Osakaite

Chemical formula: Zn<sup>2+</sup><sub>4</sub>S<sup>6+</sup>O<sub>4</sub>(OH)<sub>6</sub>·5H<sub>2</sub>O

Osakaite is a very rare, hydrated zinc sulfate, forming tiny, colorless or white crystals with a hexagonal habit.

## Characteristics Osakaite is a mineral from the sulfate group, chemically a hydrated basic zinc sulfate. It occurs as very small, hexagonal, tabular crystals, rarely exceeding 0.3 mm in size. These crystals often form rosette-like or spherical aggregates. It is colorless to white and transparent, with a vitreous luster. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It has a relatively low density of 2.85 g/cm³. It exhibits perfect cleavage in one direction, which is characteristic of its crystal structure. ## History and Name Osakaite was discovered in the Hirao mine, located in Osaka Prefecture, Japan. Its name comes directly from the discovery locality. The mineral was approved by the International Mineralogical Association (IMA) in 2011. The discovery was made by Japanese mineralogists, including H. Ohnishi, who is the first author of the publication describing the new mineral. ## Uses Due to its extreme rarity and microscopic crystal sizes, osakaite has no industrial applications. It is solely an object of scientific and collecting interest for specialized micromineral collectors.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
White
Density
2.85
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of osakaite is practically possible only through advanced analytical methods, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS), due to its microscopic size and lack of characteristic features visible to the naked eye. In a collection, it is identified based on a label with confirmed analysis. ## Distinguishing from Similar Minerals It can be confused with other secondary, white zinc sulfates, such as namuwite, lannonite, or gunningite, which form under similar conditions. Differentiation requires specialized laboratory equipment. Visually, it is practically impossible to distinguish it from these minerals. ## Crystal Forms It forms very small, hexagonal, tabular crystals, often flattened parallel to the basal plane. These crystals combine into rosette-like or spherulitic (spherical) aggregates.

Geological environment

## Genesis Osakaite is a secondary mineral, formed as a result of weathering (oxidation) processes of zinc ore deposits, mainly sphalerite. It forms in oxidation zones (iron caps) under low-temperature conditions, often as coatings and crusts in rock cavities. ## Mineral Associations It most often co-occurs with other secondary zinc and sulfur minerals. At the type locality (Hirao mine), it was found in association with gypsum, sphalerite, pyrite, quartz, and rare sulfates such as gunningite and lannonite. ## Localities The only confirmed and described occurrence of osakaite in the world is its type locality – the Hirao mine, in the Minamikawachi District, Osaka Prefecture, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of an osakaite specimen primarily depends on the confirmation of its identity through analysis. For collectors, the most desirable specimens are those with the largest (though still microscopic) and best-formed crystals or aggregates. The aesthetics of the rock matrix on which the mineral occurs are also important. ## Popular Localities The only source of collector specimens is the Hirao mine in Japan. Material from this locality is extremely difficult to acquire and appears on the market very rarely, mainly circulating among specialized micromineral collectors.

Care and storage

## Cleaning Osakaite specimens are extremely delicate and usually microscopic in size. They should not be cleaned mechanically or chemically. If cleaning is absolutely necessary, compressed air can be carefully used from a safe distance to remove loose dust particles. ## What to Avoid Avoid contact with water, as the mineral is soluble in it. It must be protected from all chemicals, acids, and detergents. It is very soft, so any touching or rubbing should be avoided. Changes in temperature and humidity can lead to its dehydration and destruction. ## Storage Specimens should be stored exclusively in specialized "micromount" boxes that protect them from physical damage, dust, and humidity changes. Store in stable room conditions, away from direct sunlight and heat sources.

Sources

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