Hexahydroborite is a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

composed of

calcium Calcium is a chemical element; it has 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 to it ...

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

, and

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

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

metaborate A metaborate is a borate anion consisting of boron and oxygen, with empirical formula . Metaborate also refers to any salt (chemistry), salt or ester of such anion (e.g. salts such as sodium metaborate or calcium metaborate , and esters such as ...

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, . 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, is a republic of Russia located in the Russian Far East. Formerly part of the Siberian Federal District, it has been administered as part of the Far Eastern Federal District since 2018. To its north ...

. The sample was recovered from 200 m underground, associated with pentahydroborite. the hexahydroborite was identified by

X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

. Gas/liquid inclusions suggest that the hexahydroborite crystallized after the pentahydroborite. The mineral was identified also at a location in Fuka,

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

. It occurred in a layer between crystalline

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and

skarn Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, ...

s, in association with olshanskyite and

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

. 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 Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

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 mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fair ...

2.5. The crystals may exhibit bluish-white

photoluminescence Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. phot ...

. 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 the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

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 (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

, contradicting an earlier report. It is promptly dissolved by hydrochloric,

sulfuric Sulfur (American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form ...

, and

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

. 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
sub>2 · 2H₂O"">(OH)₄">"Crystal structure of a new natural calcium borate,hexahydroborite CaB₂O₄ 6H₂O = Ca[B(OH)₄
sub>2 · 2H₂O" ''Dokladi Akademii nauk SSSR'', volume 228, pages 1337-1340. PACS numbers: 61.50.Qy Boron minerals