Hexahydroborite

Hexahydroborite is a mineral composed of calcium, boron, oxygen, and hydrogen, with formula CaB₂H₁₂O₆, more precisely [Ca²⁺]([B(OH₄)]⁻)₂·2H₂O.^[1][2]

Hexahydroborite is the end member of a series of natural metaborates with the general formula CaB₂O₄·nH₂O or CaO·B₂O₃·nH₂O with n varying from 0 to 6, the other end member being anhydrous calciborite CaB₂O₄.^[3][4]

The anion in the structure is tetrahydroxyborate, [B(OH₄)]⁻. The name comes from an interpretation of the formula as CaBO₃·6H₂O.

History and occurrence

The mineral was originally described by M. A. Simonov in 1976 from a sample found in a kurchatovite-sakhaite ore deposit in Solongo, Buryat Republic.^[5][2] The sample was recovered from 200 m underground, associated with pentahydroborite. the hexahydroborite was identified by X-ray diffraction. Gas/liquid inclusions suggest that the hexahydroborite crystallized after the pentahydroborite.^[2]

The mineral was identified also at a location in Fuka, Japan. It occurred in a layer between crystalline limestone and skarns, in association with olshanskyite and calcite. It was conjectured to be formed by hydrothermal alteration of takaedite at low temperatures, around 250 °C.^[6]

The mineral was synthesized in 2011.^[3]

Structure

The structure (observed in the synthetic material) consists of infinite columns parallel to the c axis, identically oriented, displaced along the other two axes and linked only by hydrogen bonds. The columns are formed by calcium polyhedra linked together and to B(OH)₄ orthotetrahedra by sharing edges.^[3]

The crystal cell is monoclinic with the following parameters:

a (pm)	b (pm)	c (pm)	β (°)	V (nm3)	Sample
800.6	664.9	801.9	104.2	0.4134	Solongo [5]
1601.1	668.8	795.4	103.8	-	Fuka[6]
799.41	663.21	798.71	104.16	0.41058	Synthetic[3]

The density calculated for the synthetic version is 1.891 g/cm³.^[3]

Properties

The mineral occurs in nature as flattened prismatic crystals, clear and transparent, with density 1.84 to 1.87 g/cm³ and Mohs hardness 2.5.^[6][1] The crystals may exhibit bluish-white photoluminescence.^[2]

The mineral is optically biaxial positive with 2V = 83°.^[2] The refractive index is 1.50, with slightly increasing values for α, β, and γ; the difference γ−α is 0.012 by Simonov,^[2] and 0.007 by Kusachi.^[6] It has strong dispersion, increasing from violet to red.^[2]

The compound is slowly soluble in water and insoluble in ethanol,^[6] contradicting an earlier report.^[2] It is promptly dissolved by hydrochloric, sulfuric, and acetic acid.^[2] On heating, it loses a significant amount of water between 90 and 110 °C, but dehydration is complete only over 800 °C, leaving anhydrous calcium metaborate CaB₂O₄.^[6]

The synthetic version was obtained by recrystallization of calciborite CaB₂O₄ from H₂O in presence of B₂O₃ at 250 °C and 70–80 atm.^[3]