Foitite

Chemical formula: &#₉₇₂₃;(Fe²⁺₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃OH

Foitite is a rare mineral from the tourmaline group, distinguished by its dark, blue-black color and the characteristic prismatic crystal habit of this group.

## Characteristics Foitite is a mineral belonging to the tourmaline group, characterized by a vacancy at the X-site in its crystal structure. Typical specimens form elongated, prismatic crystals with striated side faces, terminated by flat or pyramidal faces. Its color is usually very dark, ranging from indigo with a violet hue to blue-black, which makes it difficult to distinguish from other dark tourmalines, such as schorl. ## Physical Properties Foitite has a hardness of 7 on the Mohs scale, which is typical for tourmalines. The luster is vitreous, sometimes transitioning to greasy. This mineral is transparent to translucent, although its very dark coloration can give the impression of opacity in more massive fragments. It exhibits no cleavage, and its fracture is conchoidal or subconchoidal (similar to conchoidal, but less regular). ## Colors and Varieties The dominant and virtually only observed coloration of foitite is a very dark garnet, tinged with violet, to almost black with a bluish hue. No color or commercial varieties are distinguished. ## History and Name The mineral was formally described and recognized by the International Mineralogical Association (IMA) in 1993. Its name honors Franklin F. Foit, Jr. (1942–), an American mineralogist from Washington State University, for his contributions to research on the tourmaline group. ## Applications Due to its rarity and typically small crystal sizes, foitite has no industrial applications. Its significance is almost exclusively scientific and collectible, being sought after by specialists and advanced mineral collectors of the tourmaline group.

Properties

Mohs hardness: 7
Luster: Vitreous to oily.
Streak: Greyish-white
Density: 3.17
Cleavage: None.
Fracture: Conchoidal,Sub-Conchoidal
Transparency: Transparent,Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Identifying foitite in the field is very difficult and requires chemical analysis. In a collection, it can be preliminarily recognized by its typical tourmaline prismatic habit, striations along the main crystal axis, lack of cleavage, and characteristic, very dark blue-black color. A hardness of 7 on the Mohs scale is also an important clue. ## Distinguishing from Similar Minerals Foitite is visually almost impossible to distinguish from schorl (black tourmaline) and other dark tourmalines without advanced analysis. Schorl is much more common. Definitive differentiation requires chemical composition analysis, particularly determining the iron, aluminum, and sodium content, to confirm the vacancy at the X-site in the structure. ## Crystal Forms Foitite forms prismatic or acicular crystals, often gathered in radial or parallel aggregates. Crystals are usually strongly striated along the vertical axis, and in cross-section, they may have a rounded, triangular outline.

Geological environment

## Genesis Foitite forms in lithium- and boron-rich granitic pegmatites, as well as in some metamorphic rocks. Its formation is associated with specific chemical conditions where there is a deficiency of alkalis (especially sodium), which favors the formation of a vacancy in the tourmaline structure. ## Mineral Associations This mineral often co-occurs with other tourmalines (such as elbaite, schorl, liddicoatite), as well as with quartz, feldspars (albite, microcline), lepidolite, and other pegmatitic minerals. ## Localities The type locality (the place of first finding and description) is the Himalaya Mine in San Diego County, California, USA. Other known occurrences include pegmatites in the Czech Republic, Italy, Brazil, and Madagascar. However, it is a rare mineral everywhere.

Rarity

Rare

For collectors

## Quality Criteria The most valued by collectors are well-formed, terminated crystals with a distinct prismatic habit and visible striations. Specimens exhibiting partial transparency and an intense, bluish or violet color (even if very dark) are more desirable than those completely black and opaque. The aesthetic composition with other minerals, such as quartz or albite, is also important. ## Popular Localities Specimens from the type locality in California (Himalaya Mine) are of particular value to collectors specializing in minerals from type localities. High-quality crystals also come from some pegmatites in Brazil and the Czech Republic.

Care and storage

## Cleaning Foitite specimens can be safely cleaned using a soft brush and lukewarm water, possibly with a mild soap. After washing, they should be thoroughly rinsed with distilled water to avoid leaving residue. ## What to Avoid Contact with strong acids and bases should be avoided. Despite its relatively high hardness, the mineral is brittle and sensitive to impacts and sudden temperature changes, which can cause cracks. Prolonged exposure to intense sunlight is not recommended. ## Storage Specimens should be stored in individual boxes or on soft pads to prevent scratching by harder minerals or damage to more delicate specimens. Protect from dust and impacts.