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Formula: Fe2+3B7O13Cl
Anhydrous borate containing halogen,
boracite group, trigonal paramorph
of orthorhombic ericaite
Crystal System: Trigonal
Specific gravity: 3.58 measured, 3.52 calculated
Hardness: 6½ to 7½
Colour: Pale red, pink
Environments
Localities
At the Penobsquis and Millstream deposits, Kings county, New Brunswick, Canada, congolite occurs as zones within
trembathite crystals in a marine evaporite deposit. Although it occurs at both
deposits, it is much more common at Penobsquis. It is found as perfect pseudocubes a few mm across and is commonly
colourless to grey or mauve. In the Penobsquis deposit, congolite is associated with
volkovskite, hilgardite,
trembathite, hydroboracite,
danburite and strontioginorite. At Millstream, congolite is associated with hilgardite,
strontioginorite, anhydrite
and magnesite.
(CM 43.1469-1487).
At the type locality, Kouilou Department, Republic of the Congo, congolite occurs as part of insoluble residue from
a drill core into sedimentary evaporites
(HOM).
At the Kłodawa salt deposit, Gmina Kłodawa, Koło County, Greater Poland Voivodeship, Poland, pseudocubic crystals of
borate, ≤ 1.2 mm in size and yellowish through pale-violet to pale-violetbrown and brownish in colour, were found in the
salt dome. The crystals occur in halite units bordering the
anhydrite pegmatite
unit. The internal texture of the crystals reflects phase-transitions from rhombohedral congolite through
orthorhombic ericaite to a cubic, high-temperature, mainly Fe-dominant
analogue of boracite, indicating formation of the phases under increasing
temperature. The phase-transition from congolite to ericaite occurred at
230 to 250oC, and the transition of ericaite to the high temperature
analogue of boracite occurred at 310 to 315oC. These phase-transitions
have been forced by inflows of Mg-bearing fluids sourced from relatively thick loads of potash salts containing inclusions
of carnallite, kainite and
kieserite. The brines and fluids have utilised cracks in
pegmatite anhydrite as
convenient pathways for migration in the dome, in contrast to the host rock-salts, which are too solid to be penetrated by
the fluids for distances greater than a few metres
(CM 34.881-892).
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