Hydroboracite

Hydroboracite is a hydrated borate mineral (hence the name) of calcium and magnesium, whose chemical composition is CaMgB₆O₈(OH)₆·3H₂O. It was discovered in 1834 in the Inder lake, Atyrau Province, Kazakhstan. Hydroboracite is a minor borate ore mineral.

Hydroboracite
General
Category	Inoborates
Formula (repeating unit)	CaMgB6O8(OH)6·3H2O
IMA symbol	Hbo[1]
Strunz classification	6.CB.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P2/c
Unit cell	a = 11.76, b = 6.68 c = 8.23 [Å]; β = 102.59°; Z = 2
Identification
Colour	Colourless, white
Mohs scale hardness	2
Luster	Vitreous, silky
Diaphaneity	Transparent
Optical properties	Biaxial (+)
Refractive index	nα = 1.520 – 1.523 nβ = 1.534 – 1.535 nγ = 1.569 – 1.571
Birefringence	δ = 0.049
2V angle	Measured: 60° to 66°, calculated: 62° to 66°
Dispersion	Relatively weak
Solubility	Very poorly soluble in cold water. Partially dissolved by prolonged submersion in boiling water.
References	[2][3]

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. Webmineral data
3. Mindat