sub>O

Brownish-yellow iron oxyhydroxide, formed in chloride-rich environments, known for its occurrence in iron meteorites.

## Characteristics Akaganeite is a mineral from the iron oxyhydroxide group, representing one of four polymorphic varieties alongside goethite, lepidocrocite, and feroxyhyte. It typically forms very fine-grained, powdery or earthy aggregates, coatings, and crusts. It is rarely observed as well-formed crystals, which appear as microscopic needles or rods, often forming radial or stellar aggregates. Its color is characteristic – from yellow, through rusty brown to brown, and the mineral itself is opaque with a dull or earthy luster. ## Physical Properties Akaganeite is a relatively soft mineral, with a Mohs hardness of 2-3. Its density varies depending on the porosity of the aggregates, ranging from approximately 3.0 to 3.6 g/cm³. Due to the microscopic size of its crystals, features such as cleavage are difficult to observe, although theoretically, it is good in two directions. ## Colors and Varieties This mineral does not form color varieties or commercial types. Its coloration is constant and falls within the range of brown and yellow shades, which is directly related to its iron-based chemical composition. The presence of nickel in its structure, as indicated in some sources, does not significantly affect its visual characteristics. ## History and Name The name akaganeite comes from the Aka-gane mine in Iwate Prefecture, Japan, where it was first identified and described. The name, meaning "red metal" in Japanese, was given to it in 1962 by Scottish crystallographer Alexander T. Mackay. The type locality (locus typicus) is an iron ore deposit where this mineral occurred in the oxidation zone. ## Uses Akaganeite has no direct industrial application and is not extracted as iron ore. However, it is an important subject of research in materials science as a product of steel and iron corrosion in the presence of chlorides. Its identification on the surface of Mars by Martian rovers makes it a significant indicator of the planet's paleoenvironment. For collectors, it holds mainly scientific significance.

Properties

Mohs hardness
2-3
Luster
Earthy
Streak
Brownish yellow
Density
3.0-3.6
Cleavage
Good on {100}
Fracture
Uneven
Transparency
Translucent to Opaque
Crystal system
Tetragonal

Diagnostic features

## Identification Characteristic features of akaganeite include its brownish-yellow color, earthy or powdery aggregates, and its specific occurrence environment, rich in chlorides. In collections, it is often found as a weathering product of iron meteorites, where it forms characteristic coatings in cracks. ## Distinguishing from Similar Minerals Macroscopically, akaganeite is almost impossible to distinguish from other iron oxyhydroxides, such as goethite, lepidocrocite, or amorphous mixtures referred to as limonite. Goethite usually has a darker, brown color and a distinctly yellowish-brown streak. Lepidocrocite tends to be more reddish. A definitive distinction between these minerals is possible almost exclusively through advanced analytical methods, primarily X-ray diffraction (XRD). ## Crystal Forms Akaganeite crystals are microscopic, acicular or rod-like. They form bundles and sheaves, which aggregate into rosette-like, star-like, or spherical forms (spherulites). Most often, however, it occurs as compact, earthy masses or thin crusts.

Geological environment

## Genesis Akaganeite is a secondary mineral, formed by the oxidation of iron-containing minerals (e.g., pyrite, siderite, magnetite) under conditions of high chloride ion content. Its formation is characteristic of several specific environments: weathering zones of ore deposits, sediments from submarine and terrestrial hot springs (geothermal brines), and as a product of natural and industrial corrosion. It is also a typical weathering product of iron meteorites under terrestrial conditions. ## Mineral Associations This mineral often co-occurs with other iron oxidation products. Its most common associations include goethite, hematite, lepidocrocite, jarosite, and in the case of meteorites – native iron (kamacite, taenite) and hibbingite. ## Localities Beyond its type locality in Japan (Aka-gane mine), akaganeite is found in many places worldwide. Significant occurrences include geothermal areas, such as Salton Sea in California (USA). It is commonly identified as a component of patina on ancient iron and bronze artifacts, e.g., in Laurion, Greece. It is also widely distributed as a weathering product in numerous iron meteorites found globally, such as Canyon Diablo.

Rarity

Not very common

For collectors

## Quality Criteria The collector's value of akaganeite is not related to its aesthetics in the classical sense. Specimens of scientific and documentary value are most prized. Precise locality information is crucial, especially if it comes from a rare site or a known meteorite. Specimens with microscopically visible crystalline aggregates are more highly valued than uniform earthy masses. Specimen stability is also an important factor – specimens that do not show active degradation are preferred. ## Popular Localities For specialist collectors, specimens from the type locality (Aka-gane, Japan) are most desirable, although they are extremely rare in circulation. Fragments of iron meteorites (e.g., Sikhote-Alin, Canyon Diablo, Nantan) with clearly visible akaganeite crusts, documenting their terrestrial weathering process, are very popular.

Care and storage

## Cleaning Akaganeite specimens are very sensitive. To remove dust and loose contaminants, use only a soft brush or compressed air from a safe distance. Avoid any contact with water and other liquids, which can initiate irreversible chemical reactions. ## What to Avoid The greatest threat to akaganeite is moisture. The chloride ions contained in its structure can lead to the formation of hydrochloric acid in the presence of water, causing chemical decomposition of the mineral, manifested by "sweating" and ultimately its disintegration. It should also be protected from direct sunlight and high temperatures, which can lead to dehydration and structural changes. ## Storage Storage in an environment with the lowest possible humidity is absolutely required. The best solution is tightly sealed containers (e.g., membrane boxes, gasketed display cases) with a moisture-absorbing agent, such as silica gel, placed inside. Regular inspection and replacement of the moisture absorber are crucial for the long-term preservation of the specimen.

External references

Sources

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