Pentahydroborite

specimen number:
2354301

location:
Fuka Mine, Okayama, Japan

description:
An open pocket approximately 3.0 x 2.8 cm in size is lined with colorless crystals of pentahydroborite. In good condition. This is excellent for the species.

Located in Okayama Prefecture, Japan, the Fuka Mine is a world-renowned mineral locality famous for its rare skarn deposits, which formed through high-temperature contact metamorphism between Cretaceous granitic rocks and Paleozoic limestones. Active mining in the region historically targeted limestone and marble, but mid-twentieth-century mineralogical explorations revealed a remarkably rich assembly of unusual borate minerals generated by boron-rich hydrothermal fluids altering the surrounding rock. Among these rare occurrences is pentahydroborite, a soft, colorless to white hydrated calcium borate mineral that forms delicate prismatic crystals, crusts, or granular aggregates within limestone cavities. Discovered at Fuka during detailed mineralogical studies of the skarn zones, pentahydroborite represents a classic example of the complex low-temperature secondary reactions occurring in boron-rich environments. Today, specimens from the Fuka Mine are highly prized by systematic mineral collectors worldwide due to the locality being the primary source for exceptionally fine examples of this scarce borate species.

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