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
800.6664.9801.9104.20.4134Solongo [5]
1601.1668.8795.4103.8-Fuka[6]
799.41663.21798.71104.160.41058Synthetic[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]

References
  1. M. A. Simonov (1977): "The new mineral hexahydroborite, Ca[B(OH)4]2 · 2H2O". Zap. Vsesojuz. Mineral. Obshchest. SSSR, volume 106, issue 6, pages 691-697
  2. M. A. Simonov (1979): "The new mineral hexahydroborite, Ca[B(OH)4]2 · 2H2O". International Geology Review, volume 21, issue 4, pages 491-496. doi:10.1080/00206818209467082
  3. N. A. Yamanova, E. Yu. Borovikova, and O. V. Dimitrova (2011): "Crystallization, crystal-structure refinement, and IR spectroscopy of a synthetic hexahydroborite analog". Crystallography Reports, volume 56, pages 1019–1024. doi:10.1134/S1063774511060289
  4. Shoichi Kobayashi, Tamami Ando, Akiko Kanayama, Mitsuo Tanabe, Shigetomo Kishi, Isao Kusachi (2014): "Calciborite from the Fuka mine, Okayama Prefecture, Japan". Journal of Mineralogical and Petrological Sciences, volume 109, issue 1, pages 13-17. doi:10.2465/jmps.130619
  5. M. A. Simonov, N. A. Yamanova, E. V. Kazanskaya, Yu. K. Egorov-Tismenko, and N. V. Belov (1976) "Crystal structure of a new natural calcium borate,hexahydroborite CaB2O4 6H2O = Ca[B(OH)42 · 2H2O"]. Dokladi Akademii nauk SSSR, volume 228, pages 1337-1340. PACS numbers: 61.50.Qy
  6. Isao Kusachi, Yasushi Takechi, Shoichi Kobayashi, Junji Yamakawa, Yoshihiro Nakamuta, Kyue-Hyung Lee, Shoji Motomizu (1999): "Hexahydroborite from Fuka, Okayama Prefecture, Japan". J-STAGE Mineralogical Journal, volume 21, issue 1, pags 9-14. doi:10.2465/minerj.21.9
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