Boracite

minerals

hilgardite

Formula: Mg3B7O13Cl
Anhydrous borate containing halogen, boracite group, orthorhombic paramorph of trembathite, which is hexagonal. Boracite forms a series with ericaite
Specific gravity: 2.91 to 3.1
Hardness: 7 to 7½
Streak: White
Colour: Green, blue, colourless, grey, white; dark green (ferroan); colourless in transmitted light
Solubility: Slowly but completely soluble in HCl. Very slowly decomposed by water.
Environments

Evaporite deposits

Boracite occurs in bedded sedimentary salt and potash deposits of marine origin, the boron probably derived from nearby volcanic activity (HOM). Associated minerals include anhydrite, gypsum, halite, sylvite, carnallite, kainite, danburite, magnesite and hilgardite (HOM, Mindat).

Localities

At the type locality, Lüneburger Kalkberg, Lüneburg, Lower Saxony, Germany, boracite occurs in a bedded sedimentary deposit of gypsum and anhydrite (Mindat).

At the Boulby potash mine, Loftus, North Yorkshire, England, UK, exceptional blue boracite crystals or nodules occur within massive sylvite and halite ore. The blue colour is due to Fe2+ substituting for Mg2+ (Minrec 41-1 Supplement p57-58). There is extensive substitution of Fe2+ for Mg2+ here, forming a solid solution of boracite up to ericaite, which is stable only above about 70oC. At room temperature boracite and trembathite are stable, but ericaite is not (M&M 25.31-32).
Hilgardite pseudomorphs after boracite have been found here (KL p186, Min Rec 41-1 Supplement p56).

Alteration

At 265oC the crystal system reverts to a high-temperature phase and the material becomes isotropic (Mindat).

Common impurities: Fe2+

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