Magnesio-foitite

Chemical formula: &#₉₇₄₄;(Mg₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃OH

Magnesiofoit is a rare mineral from the tourmaline group, characterized by the dominance of magnesium and a vacancy at the X site, forming acicular or radial crystals.

## Characteristics Magnesiofoit is a mineral belonging to the tourmaline supergroup. It forms very small, acicular or hair-like crystals, often gathered in radial or chaotic aggregates. Individual crystals rarely exceed 1 mm in length. Due to its habit, it can be mistaken for other tourmalines of similar form, and its precise identification requires advanced analytical methods. ## Physical Properties The mineral is characterized by a hardness of 7 on the Mohs scale, typical for tourmalines. It has a vitreous luster. It is brittle and exhibits an uneven, conchoidal fracture. Its density is approximately 3.03 g/cm³. ## Colors and Varieties The observed colors of magnesiofoit are most often dark blue, bluish-black, or black. It exhibits strong pleochroism, changing color from almost colorless or light blue to dark blue depending on the observation direction. ## History and Name The mineral's name refers to its chemical composition – the dominance of magnesium (magnesio) and its relation to foitite, another mineral from the tourmaline group. It was formally described as a new mineral in 1999 by a research team led by Frank C. Hawthorne. The type locality (locus typicus) is Kyonosawa, Gunma Prefecture, Japan.

Properties

Mohs hardness: 7
Luster: Vitreous
Streak: White
Density: 3.03
Cleavage: None
Fracture: Uneven, Conchoidal
Transparency: Transparent to translucent
Crystal system: Trigonal

Diagnostic features

## Identification Identification of magnesiofoit under collector conditions is practically impossible and requires specialized studies, such as chemical analysis (EDS/WDS) and X-ray diffraction (XRD). It can be preliminarily classified as a tourmaline based on its hardness, luster, and crystal form (trigonal, prismatic with characteristic striations). ## Distinguishing from Similar Minerals It can be mistaken for other acicular tourmalines, such as schorl, elbaite, or foitite. Final differentiation is only possible based on precise chemical analysis, which will show the dominance of magnesium at the Y site and a vacancy at the X site. ## Crystal Forms It forms slender, prismatic to acicular crystals, often terminated by pyramids. Crystals are strongly striated along the main axis. It occurs in the form of radial aggregates, rosettes, and also as inclusions in other minerals, e.g., in quartz.

Geological environment

## Genesis It is a mineral of hydrothermal origin, associated with low-grade metamorphic processes. It occurs in altered sedimentary rocks, rich in magnesium and boron, as well as in quartz veins cutting these rocks. ## Mineral Associations It most often co-occurs with quartz, albite, chlorite, muscovite, dravite, and other tourmalines. ## Localities The most important localities worldwide include Kyonosawa in Japan (type locality), as well as some locations in the Czech Republic (e.g., Kutná Hora), Austria (Styria), Italy (Elba), and the USA (California). However, these occurrences are primarily of scientific importance.

Rarity

Very rare

For collectors

## Quality Criteria As a rare and microscopic mineral, magnesiofoit is not a typical collector's item for hobbyists. In specialized and scientific collections, well-formed specimens that create distinct aggregates on a contrasting matrix (e.g., on white quartz or albite) are particularly valued. Precise documentation and confirmation of analytical identification are crucial. ## Popular Localities The most sought-after specimens, although still microscopic, come from the type locality in Japan and from several localities in Europe, where they formed slightly larger, well-defined crystals.

Care and storage

## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to their fragility and small crystal size, mechanical cleaning, including brushes and ultrasonic cleaners, should be avoided. ## What to Avoid Avoid contact with chemicals, especially strong acids and bases. The mineral is stable under normal conditions, but it should be protected from sudden temperature changes and mechanical damage. ## Storage It is recommended to store specimens in closed display boxes or mineral cabinets to protect them from dust and damage. Delicate, acicular aggregates are very prone to crumbling.

Sources

IMA Mineral List (CNMNC)