Magnesio-foitite

magnesio-foitite

sudoite

uraninite

dumortierite

Images

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 biotitesillimanite gneiss and felsic granulite. The tourmaline crystals and their aggregates, up to 30 cm across, from pockets, exhibit striking zoning.
Core: foititeschorlmagnesio-foitite
Intermediate zone: schorldravite
Narrow outer rim: schorldravite
Tourmaline from muscovite-rich veins is relatively homogeneous: dravitemagnesio-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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