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Formula: ☐(Fe2+2Al)Al6(Si6O18)(BO3)3(OH)3(OH)
Cyclosilicate (ring silicate), tourmaline group,
boron-bearing mineral
Crystal System: Trigonal
Specific gravity: 3.17 measured, 3.14 calculated
Hardness: 7
Streak: Greyish white
Colour: Dark indigo with purple tints to bluish black
Luminescence: Not fluorescent under UV
Common impurities: Mn,Mg,Ca,Na,Li,B,H2O
Environments
Volcanic igneous environments
Pegmatites
Hydrothermal environments
Localities
At the Cruzeiro mine, São José da Safira, Minas Gerais, Brazil, silver-grey
tourmaline fibres intergrown with a deep pink
elbaite host provide evidence for the compositional evolution of magmatic and
hydrothermal fluids involved in pegmatite formation. The
fibres, 0.05 to 0.3 mm in width, are complexly zoned and developed in four distinct generations marked by discrete
compositions and replacement textures. Fibre growth is punctuated by periods of dissolution.
The first generation, preserved in the fibre interior, is a dark blue
the blue-gray second generation varies from iron-rich
elbaite to lithium-rich
schorl;
the third generation is a yellowish-green fluor-elbaite.
Volumetrically the most abundant, generation-three fibres poikiloblastically (having small grains of one
mineral embedded in crystals of another mineral) replace the earlier generations as well as the host;
a fourth generation of fibrous tourmaline fills fractures that cut all
previous generations and the host, but is unrelated to growth of the previous fibres.
Compositionally, the last generation is indistinguishable from the second-generation
lithium-rich schorl fibres.
Textural and compositional discontinuities of each generation record periods of stability followed by reaction(s) in
which the tourmaline was initially unstable, partially dissolved owing to
interaction with fluids, and then redeveloped in response to interactions with evolving orthomagmatic or hydrothermal
fluids. The general progression of the first three generations implies that reacting fluids were generally undergoing
fractionation, becoming successively enriched in Na, Li, Ca, and F during late crystallisation of the
pegmatite. The composition was reset to a
lithium-rich schorl during late-phase
fracturing
(CM 38.131-143).
Foitite from the Cruzeiro Mine - Image
At the Koktokay No. 3 pegmatite, Altay Mine, Koktokay pegmatite field, Fuyun County, Aletai Prefecture, Yili Hasake
Autonomous Prefecture, Xinjiang, China, two species of X-site-vacant
tourmaline, foitite and
rossmanite, occur in the
spodumene-subtype
pegmatite.
Foitite–schorl crystals develop as fillings in the interstices among
iron-rich dravite crystals in the
endocontact zone between the pegmatite dyke and the
metagabbro country-rock. The evolution of the
tourmaline crystals in this zone occurred in two stages. The first stage is
typified by the formation of iron-rich
dravite with variable compositions. In the second-stage
foitite–schorl-dravite may
develop as a result of pegmatite-derived fluids reacting with
fluids from the country rock, whereas the formation of foitite–schorl is
mainly attributed to pegmatite-derived fluids, the occurrence
of foitite further reflecting the low Ca and Na concentrations in the fluids. In the
cleavelandite–spodumene zone,
rossmanite occurs as veinlets within the main mass of
elbaite crystals
(CM 42.3.873–882).
Foitite from Koktokay - Image
Rosina vein, San Piero in Campo, Campo nell'Elba, Livorno Province, Tuscany, Italy. At Elba Island a
monzogranitic pluton outcrops at the western end of the island.
There are numerous aplite
pegmatite dikes along its eastern margin, and these dikes
contain miarolitic cavities lined with euhedral crystals of several minerals.
At the Rosina vein there are many large pockets rich in accessory minerals, including
schorl, elbaite,
spessartine, blue and pink beryl,
petalite, pollucite and
(Nb,Ta)-oxide minerals. In the upper part of the aplitic zone of the dike,
there is a medium-grained phase rich in small miarolitic cavities lined with small crystals of
quartz and feldspars. Projecting into
these cavities are acicular and hairlike crystals of foitite. Associated with these needles and hairs is an
aggregate of platy material shown by SEM to be biotite, that seems to have
crystallised essentially contemporaneously with foitite. Other minerals associated with foitite in these
cavities are colourless and blue beryl,
cassiterite,
columbite-(Mn), euxenite,
ilmenite, zircon and
zeolites (mordenite and altered
laumontite). In some of the cavities" the foitite needles can be
observed growing directly on and in crystallographic continuity with coarsely crystalline
tourmaline. Large terminated crystals of
tourmaline also project into the cavities, and both habits of
tourmaline can be present in the same cavity. Fibrous foitite only
develops as an overgrowth on the analogous end of the crystals, and the antilogous end consists of coarsely
crystalline schorl-elbaite
(CM 34.74l-7M).
Foitite from the Rosina Vein - Image
At the Asarcık deposit, Şebinkarahisar District, Giresun Province, Turkey, the plutonic rocks that outcrop range from
alkali granite to diorite,
but syenite,
quartz syenite,
monzonite,
quartz monzonite,
rhyolite, rhyodacite
and trachyandesite are dominant.
These rocks were separated into two groups, the lower and upper series. The upper series is composed mainly of
dacite, andesite,
basalt and related tuffs and
pyroclastic units. The lower series contains calc-alkaline rocks igneous rocks and interbedded sedimentary units. The
upper series and part of the lower series are characterised by
intensive formation of tourmaline, hydrothermal alteration and
Pb-Zn-Cu mineralisation.
Monzonite,
quartz monzonite and
quartz syenite are cut by later
quartz veins (1 to 15 cm) and
aplite dikes up to 5 cm in thickness. These
quartz veins contain cavities up to 2 cm across and about 1 cm in depth. Black
schorl, about 0.5 cm thick, rims these cavities. The light grey acicular and
hairlike fibrous tourmaline crystals project inward from the
schorl rim.
Accessory minerals in the foitite-bearing quartz veins are
K-feldspar, chrysoprase and
hematite. Hematite is
characteristic of an oxidised environment; its presence may suggest Fe3+ in the structure of the coexisting
tourmaline
(CM 37.155-161).
From the type locality, California, USA, the type material was found as museum specimens designated only as from
southern California. The foitite specimens were there as single crystals with no matrix.
Tourmaline is a common constituent of a large number of
granitic
pegmatites in southern California, and it seems reasonable to
conclude that foitite was originally collected from such an environment. The composition of foitite would
support this conclusion, as alkali depletion is characteristic of some
pegmatitic environments
(AM 78.1299-1303).
Foitite from California - Image
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, foitite occurs where
schorl has been partially replaced or dissolved, then a late overgrowth of
foitite fibres has formed. The Emmons pegmatite is an
example of a highly evolved boron -
lithium - cesium -
tantalum enriched
pegmatite
(R&M 94.6.508).
Foitite from the Emmons Quarry -
Image
At the Tungsten Mine, Copper Mountain, Picuris District, Taos County, New Mexico, USA, metamorphic assemblages are in
the amphibolite facies:
kyanite, andalusite and
sillimanite coexist; schists
contain muscovite, biotite,
garnet and staurolite.
Foitite occurs on the western slope of Copper Mountain. The mine is associated with a steeply dipping
quartz vein. Foitite was observed as veinlets, 1 to 2 cm wide, in
quartz with muscovite,
scheelite and wolframite, as
well as in minor films and patches of green secondary copper minerals
(CM 37.6.1431-1438).
Foitite from Copper Mountain - Image
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