Foitite

foitite

muscovite

scheelite

wolframite

Images

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. Foititeschorl 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 foititeschorl-dravite may develop as a result of pegmatite-derived fluids reacting with fluids from the country rock, whereas the formation of foititeschorl is mainly attributed to pegmatite-derived fluids, the occurrence of foitite further reflecting the low Ca and Na concentrations in the fluids. In the cleavelanditespodumene 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

Back to Minerals