Hexahydroborite is a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

composed of

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

, boron,

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

, and

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

, with formula , more precisely . Hexahydroborite is the end member of a series of natural

metaborate A metaborate is an anion (negative ion) consisting of boron and oxygen, with empirical formula ; or any salt with such anions, such as sodium metaborate, or calcium metaborate . It is one of the boron oxoanions or borates In aqueous solution ...

s with the general formula or with ''n'' varying from 0 to 6, the other end member being anhydrous calciborite . The anion in the structure is

tetrahydroxyborate Tetrahydroxyborate is an inorganic anion with the chemical formula or . It contributes no colour to tetrahydroxyborate salts. It is found in the mineral hexahydroborite, , originally formulated . It is one of the boron oxoanions, and acts as ...

, . The name comes from an interpretation of the formula as .

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 Buryatia, officially the Republic of Buryatia (russian: Республика Бурятия, r=Respublika Buryatiya, p=rʲɪsˈpublʲɪkə bʊˈrʲætʲɪjə; bua, Буряад Улас, Buryaad Ulas, , mn, Буриад Улс, Buriad Uls), is ...

. 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. The mineral was identified also at a location in

Fuka Fuka, Fūka or Fuuka may refer to: * Fūka (given name), a feminine Japanese given name * ''Fuuka'' (manga), a Japanese manga series * Fukah, a village in northern Egypt, referred to as Fuka in a World War II context **Sidi Haneish Airfield Si ...

, Japan. It occurred in a layer between crystalline

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and

skarn Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate mineral ...

s, in association with olshanskyite and calcite. It was conjectured to be formed by hydrothermal alteration of takaedite at low temperatures, around 250 °C. The mineral was synthesized in 2011.

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 orthotetrahedra by sharing edges. The crystal cell is

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

with the following parameters: The density calculated for the synthetic version is 1.891 g/cm³.

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 The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by t ...

2.5. The crystals may exhibit bluish-white photoluminescence. The mineral is optically biaxial positive with 2V = 83°. The

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

is 1.50, with slightly increasing values for α, β, and γ; the difference γ−α is 0.012 by Simonov, and 0.007 by Kusachi. It has strong

dispersion Dispersion may refer to: Economics and finance * Dispersion (finance), a measure for the statistical distribution of portfolio returns * Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variat ...

, increasing from violet to red. The compound is slowly soluble in water and insoluble in

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

, contradicting an earlier report. It is promptly dissolved by

hydrochloric Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the digestiv ...

, sulfuric, and acetic acid. 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 . The synthetic version was obtained by recrystallization of calciborite from in presence of at 250 °C and 70–80 atm.

References

M. A. Simonov (1979): "The new mineral hexahydroborite, Ca (OH)₄sub>2 · 2H₂O". ''International Geology Review'', volume 21, issue 4, pages 491-496. M. A. Simonov (1977): "The new mineral hexahydroborite, Ca (OH)₄sub>2 · 2H₂O". ''Zap. Vsesojuz. Mineral. Obshchest. SSSR'', volume 106, issue 6, pages 691-697 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. 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. 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 M. A. Simonov, N. A. Yamanova, E. V. Kazanskaya, Yu. K. Egorov-Tismenko, and N. V. Belov (1976
2O₄ 6H₂O = Ca "Crystal_structure_of_a_new_natural_calcium_borate,hexahydroborite_CaB₂O₄_6H₂O_=_Ca[B(OH)₄
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sub>2 · 2H₂O" ''Dokladi Akademii nauk SSSR'', volume 228, pages 1337-1340. PACS numbers: 61.50.Qy Boron minerals