Formula: CaMg[B3O4(OH)3]22O
Hydrated borate containing hydroxyl
Specific gravity: 2
Hardness: 2
Streak: White
Colour: Colourless, white
Solubility: Virtually insoluble in cold water, partially dissolved by prolonged submersion in boiling water

Evaporite deposits

Hydroboracite typically is formed by groundwater alteration of colemanite, or it may be primary. Associated minerals include colemanite and tunellite (HOM).


At Penobsquis, Cardwell Parish, Kings county, New Brunswick, Canada, hydroboracite is very common as crystals to 2 cm in length, usually associated with volkovskite, hilgardite and trembathite. It may be cavernous with large fluid or solid inclusions of boracite, clay and mica. Strontioginorite has been observed impaling hydroboracite (CM 43.1469-1487).

At the Salt Springs evaporite deposit, Hammond Parish, Kings county, New Brunswick, Canada, boracite, the major borate mineral, occurs in association with hydroboracite, hilgardite, szaibélyite, priceite, ulexite, colemanite and danburite.
Halite has been found in one intersection as soft, chalky white nodules to 1 mm, associated with boracite and hydroboracite (CM 19.291-301).

At the Gharah Gol Boron mine, Dandi, Mahneshan County, Zanjan, Iran, the major mineral is hydroboracite, with minor presence of other borates such as colemanite, inyoite, meyerhofferite and szaibélyite. The hydroboracite is mostly massive, but microcrystals are quite common. Yellow to yellowish-brown and rarely reddish-brown crystals of colemanite to 3 cm have been found. They most likely formed as a later stage mineral after some of the hydroboracite was dissolved (R&M 92.6.542-544).

The type locality is the Inder B deposit and salt dome, Inder, Atyrau Region, Kazakhstan.

At Boulby mine, Loftus, Redcar and Cleveland, England, UK, hydroboracite has been found in core samples from the evaporites as glassy beige crystals to 0.7 cm in length associated with salmon-orangepink volkovskite crystals and frosted grey-white boracite microcrystals. Szaibélyite occurs intimately associated with volkovskite, boracite, hilgardite and hydroboracite (R&M 91.5.434-441).

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