Manjiroite

Chemical formula: Na(Mn⁴⁺₇Mn³⁺)O₁₆

Manjiroite is a rare manganese and sodium oxide from the coronadite group, forming aggregates of fine, acicular crystals with a dark color.

## Characteristics Manjiroite is a mineral belonging to the coronadite group, being a hydrated manganese and sodium oxide. It most commonly occurs as aggregates of very fine, acicular or fibrous crystals. Its aggregates typically have a dark gray or brownish-black color and a dull appearance. Due to the small size of the crystals, individual features are difficult to observe with the naked eye. ## Physical Properties This mineral is characterized by a hardness ranging from 3-3.5 on the Mohs scale, making it relatively soft. Its density is approximately 4.29 g/cm³. It is opaque and has a dull luster. The streak, which is the color of the powdered mineral, is brownish-black. ## Colors and Varieties Manjiroite occurs in uniform, dark colors, most often dark gray, brownish-gray to brownish-black. No color varieties or commercial varieties are distinguished. ## History and Name Manjiroite was discovered and described in 1967. Its name comes from Manjiro Watanabe (1891-1980), a Japanese professor of economic geology at Tohoku University, who made significant contributions to the study of Japanese manganese ore deposits. The type locality where this mineral was first identified is the Kohare mine in Iwate Prefecture, Japan.

Properties

Mohs hardness: 3-3.5
Luster: Dull
Streak: Brownish black
Density: 4.29
Cleavage: Probably has a good cleavage similar to other minerals in its group.
Fracture: Conchoidal,Sub-Conchoidal
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Manjiroite is difficult to identify without specialized tests. It can be preliminarily recognized by its fibrous or acicular aggregate form, dark color (brownish-gray to black), dull luster, and brownish-black streak. Its relatively low hardness (3-3.5) is also a helpful characteristic. ## Distinguishing from Similar Minerals Manjiroite can be easily confused with other manganese oxides, such as pyrolusite, hollandite, or cryptomelane, which often form similar, dark, reniform or fibrous aggregates. Definitive differentiation from these minerals requires chemical analysis (to confirm the presence of sodium) or X-ray diffraction (XRD) studies. ## Crystal Forms Manjiroite crystals are very small, usually microscopic, and have an acicular or fibrous form. They form dense, felted aggregates, as well as radial or spherulitic clusters.

Geological environment

## Genesis Manjiroite is a mineral of hydrothermal origin. It forms at low temperatures in veins cutting through manganese deposits that have undergone metamorphic processes. It is a product of the alteration of other, earlier manganese minerals. ## Mineral Associations This mineral often co-occurs with other manganese oxides, such as pyrolusite, birnessite, todorokite, and nsutite. At the type locality (Kohare mine), it was found in association with pyrolusite and cryptomelane. ## Localities The most important and typical locality for manjiroite is the Kohare mine in Iwate Prefecture, Japan. Its presence has also been reported in several other locations worldwide, including manganese deposits in South Africa and the United States (Arizona), but it remains a very rare mineral.

Rarity

Very rare

For collectors

## Quality Criteria The quality of manjiroite specimens is primarily assessed based on the richness and development of the aggregates. Specimens with well-defined, radial or spherulitic clusters of acicular crystals are most valued. Association with other minerals is also important, as it can enhance the visual and scientific value of the specimen. Due to the microscopic size of the crystals, purity and individual crystals are not typical evaluation criteria. ## Popular Localities By far the most known and valued source of manjiroite specimens is its type locality – the Kohare mine in Japan. Specimens from this location hold the greatest historical and reference value for collectors specializing in rare minerals or systematics.

Care and storage

## Cleaning Manjiroite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its low hardness and potential fragility, avoid washing under running water and using any mechanical tools. If liquid is necessary, distilled water is recommended, followed by very gentle drying. ## What to Avoid Avoid contact with acids and other chemicals that may react with manganese oxides. The mineral is susceptible to scratching, so it should not be stored in contact with harder minerals. Ultrasonic cleaners should also be avoided. ## Storage Manjiroite should be stored in a separate, padded collector's box to protect it from mechanical damage and abrasion. It should be kept in stable conditions, away from moisture and extreme temperatures.

External references

https://www.mindat.org/min-2566.html