Oxy-foitite

Chemical formula: &#₉₇₄₄;(Fe₂+Al₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O

A rare mineral from the tourmaline group, an oxygen analog of foitite, forming black, prismatic crystals.

## Characteristics Oxy-foitite is a rare member of the tourmaline supergroup, chemically defined as the oxygen analogue of foitite. It forms elongated, prismatic crystals with a characteristic triangular or rounded cross-section typical of tourmalines and longitudinal striations on the faces. It usually occurs as opaque, black or very dark brown crystals, which are visually indistinguishable from the more common schorl. ## Physical Properties This mineral is characterized by a significant hardness of 7 on the Mohs scale, which is typical for tourmalines. It has a vitreous luster on crystal faces and fracture surfaces. The density is approximately 3.18 g/cm³. It is a brittle mineral, does not exhibit cleavage, and its fracture is conchoidal or uneven. It is usually opaque, only in very thin fragments can it be translucent. ## Colors and Varieties The dominant and essentially only color found in oxy-foitite is black. Specimens with a deep brown or purplish-black hue are also found. As a mineral defined by its chemical composition rather than color, it does not form named color varieties. ## History and Name The name of the mineral is two-part: the "Oxy-" prefix indicates the dominance of oxygen (O²⁻) in a specific position in the crystal structure, and the "-foitite" suffix refers to its close relationship with the mineral foitite. Foitite itself was named after the American mineralogist Franklin F. Foit, Jr. (1942-2017). Oxy-foitite was officially recognized as a distinct mineral species by the International Mineralogical Association (IMA) in 2016. ## Applications Due to its rarity and the need for advanced analysis for identification, oxy-foitite has no industrial applications. It is solely an object of scientific interest and is sought after by specialized collectors of rare and systematic minerals.

Properties

Mohs hardness: 7
Luster: Vitreous
Streak: White
Density: 3.18
Cleavage: None
Fracture: Conchoidal to uneven
Transparency: Translucent to Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Identification of oxy-foitite based on visual characteristics is impossible. Its appearance is identical to that of black schorl, foitite, or dravite. The only reliable method of identification is advanced chemical analysis (e.g., electron microprobe analysis - EMPA) combined with structural studies (e.g., X-ray diffraction - XRD), which can confirm the dominance of oxygen in the appropriate structural position and a vacancy at the X site. ## Distinguishing from similar minerals It is most often confused with other black tourmalines: - **Schorl**: The most common black tourmaline. It differs chemically – it has sodium (Na) at the X site, whereas oxy-foitite mostly has a vacancy (empty space) there. - **Foitite**: It is its direct analogue. The difference is that in foitite, the hydroxyl group (OH) dominates at the anionic site, while in oxy-foitite, oxygen (O) dominates. - **Dravite**: A magnesium tourmaline, which can also be black. It differs by the dominance of magnesium over iron. ## Crystal forms Crystals are typical for tourmalines: columnar, prismatic, often with vertical striations on the faces. The cross-section of the crystal is characteristically triangular, often with rounded corners. It also occurs in radial aggregates or as granular masses embedded in rock.

Geological environment

## Genesis Oxy-foitite is a mineral associated with granitic pegmatite environments, especially those characterized by low lithium and sodium content. It forms as a primary mineral during magmatic crystallization processes under specific geochemical conditions. ## Mineral associations It most often co-occurs with minerals typical of granitic pegmatites, such as quartz, albite (and other plagioclase feldspars), and muscovite. ## Localities The most important locality, which is the type locality, is Zlatá Idka in the Košice District, Slovakia. Other confirmed occurrences include pegmatites in the Mirošov area in the Czech Republic and in Henderson County, North Carolina, USA.

Rarity

Rare

For collectors

## Quality criteria The most prized by collectors are specimens featuring well-formed, terminated, lustrous crystals with a distinct prismatic habit and a typical tourmaline cross-section. Specimens from the type locality (Slovakia) are particularly desirable. The value of a specimen is enhanced by its analytically confirmed identity, which is crucial for this mineral. Specimens on a rock matrix (e.g., with quartz or albite) are also valued higher than single, loose crystals. ## Popular localities For specialized collectors, specimens from the type locality – Zlatá Idka in Slovakia – are most important, as they hold the greatest scientific and historical significance. Specimens from other confirmed localities, such as the Czech Republic or the USA, are also sought after, provided their identity has been laboratory verified.

Care and storage

## Cleaning Oxy-foitite specimens can be safely cleaned with a soft brush under running lukewarm water. For heavier soiling, a small amount of mild soap may be used, followed by thorough rinsing, preferably with distilled water. Avoid strong detergents. ## What to avoid The mineral is relatively resistant, but ultrasonic cleaners should be avoided, as they can enlarge internal fractures or damage the brittle crystal. It should be protected from contact with strong acids, especially hydrofluoric acid. It is resistant to sunlight and normal temperature changes, but thermal shock (sudden, extreme changes) should be avoided. ## Storage Due to its hardness of 7, it is resistant to scratching by most common minerals (e.g., quartz). Nevertheless, it is recommended to store it in a way that prevents friction with harder materials (such as corundum or diamond). It is best to store it in a separate display box or on a stand, away from dust.

Sources

IMA Mineral List (CNMNC)