Hilgardite Locality: Boulby Mine , Loftus , North Yorkshire , England , UK ( Locality at mindat.org ) Pink Hilgardite crystals on pale green Boracite. Field of view 6 mm. Specimen and photo Leon Hupperichs. — Photo: Leon Hupperichs · CC BY-SA 3.0
Hilgardite
A transparent colorless-to-pink hydrated calcium chloroborate, hardness 5.
Intro adapted from Wikipedia (CC BY-SA).
Hilgardite is an IMA-recognized mineral species in the borate class, with the chemical formula Ca₂B₅O₉Cl·H₂O.
It crystallizes in the triclinic crystal system (space group P1). It is a member of the Hilgardite structural group.
Key properties: Mohs hardness 5; specific gravity 2.67–2.71; White streak; Vitreous luster; refractive index 1.643; optically biaxial.
IMA symbol Hgr, grandfathered status — known before the IMA’s founding, named in 1937.
Found in marine evaporite deposits.
It is found associated with Boracite, anhydrite, danburite, dolomite, magnesite, pyrite, hauerite, calcite, quartz, sulfur, gypsum (Choctaw salt dome, Louisiana, USA); boracite.
The mineral was named to honor Eugene Woldemar Hilgard (1833–1916), German–American geologist, one of the first to study the Louisiana salt diapir deposits.