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Microcrystalline red-orange hilgardite encasing white boracite, from Boulby Mine, Loftus, North Yorkshire, England. Size: 5.5 × 4.5 × 3.4 cm.
Category Tektoborates
(repeating unit)
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
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]

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]

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[edit]


  • 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