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Formula: Ca2B5O9Cl.H2O
Hydrated borate containing halogen, hilgardite group
Crystal System: Triclinic
Specific gravity: 2.67 to 2.71 measured, 2.69 calculated
Hardness: 5
Streak: White
Colour: Colourless, light pink; colourless in transmitted light
Environments
Hilgardite is a minor borate mineral in marine evaporite deposits
(Webmin, HOM).
Localities
At New Brunswick, Canada, hilgardite is associated with boracite and
szaibélyite
(HOM).
At the Boulby potash mine, Loftus, North Yorkshire, England, UK, hilgardite occurs with
volkovskite (R&M 91-5.434-441), and hilgardite
pseudomorphs after boracite have been found
(KL p186).
At the type locality, the Choctaw Salt Dome, Plaquemine, Iberville Parish, Louisiana, USA, hilgardite occurs in the salt dome
associated with boracite, anhydrite,
danburite, dolomite,
magnesite, pyrite,
hauerite, calcite,
quartz, sulphur and
gypsum
(AM 64.187-195, Mindat, HOM).
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