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Formula: Mg3B7O13Cl
Anhydrous borate containing halogen, boracite group, orthorhombic
paramorph
of trembathite, which is hexagonal. Boracite forms a series with
ericaite
Crystal System: Orthorhombic
Specific gravity: 2.91 to 3.1 measured, 2.97 calculated
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.
Common impurities: Fe2+
Environments
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).
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