Nagashimalite

Chemical formula: Ba<sub>4</sub>(V<sup>3+</sup>,Ti<sup>4+</sup>)<sub>4</sub>(O,OH)<sub>2</sub>[B<sub>2</sub>Si<sub>8</sub>O<sub>27</sub>]Cl

Nagashimalite is an extremely rare barium, vanadium, and titanium borosilicate, forming dark green to black tabular crystals.

## Characteristics Nagashimalite is a complex borosilicate from the hyalotekite group, containing barium, vanadium, and titanium. It forms very small, thin, tabular or platy crystals, rarely exceeding 1 mm in length. These crystals often group into subparallel or rosette aggregates. The mineral is dark green to almost black and has a vitreous luster. ## Physical Properties Its Mohs hardness is 6. It is a relatively dense mineral, with a specific gravity of approximately 4.15 g/cm³. It exhibits translucency on thin edges or in the case of very small crystals. ## Colors and Varieties This mineral occurs in uniform colors, from dark green to black. Due to its extreme rarity, no color or commercial varieties have been distinguished. ## History and Name Nagashimalite was described in 1999 by Izumi Nakai and his team. The name honors Otokichi Nagashima (1907-1981), a Japanese mineral collector who found the first samples of this mineral in the 1970s. The type locality is the Mogurazawa mine in Gunma Prefecture, Japan. ## Uses Nagashimalite has no industrial applications. Its significance is purely scientific and collectible, being of interest to specialized institutions and advanced collectors of rare minerals.

Properties

Mohs hardness
6
Luster
Vitreous
Streak
Pale green
Density
4.15
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Characteristic features of nagashimalite include its dark green to black color, thin, tabular crystal habit, hardness of 6, and a pale green streak. Its unique occurrence environment and paragenesis with other rare barium minerals are of key diagnostic importance. ## Distinguishing from Similar Minerals It can be confused with dark tourmalines (schorl), some amphiboles (hornblende), or pyroxenes (aegirine). However, it is distinguished by its specific, thin-tabular habit, relatively high density, pale green streak, and co-occurrence with celsian and cymrite in manganese deposits. ## Crystal Forms Nagashimalite forms thin, tabular crystals with a rectangular or hexagonal outline. These crystals often occur as small aggregates with a rosette or subparallel structure.

Geological environment

## Genesis Nagashimalite forms as a result of low-temperature metamorphism in manganese ore deposits. Its crystallization is associated with hydrothermal processes in rocks rich in barium, vanadium, and boron. ## Mineral Associations It most commonly co-occurs with celsian, cymrite, tamaiite, ganophyllite, quartz, and braunite. This paragenesis is very characteristic of its only known locality. ## Localities The only confirmed locality of nagashimalite in the world is the Mogurazawa mine, near Kiryū City, Gunma Prefecture on Honshu Island, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria The most highly valued specimens are those that contain well-formed, sharp crystals with a distinct tabular habit, forming aesthetic groups on the rock matrix. The presence of other rare associated minerals, such as celsian or tamaiite, adds significant value. Due to the small size of the crystals, specimens suitable for microscopic observation (so-called micromounts) are most common in collections. ## Popular Localities The only source of collector specimens is the historical type locality – the Mogurazawa mine in Japan. This mine is now inactive, meaning new specimens are not being acquired, and material on the market comes exclusively from old collections.

Care and storage

## Cleaning Nagashimalite specimens should only be cleaned mechanically, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided, and the specimen should be dried immediately. ## What to Avoid Ultrasonic cleaners, steam cleaning, and contact with acids and other chemicals must be strictly avoided. The mineral is brittle, so it must be protected from impacts, scratching, and sudden temperature changes. ## Storage It is recommended to store specimens in separate, padded display boxes to prevent abrasion and contact with other minerals. It should be protected from dust and moisture.

Sources

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