Fluor-tsilaisite

Chemical formula: NaMn₂+₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃F

Fluor-tsilaisite is a rare mineral from the tourmaline group, characterized by the dominance of fluorine and manganese in its composition.

## Characteristics Fluor-tsilaisite is a mineral belonging to the tourmaline group. It has been defined as a tourmaline in which sodium dominates at the X site, manganese at the Y site, and fluorine at the W site. Its appearance is typical for tourmalines, forming prismatic crystals, often with a characteristic triangular cross-section and rounded side faces. Specimens are usually small. ## Physical Properties This mineral is characterized by a vitreous luster. It is brittle, and its fracture is uneven to conchoidal. Its Mohs hardness is approximately 7, which is typical for minerals of the tourmaline group. ## Colors and Varieties The color of fluor-tsilaisite ranges from yellowish-brown to dark brown, almost black. Variations in coloration result from slight differences in chemical composition, especially iron content. ## History and Name The name "fluor-tsilaisite" refers to its chemical composition – the dominance of fluorine (Fluor) and the historical name "tsilaisite," introduced in 1914 by A. E. Fersman for a manganese variety of tourmaline from the Tsilaisina locality in Madagascar. The mineral was formally approved as a distinct species by the International Mineralogical Association (IMA) in 2011 (IMA2011-038). The first description comes from material collected in Nová Ves, Czech Republic. ## Uses Due to its rarity and generally small crystal sizes, fluor-tsilaisite has no industrial applications. It is exclusively an object of interest for collectors specializing in mineral systematics or the tourmaline group.

Properties

Mohs hardness: 7
Luster: Vitreous
Streak: White
Density: 3.14
Cleavage: None
Fracture: Uneven to Conchoidal
Transparency: Translucent to Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Identification of fluor-tsilaisite based on visual characteristics is practically impossible and requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to confirm the dominance of sodium, manganese, and fluorine at the appropriate structural sites. In a collection, it can be preliminarily identified based on the typical tourmaline crystal habit and confirmed locality. ## Distinguishing from Similar Minerals Fluor-tsilaisite is indistinguishable to the naked eye from other dark, brown, or black tourmalines, such as schorl, dravite, or other manganese varieties. All of them have similar hardness, luster, and crystal form. Only chemical analysis provides certain differentiation. ## Crystal Forms Fluor-tsilaisite crystals are typically prismatic, elongated along the crystallographic c-axis. They have a characteristic rounded triangular cross-section typical of tourmalines. They occur as single, poorly formed crystals or in radial aggregates.

Geological environment

## Genesis Fluor-tsilaisite is a mineral of metamorphic origin. It forms in manganese- and boron-rich rocks that have undergone transformation under high pressure and temperature conditions. In the type locality (Nová Ves, Czech Republic), it occurs in quartzites containing spessartine and rhodochrosite. ## Mineral Associations This mineral coexists with other manganese minerals. In the Czech Republic, these include spessartine, rhodochrosite, quartz, and other tourmalines from the schorl-dravite series. At the locality in Sweden (Stora Vika), it is associated with tephroite, alleghanyite, galaxite, magnetite, and johannsenite. ## Localities Confirmed occurrences of fluor-tsilaisite are very few. The most important ones include: - **Nová Ves near České Budějovice, Czech Republic** - type locality (locus typicus). - **Stora Vika Mine, Nynäshamn, Sweden** - occurs in manganese skarns. - **San Piero in Campo Mine, Elba, Italy** - findings in pegmatites.

Rarity

Very rare

For collectors

## Quality Criteria The collector appeal of fluor-tsilaisite is low for general collectors but high for specialists. Since specimens are usually visually unimpressive (small, dark crystals), their value lies in their rarity and analytically confirmed identity. Specimens with well-formed, even if small, crystals, originating from the type locality (Nová Ves) and accompanied by a chemical analysis certificate, are most valued. ## Popular Localities The most sought-after by specialist collectors are specimens from the type locality in the Czech Republic. Material from the Stora Vika mine in Sweden is also significant due to its different geological environment (skarns).

Care and storage

## Cleaning It is recommended to clean specimens only with a soft, dry brush to remove dust. The use of water, especially with detergents, is not advisable due to the risk of damaging delicate aggregates and potential reaction with associated minerals. ## What to Avoid Avoid contact with chemicals, acids, and bases. The mineral is brittle, so it must be protected from impacts, falls, and sudden temperature changes. Do not clean specimens in ultrasonic cleaners. ## Storage Fluor-tsilaisite specimens are best stored in separate, padded collector boxes to prevent abrasion and mechanical damage. Protect from dust and moisture. Due to the small size of the crystals, storage in "micromount" type boxes is recommended.

Sources

IMA Mineral List (CNMNC)