White crystal aggregates of the borate mineral hexahydroborite associated to grey kurchatovite from Russia (Solongo B Deposit, Vitim Plateau, Baunt District, Buryatia Republic, Russian Federation). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Hexahydroborite
The fully hydrated end of a natural calcium metaborate series, opposite anhydrous calciborite.
Intro adapted from Wikipedia (CC BY-SA).
Hexahydroborite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Ca[B(OH)₄]₂·2H₂O.
It crystallizes in the monoclinic crystal system (space group P2/a).
Key properties: Mohs hardness 2.5; specific gravity 1.84–1.87; Vitreous luster; optically biaxial.
IMA symbol Hhb, IMA status: approved, named in 1977.
A rare component of boron-enriched skarn deposits in metasomatized limestones.
It is found associated with Pentahydroborite, frolovite, kurchatovite, sakhaite (Solongo deposit, Russia); olshanskyite, calcite (Fuka, Japan).
The name refers to the composition, originally thought to contain six waters of hydration (later found by crystal-structure analysis to be incorrect).