Feruvite

Chemical formula: CaFe₂+₃(Al₅Mg)(Si₆O₁₈)(BO₃)₃(OH)₃OH

Feruvite is a rare, dark brown or black mineral from the tourmaline group, distinguished by its high iron and calcium content.

## Characteristics Feruvite is a mineral belonging to the tourmaline group, specifically to its calcium subgroup. It forms crystals with habits ranging from short prismatic to almost tabular, often with the triangular cross-section characteristic of tourmalines and rounded side faces. It usually occurs in the form of radial aggregates or as single, well-formed crystals. Its color is almost always very dark, from dark brown to black, which is a result of its high iron content. ## Physical Properties Feruvite is characterized by a hardness of 7 on the Mohs scale, which is typical for tourmalines. It has a vitreous luster, and its fracture is uneven to conchoidal. It is a relatively dense mineral, with a density in the range of 3.19 - 3.27 g/cm³. It is usually opaque; only in very thin fragments can it be translucent in shades of brown or green. ## Colors and Varieties The dominant and essentially only color found in feruvite is dark brown to black. No color or commercial varieties are distinguished. Variation in shade is minimal and related to slight fluctuations in chemical composition. ## History and Name The name "feruvite" refers to its chemical composition – its high iron content (Latin: *ferrum*) and its structural similarity to uvite, another mineral from the tourmaline group. The mineral was first described in 1989 by D.J. Grice and T.S. Ercit based on material from the type locality in Cuvier Township, Quebec, Canada. ## Uses Due to its rarity and usually unattractive, dark color, feruvite has no use in jewelry. It is solely an object of interest for collectors specializing in the tourmaline group or rare minerals.

Properties

Mohs hardness: 7
Luster: Vitreous
Streak: Light brown
Density: 3.19-3.27
Cleavage: None
Fracture: Uneven to conchoidal
Transparency: Opaque, translucent in thin splinters
Crystal system: Trigonal

Diagnostic features

## Identification The key diagnostic feature of feruvite is its characteristic tourmaline crystal habit – prismatic, with a rounded, triangular cross-section and vertical striations on the prism faces. The dark brown to black color combined with high hardness (7 on the Mohs scale) and vitreous luster are strong indicators. However, final confirmation requires advanced chemical analyses (EDS/WDS) to confirm the dominance of calcium and iron. ## Distinguishing from Similar Minerals Feruvite is visually indistinguishable from other black tourmalines, such as schorl (the most common), dravite, or uvite, without specialized examination. Schorl is a sodium tourmaline, dravite is a sodium-magnesium tourmaline, and uvite is a calcium-magnesium tourmaline. All can form identical-looking black crystals. They differ in density and optical parameters, but these differences are too small to be assessed under amateur conditions. ## Crystal Forms Feruvite most often forms short, prismatic crystals, often terminated by a flat trigonal pyramid. It also occurs in the form of radial or fibrous aggregates. Crystals often show vertical striations on the prism faces.

Geological environment

## Genesis Feruvite is a relatively rare mineral, formed under specific geochemical conditions. Its formation is associated with metamorphic and metasomatic processes in rocks rich in calcium and boron. It is most commonly found in skarns, which are rocks formed at the contact of a magmatic intrusion with carbonate sedimentary rocks (limestones, dolomites). ## Mineral Associations Minerals co-occurring with feruvite include other minerals typical of the skarn environment, such as diopside, vesuvianite, grossular (garnet), wollastonite, as well as other tourmalines (e.g., dravite, uvite), calcite, and titanite. ## Localities The most important and best-documented feruvite localities in the world include its type locality in Cuvier Township, Quebec (Canada). It is also known from Franklin, New Jersey (USA), where it occurs in famous metamorphic deposits, as well as from some localities in Italy and Russia. It is a rare mineral everywhere.

Rarity

Rare

For collectors

## Quality Criteria The most valued feruvite specimens by collectors are those with well-formed, sharp crystals with a strong luster. An ideal specimen should exhibit the typical tourmaline habit with a distinct triangular cross-section. The attractiveness is enhanced by the contrasting placement of black feruvite crystals on a light rock matrix, for example, on white calcite. Crystal size is also an important factor, although even small but perfectly formed crystals are sought after. ## Popular Localities Specimens from the type locality in Quebec, Canada, and from Franklin, New Jersey, USA, are the most classic and desired by specialists. Each of these localities provided material that was used for research and definition of this mineral, which gives them historical value.

Care and storage

## Cleaning Feruvite specimens can be safely cleaned with a soft brush and distilled water. The use of mild soap is permissible, but thorough rinsing of the mineral is essential. Ultrasonic cleaners can be used, but with caution, especially for specimens with inclusions or fractures. ## What to Avoid Avoid contact with strong acids, especially hydrofluoric acid, which can damage the crystal surface. Although it is a hard mineral, it should be protected from impacts and contact with harder minerals (e.g., diamond, corundum) to prevent scratches. ## Storage Feruvite is a stable mineral that does not require special storage conditions. It can be displayed at room temperature, away from dust. To prevent potential mechanical damage, collector specimens should be stored in separate boxes or on stands, avoiding mutual friction.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FTourmaline