Foitite
□(Fe2+2Al)Al6(Si6O18)(BO3)3(OH)3(OH)
A rare alkali-deficient tourmaline whose X crystal site is largely vacant, giving slender dark blue to bluish-black crystals.
- Mohs hardness
- 7-7.5
- Color
- Bluish-black to dark blue
- Type
- mineral
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Overview
Foitite is a member of the tourmaline supergroup distinguished by having its X structural site predominantly vacant rather than filled with sodium or calcium. This makes it one of the "X-site vacant" tourmalines. It was named in 1993 for American crystallographer Franklin F. Foit Jr.
Its ferrous-iron and aluminum chemistry produces a dark blue to bluish-black color. Crystals are usually small, slender, and often occur as overgrowths or late-stage needles on or within earlier-formed tourmalines and quartz.
Foitite is mainly a specimen and research mineral that illustrates how tourmaline can crystallize from alkali-poor fluids.
Formation & geology
Foitite forms from boron-rich but alkali-poor fluids, typically in the late stages of granitic pegmatite or hydrothermal activity. Because the available sodium and calcium are depleted, the X site is left largely empty, charge-balanced by adjustments in the iron and aluminum sites.
It commonly appears as thin blue needles and overgrowths on schorl or elbaite, or as fine sprays within quartz. The type locality is in San Diego County, California, with other occurrences reported in pegmatite districts worldwide.
Its presence signals evolved, alkali-exhausted residual fluids in a pegmatite system.
How to identify it
Foitite shows as dark blue to bluish-black slender prisms or needles, often as late overgrowths, with the striated prism faces and triangular cross-section of tourmaline. Hardness is 7-7.5, with no cleavage and a vitreous luster.
It is very difficult to distinguish visually from schorl and dark indicolite; confirmation requires microprobe analysis showing X-site vacancy. A clue is its habit as thin blue needles in or on quartz and earlier tourmaline.
Look-alikes include blue indicolite (sodium-rich), schorl (sodium-rich, blacker), and dumortierite (which is fibrous and softer in mats).
Uses & significance
Foitite has no commercial or industrial application. Its value lies in mineralogy: it documents the X-site-vacant branch of the tourmaline supergroup and the conditions under which alkali-deficient tourmalines crystallize.
Collectors seek attractive quartz specimens hosting blue foitite needles. It is not used as a gemstone because crystals are typically too small and dark to facet.
Frequently asked questions
Why is foitite called "X-site vacant"?
Its X structural site, normally occupied by sodium or calcium, is mostly empty (vacant), which defines the species.
Who is foitite named after?
It honors Franklin F. Foit Jr., an American crystallographer who studied tourmaline chemistry.
What color is foitite?
It is dark blue to bluish-black, often appearing as slender needles or overgrowths on other tourmalines.
Can foitite be cut as a gem?
Generally no; its crystals are usually too small, dark, and thin to facet, so it remains a collector's mineral.
Foitite guides
In-depth guides for identifying, valuing, and understanding Foitite.
