Hilgardite

Hilgardite is a borate mineral with the chemical formula Ca₂B₅O₉Cl·H₂O. It is transparent and has vitreous luster. It is colorless to light pink with a white streak. It is rated 5 on the Mohs Scale. It crystallizes in the triclinic crystal system. Crystals occur as distorted tabular triangles and are hemimorphic, polytypes exist.[1][2]

Hilgardite
Microcrystalline red-orange hilgardite encasing white boracite, from Boulby Mine, Loftus, North Yorkshire, England. Size: 5.5 × 4.5 × 3.4 cm.
General
Category	Tektoborates
Formula (repeating unit)	Ca2B5O9Cl·H2O
Strunz classification	6.ED.05
Crystal system	Triclinic
Crystal class	Pedial (1) (same H-M symbol)
Space group	P1
Unit cell	a = 6.297, b = 6.464 c = 6.565 [Å]; α = 74.24° β = 61.68°, γ = 61.26°; Z = 1
Identification
Color	Colorless, light pink to reddish brown
Crystal habit	Tabular triangular crystals
Cleavage	{010}, perfect; {100}, good
Fracture	Conchoidal
Mohs scale hardness	5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.67–2.71
Optical properties	Biaxial (+)
Refractive index	nα = 1.630 nβ = 1.636 nγ = 1.664
Birefringence	δ = 0.034
2V angle	Measured: 35°
Other characteristics	Piezoelectric
References	[1][2][3]

It was named for geologist Eugene W. Hilgard (1833–1916). It was first described in 1937 for an occurrence in the Choctaw Salt Dome of Iberville Parish, Louisiana, US.[1] It occurs as an uncommon accessory mineral in evaporite deposits and salt domes worldwide. In addition to the type locality it has been reported in Wayne County, Mississippi and in the Louann Salt Formation, Clarke County, Alabama in the United States and at the Penobsquis and Salt Springs evaporites, near Sussex, New Brunswick, Canada. In Europe it is reported from the Konigshall-Hindenburg potash mine near Göttingen, Lower Saxony, Germany and in the Boulby potash mine, Whitby, Yorkshire, England. In Asia it is reported from the Chelkar salt dome, Uralsk district, Kazakhstan; the Ilga Basin, eastern Siberia, Russia and the Sedom Formation, Mount Sedom, Dead Sea, Israel.[1][2]

See also

• List of minerals

References

1. Mindat.org
2. Handbook of Mineralogy
3. Webmineral data

• S. Ghose and C. Wan, Hilgardite, Ca₂[B₅O₉]Cl·H₂O; a piezoelectric zeolite-type pentaborate, American Mineralogist; February 1979; v. 64; no. 1-2; p. 187-195
• Burns, P. C. and F. C. Hawthorne, Refinement of the structure of Hilgardite-1A, Acta crystallographica. Section C, 1994, vol. 50 (5), pp. 653–655