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Formula: Ba(Al2Si6)O16.5H2O
Phyllosilicate (sheet silicate), serpentine subgroup,
kaolinite-serpentine group,
barium-bearing mineral
Crystal system: Monoclinic
Specific gravity: 3.17 measured, 3.16 calculated
Hardness: 2½ to 3
Colour: Dark yellow-green
Environments
Sedimentary environments
Hydrothermal environments
Localities
At the type locality Marmara bauxite deposit, Megara, West Attica, Attica, Greece, brindleyite occurs as
green coatings a few millimetres thick, with sporadic thickening up to 12 mm, on the footwall
limestone. It has a
clayey appearance, but often is compact with a pronounced schistosity
and lustrous surface. Brindleyite also forms thin veinlets and irregular accumulations in the red
kaolinitic clay above the
footwall limestone.
The mineralogical composition of the Marmara bauxite layer is:
böhmite 60 percent, hematite 30
percent, kaolinite 6 percent,
anatase 2.4 percent and chromite
1.2 percent. In the lowermost part of the deposit near the footwall
limestone, kaolinite is
the most abundant mineral, hematite occurs in very reduced amounts, and
böhmite is present only in traces. Other minerals also appear, forming an
unusual association: brindleyite, bastnäsite,
malachite, bayerite and
calcite.
There is a progressive downward enrichment of all lanthanide elements
in the deposit, with maximum concentrations of about 6400 ppm total
lanthanides in brindleyite. The same concentration trend of
downward enrichment was observed for Ni, Cu, Co, Mn, and Y. This enrichment is considered to be contemporaneous with
the removal of silica during the bauxitisation process in a
karst environment.
The enrichment of nickel and the formation of brindleyite were the
result of several factors. The high initial nickel content of the
clay material washed down into the
karstic depression resulted from the weathering of
ultramafic rocks. The strong downward migration of
nickel was caused by percolating waters in a highly drained
karst environment. Finally, the footwall
limestone acted as an effective alkaline barrier for descending
solutions and caused the precipitation and concentration of nickel,
rare earth elements and copper,
and the formation of brindleyite, bastnäsite
and malachite
(AM 63.484-489).
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