Shigaite

Chemical formula: Mn<sup>2+</sup><sub>6</sub>Al<sub>3</sub>(OH)<sub>18</sub>[Na(H<sub>2</sub>O)<sub>6</sub>](S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O

A rare mineral from the hydrotalcite group, forming characteristic hexagonal, tabular crystals of intense, reddish-brown color.

## Characteristics Shigaite is a rare, complex sulfate belonging to the hydrotalcite group. Its most recognizable feature is its well-formed, thin, tabular crystals with a hexagonal outline. They usually occur as single, sharply defined plates or their radial aggregates, forming rosette-like clusters. Specimens have an intense, reddish-brown to orange-brown color and a vitreous luster, making them visually very attractive. ## Physical Properties Shigaite is a very soft mineral, with a hardness of only 2 on the Mohs scale. Its crystals are brittle and easily damaged. It has a vitreous luster, and a slightly pearly luster on cleavage surfaces. It is transparent to translucent. The density is approximately 2.32 g/cm³. ## Colors and Varieties This mineral occurs in a characteristic range of colors, from dark red, through reddish-brown and orange-brown, to yellowish-brown. No named varieties are distinguished, and its coloration is relatively uniform, mainly dependent on the thickness of the crystals. ## History and Name Shigaite was discovered in the Ioi mine in Shiga Prefecture, Japan. Its name comes directly from the name of this prefecture. The mineral was first described in 1985 by a team of mineralogists: Akira Kato, Kozo Sugiura, Shigeo Shimada, and Yasutaka Okada, and its status as a new mineral species was approved by the International Mineralogical Association (IMA). ## Uses Due to its rarity and small crystal size, shigaite has no industrial applications. However, it is a highly valued and sought-after collector's mineral.

Properties

Mohs hardness
2
Luster
Vitreous, Pearly
Streak
Pale brown
Density
2.32
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification The key diagnostic feature of shigaite is its characteristic crystal form: thin, hexagonal (six-sided) tablets. The intense reddish-brown color, low hardness (can be scratched with a fingernail), and perfect cleavage in one direction are also very helpful in identification. ## Distinguishing from Similar Minerals Shigaite is sometimes confused with other minerals of similar color and tabular appearance, such as hematite or some micas (e.g., biotite). However, it is distinguished by its significantly lower hardness, hexagonal crystal shape (in contrast to the often irregular plates of hematite), and specific occurrence environment. ## Crystal Forms Shigaite crystallizes in the form of thin, sharply defined tablets with a hexagonal outline. These crystals often form fan-shaped or rosette-like aggregates, where individual plates radiate from a common center. Single, isolated crystals are less common.

Geological environment

## Genesis Shigaite is a secondary mineral, formed in the oxidation zones of manganese deposits. It forms in rock fractures and voids as a result of low-temperature hydrothermal processes occurring in manganese-rich metamorphic rocks, such as rhodochrosite and tephroite ores. ## Mineral Associations This mineral often co-occurs with other manganese minerals. Its most common associations include rhodochrosite, tephroite, sonolite, galaxite, as well as quartz, alabandite, and pyrochroite. It also occurs in association with other rare minerals, such as leucophoenicite and caryopilite. ## Localities The most important and type locality is the Ioi mine in Shiga Prefecture, Japan. Significant specimens also come from the famous manganese deposits in the Republic of South Africa, particularly from the N'Chwaning II and Wessels mines in the Kuruman area (Kalahari Desert). It is also known from the Kombat mine in Namibia.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized shigaite specimens are characterized by sharp, well-formed crystals of intense, ruby-red color and high transparency. Rosette-like aggregates of large diameter, as well as single, large (over 1 cm) hexagonal crystals without damage, are particularly sought after. A contrasting combination with a white or pink matrix (e.g., with rhodochrosite) significantly increases the value and aesthetics of the specimen. ## Popular Localities Specimens from the N'Chwaning and Wessels mines in South Africa are considered the best in the world in terms of crystal size, color, and form. Japanese specimens from the type locality (Ioi mine) are of great historical and scientific importance, although they are often smaller.

Care and storage

## Cleaning Shigaite specimens should be cleaned with the utmost care. Due to its extreme softness and brittleness, the best method is to gently blow off dust with compressed air from a safe distance. The use of water is not recommended, as it is a hydrated mineral and may undergo changes. Absolutely avoid ultrasonic cleaners. ## What to Avoid Avoid all contact with chemicals, including acids and bases. The mineral is very soft, so it must be protected from scratching by harder minerals. High temperatures and direct, prolonged sunlight should also be avoided, as they can lead to dehydration and loss of luster. ## Storage Shigaite should be stored separately, in a dedicated display box, away from other minerals that could damage it. It is best to keep it in stable temperature and humidity conditions to prevent its degradation. Display should be in a closed cabinet, protecting it from dust and mechanical damage.

External references

Sources

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