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Formula: ☐(Mg2Al)Al6(Si6O18)(BO3)3(OH)3(OH)
Cyclosilicate, tourmaline group
Crystal System: Trigonal
Specific gravity: 2.995 calculated
Hardness: 7
Colour: Pale bluish grey
Environments
Pegmatites
Hydrothermal environments
Localities
At Key Lake Mine, Athabasca Basin, Saskatchewan, Canada,
oxy-schörl
formed in granitic
pegmatites from hydrothermal fluids after
the peak metamorphism but before deposition of the Athabasca
sandstones, whereas magnesio-foitite is a product of
later, low-temperature hydrothermal activity. Both oxy-schörl and
oxy-dravite are coarse-grained (from 500 μm up to 1 cm), whereas
magnesio-foitite occurs as radial aggregates of fine, prismatic crystals (<15 μm in width).
Magnesio-foitite crystallised together with sudoite,
illite, and
alunite supergroup light-rare-earth-element-rich
aluminum phosphate-sulphate minerals. In the ore zone, the assemblage occurs with
uraninite and is partially overprinted by late, remobilised
uraninite and sudoite.
Magnesio-foitite is likely contemporaneous with the main stage of
uranium mineralisation, likely replacing existing high-aluminium
phases, such as kaolin and
sudoite
(CM 54.3.661–679).
The McArthur River Mine, Athabasca Basin, Saskatchewan, Canada is in a
uranium district within highly altered metamorphic rocks. Alteration
assemblages include chlorite,
illite, kaolinite,
tourmaline and hematite.
This alteration includes at least three generations of magnesio-foitite with different boron isotopic
compositions, the earliest generation containing the heaviest isotopic signature. These results are consistent
with precipitation from low-temperature, NaCl- and CaCl2-rich brine derived from an isotopically heavy boron
source, such as evaporated seawater, that interacted with
tourmaline and silicates in the basement rocks and/or fluids
derived from depth
(Geochemistry: Exploration, Environment, Analysis 22.1 geochem2021-037).
At Rabbit Lake Mine, Athabasca Basin, Saskatchewan, Canada, magnesio-foitite occurs in a
uranium deposit associated with
epidote, hematite,
quartz and dravite
(HOM).
The Moldanubian Zone pegmatites, Vlastějovice, Kutná Hora District, Central Bohemian Region, Czech Republic,
are pegmatites with pockets lined with crystals of
smoky quartz, feldspars,
muscovite and black
tourmaline that commonly occur in
migmatised biotite
– sillimanite gneiss
and felsic
granulite.
The tourmaline crystals and their aggregates, up to 30 cm across,
from pockets, exhibit striking zoning.
Core: foitite – schorl –
magnesio-foitite
Intermediate zone: schorl –
dravite
Narrow outer rim: schorl –
dravite
Tourmaline from
muscovite-rich veins is relatively homogeneous:
dravite – magnesio-foitite
(CM 50.4.895–912).
At the type locality, Kyonosawa, Yamanashi City, Yamanashi Prefecture, Japan, there are dark-coloured
andesitic tuff
breccia and volcanic
breccia of Pliocene age (5.3 to 2.6 million years ago). The
rocks contain rounded to angular fragments, up to 1 m in size, of
shale, andesite
and granite.
Along the Okusenjo-Sekisuiji shear zone there are acidic hot springs (one has pH of 1.7), fumaroles and
acid-alteration areas. The Kyonosawa area is situated along the shear zone where there is strong acid
hydrothermal alteration, producing dumortierite,
alunite, pyrophyllite,
kaolinite, magnesio-foitite,
topaz, zunyite,
pyrite and rutile.
The rocks containing magnesio-foitite or associated with it are highly altered and
andesitic to
dacitic. They are
goethite-stained crystalline altered
porphyritic rocks composed of
quartz phenocrysts and
quartz-rich groundmass, disseminated light grey to medium grey spots
and stringers of magnesio-foitite, dumortierite and
white spots of kaolinite; the maximum dimension of the stringers
is a few centimeters.
Minute needles of magnesio-foitite form radial aggregates up to a millimeter across, or felty
aggregates of similar dimensions composed of irregularly oriented extremely fine needles. Some larger prisms
in the radial aggregates are deeper grey-green owing to iron content. In many radial aggregates,
pyrite grains occur at the core. Some felted aggregates of
magnesio-foitite involve minute clusters of rutile crystals,
which are absent in the radial aggregates; these dusty-looking aggregates of
rutile occur along boundaries between magnesio-foitite and
quartz.
Magnesio-foitite is pale bluish grey and occurs as felted masses of fibrous crystals with a matte
surface; individual crystals average around 5 microns wide and 50 microns long, with the largest approaching
15 microns wide and 1 mm long
(CM 37.1439-1443).
At Magnet hill, Tisovec, Rimavská Sobota District, Banská Bystrica Region, Slovakia, distinctly chemically
zoned tourmaline was found in a
quartz vein. The
tourmaline forms radial aggregates of light grey to green thin
prismatic to acicular crystals growing in cracks in the host rock. The root zone of the aggregates has
dravitic to oxy-dravitic
compositions, shifting to magnesio-foitite in the middle parts of the crystals and to
foititic compositions in the outer parts of the aggregates
(CM 53.2.221–234).
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