Chiyokoite

Chemical formula: Ca₃Si(CO₃)[B(OH)₄]O(OH)₅·12H₂O

Chiyokoite is a very rare, colorless borosilicate mineral, discovered in Japan, forming small, platy crystals.

## Characteristics Chiyokoite is a hydrated calcium borosilicate with a complex chemical composition. It occurs as very small, colorless, platy crystals, reaching sizes up to 0.3 mm. The crystals are transparent and exhibit a vitreous luster. Due to its extreme rarity and small crystal size, it is a mineral known primarily within the scientific community and among specialized micromineral collectors. ## Physical Properties Chiyokoite crystals exhibit a hardness of approximately 3 on the Mohs scale. They are brittle, and their density, calculated from crystallographic data, is 1.98 g/cm³. This mineral does not show fluorescence under ultraviolet light. ## Colors and Varieties Chiyokoite is colorless. No colored varieties are known. ## History and Name The mineral was discovered in the Fuka mine in Okayama Prefecture on Honshu Island, Japan. It was approved by the International Mineralogical Association (IMA) in 2018 under number IMA2018-091. The name honors Chiyoko Henmi (born 1949), a Japanese mineralogist and crystallographer, professor at Okayama University, for her contributions to the study of skarn minerals, including those from the chiyokoite type locality.

Properties

Mohs hardness
3
Color
Pink to colorless
Luster
Vitreous
Streak
White
Density
1.85
Cleavage
<mi>{10_10}</mi> and {0001} good
Fracture
Step-Like
Transparency
Transparent
Crystal system
Hexagonal

Diagnostic features

## Identification Identification of chiyokoite is possible almost exclusively using advanced analytical methods, such as X-ray diffraction (XRD) and chemical spectroscopy (EDS/WDS), due to its small size and occurrence in association with other rare minerals. Visually, it is a colorless, vitreous mineral forming small platy crystals. ## Distinguishing from Similar Minerals It can be confused with other colorless minerals found in the Fuka skarns, such as calcite, thaumasite, or other borates and silicates. Reliable differentiation from these minerals under amateur conditions is practically impossible and requires specialized laboratory equipment. ## Crystal Forms Chiyokoite crystallizes in the form of thin, platy crystals with a hexagonal outline, often forming small, rosette-like aggregates.

Geological environment

## Genesis Chiyokoite forms within skarns, which are metamorphic rocks created at the contact of a magmatic intrusion with carbonate-rich sedimentary rocks (e.g., limestones). It is a product of low-temperature hydrothermal processes occurring within these skarns. ## Mineral Associations This mineral co-occurs with other minerals typical of Fuka skarns, such as calcite, thaumasite, borcarite, frolovite, olshanskyite, sibirskite, and takedaite. ## Localities The only confirmed occurrence of chiyokoite in the world is its type locality - the Fuka mine, near Takahashi City, Okayama Prefecture, on Honshu Island, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria As a micromineral, chiyokoite is primarily valued for the sharpness and development of its crystals, their transparency, and their aesthetic arrangement on the rock matrix. Specimens with clearly formed, undamaged crystals, ideally forming small, photogenic groups, are most desirable. Contrast with associated minerals also enhances the visual appeal of the specimen. ## Popular Localities The Fuka mine in Japan is the only source of chiyokoite specimens. It is a classic locality for many rare boron minerals, and specimens from there are highly prized by collectors specializing in microminerals and systematic minerals.

Care and storage

## Cleaning Due to the extreme delicacy, brittleness, and small size of the crystals, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust particles. Avoid contact with water, which may affect the hydrated structure of the mineral. ## What to Avoid Absolutely avoid contact with any chemicals, acids, and detergents. The mineral is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight. Vibrations and shocks can easily damage or destroy the tiny crystals. ## Storage Chiyokoite specimens, typically as microminerals, should be stored exclusively in specialized "micromount" boxes that protect them from dust, moisture, and mechanical damage. Display is only possible under a microscope.

External references

Sources

Read more