Coralloite

Chemical formula: Mn<sup>2+</sup>Mn<sup>3+</sup><sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>·4H<sub>2</sub>O

Coralloite is a rare manganese arsenate, forming tiny, coral-red, spherical aggregates.

## Characteristics Coralloite is a hydrated manganese arsenate that visually resembles coral clusters, hence its name. It occurs as very small, spherical aggregates (spherulites) with a radial internal structure. Individual spherulites rarely exceed 0.5 mm in diameter, forming crusty or botryoidal coatings on the host rock. Due to the small size of the crystals, its features are difficult to observe without magnification. ## Physical Properties The crystals forming the aggregates are extremely fine, acicular. The mineral is characterized by a vitreous luster. It is translucent to opaque. Its hardness and density have not been precisely determined due to the nature of the aggregates. ## Colors and Varieties Coralloite occurs in a characteristic, uniform color from coral-red to orange-red. No color varieties or commercial forms are known. ## History and Name The mineral's name, given in 2012 by its discoverers (P. Elliott, J. Brugger, A. Pring, G. Giester, T. A. Smith), comes from the Latin word *corallium* (coral), in reference to its color and aggregate form. It was discovered on the historic dump of the Gozaisho mine in Fukushima Prefecture, Japan, and approved by the International Mineralogical Association (IMA) under number 2012-034.

Properties

Luster
Vitreous
Streak
Pale orange
Cleavage
Perfect on {010}
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Practical identification of Coralloite relies on its unique appearance under a microscope: characteristic, coral-red, spherical aggregates with a radial structure. The key is also its origin – the Gozaisho mine in Japan. Final confirmation requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction. ## Distinguishing from Similar Minerals It can be confused with other secondary red arsenate minerals, such as Erythrite or some forms of Wallkillite. However, Coralloite is distinguished by its specific spherulitic aggregate form and unique chemical composition. It differs from Erythrite by the lack of distinct cleavage and its hue (more orange than pink). ## Crystal Forms It forms exclusively microscopic, spherulitic (spherical) aggregates composed of radially arranged, acicular crystals. It does not form single crystals visible to the naked eye.

Geological environment

## Genesis Coralloite is a secondary mineral, forming in the oxidation zone (gossan) of polymetallic deposits rich in manganese and arsenic. It formed as a result of the weathering of primary ore minerals under low temperature and pressure conditions. ## Mineral Associations At the type locality, it co-occurs with manganese arsenates such as Wallkillite-(Mn) and Shigaite, as well as with Quartz, which forms the host rock. ## Localities The only confirmed locality for Coralloite in the world is the Gozaisho mine, in Iwaki, Fukushima Prefecture, on Honshu Island, Japan. The mineral was found on the historic dumps of this now-inactive mine.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" mineral, its collector value depends on the richness and aesthetics of the coating on the host rock. The most prized specimens are those where the spherulitic aggregates are densely distributed, have an intense, coral-red color, and clearly contrast with the substrate. The size of the matrix specimen is of secondary importance; the quality of the coating itself, visible under magnification, is what matters.

Care and storage

## Cleaning Due to its extreme rarity and delicacy, Coralloite specimens should not be cleaned mechanically or chemically. Any attempts at cleaning may irreversibly damage the specimen. Only very careful removal of loose dust with a soft brush or a photographic blower is permissible. ## What to Avoid Avoid contact with water, chemicals, ultrasound, and sudden temperature changes. The mineral is very brittle and sensitive to vibrations and shocks. ## Storage Specimens should be stored exclusively in stable, sealed "micromount" containers, away from light, dust, and moisture. It is not suitable for display outside of a specialized protective box.

Sources

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