Colorless crystals of the rare borate mineral pentahydroborite from one of the few localities known worldwide (Solongo, Buryatia, Eastern Siberia, Russian Federation), in a capsule. — Photo: David Hospital · CC BY-SA 4.0
Pentahydroborite
Pentahydroborite is an IMA-recognized mineral species in the borate class, with the chemical formula CaB₂O(OH)₆·2H₂O.
It crystallizes in the triclinic crystal system (space group P-1).
Key properties: Mohs hardness 2.5; specific gravity 2–2.03; Vitreous luster; optically biaxial.
IMA symbol Phb, IMA status: approved, named in 1961.
Found in boron-rich iron-ore skarns (Novofrolovskoye deposit, Russia); in a volcanogenic-sedimentary borate deposit (near the Studenitsa Cloister, Yugoslavia).
It is found associated with Andradite–grossular, magnetite, szaibélyite (Novofrolovskoye deposit, Russia); colemanite, howlite, ulexite, studenitsite (near the Studenitsia Cloister.
The name refers to the composition, originally described as a hydrated borate with five molecules of water per formula unit.