Ferro-ferri-fluoro-leakeite

Chemical formula: NaNa₂(Fe₂+₂Fe₃+₂Li)Si₈O₂₂F₂

A rare sodium amphibole, distinguished by its complex chemical composition containing both ferrous and ferric iron, and fluorine.

## Characteristics Ferro-ferri-fluoro-leakeite is a mineral belonging to the sodium amphibole group. It forms prismatic crystals, usually elongated, reaching up to 1 mm in length. It occurs as fibrous aggregates or as single crystals embedded in rock. It is opaque and black in color. ## Physical Properties This mineral has a hardness of 5-6 on the Mohs scale. It exhibits a vitreous to submetallic luster. Its density is approximately 3.36 g/cm³. It shows perfect cleavage in two directions, typical for amphiboles. ## Colors and Varieties Ferro-ferri-fluoro-leakeite is an idiochromatic mineral, meaning its black color results directly from its chemical composition, specifically from the presence of iron ions (Fe²⁺ and Fe³⁺). No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name reflects its chemical composition: the "ferro" prefix indicates the dominance of divalent iron (Fe²⁺) over magnesium, "ferri" indicates the dominance of trivalent iron (Fe³⁺) over aluminum, and "fluoro" indicates the dominance of fluorine over the hydroxyl group (OH). The second part of the name, "leakeite", honors Bernard Elgey Leake (born 1932), a British geologist and retired professor at the University of Glasgow, for his contributions to amphibole research. The mineral was approved by the IMA in 2009. ## Uses Due to its extreme rarity, ferro-ferri-fluoro-leakeite has no industrial applications. It is solely an object of scientific interest and a collector's item for specialized collectors of rare minerals.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Gray
Density: 3.36
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying ferro-ferri-fluoro-leakeite based on visual characteristics is practically impossible. This mineral is black, opaque, and forms small, prismatic crystals. Identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to determine chemical composition and X-ray diffraction (XRD) to confirm crystal structure. ## Distinguishing from Similar Minerals It can be confused with many other black amphiboles (e.g., arfvedsonite, riebeckite) or pyroxenes (e.g., aegirine). All these minerals form similarly shaped, dark, prismatic crystals. Definitive differentiation is only possible through detailed laboratory analyses that confirm the unique combination of dominant iron (Fe²⁺ and Fe³⁺), lithium, and fluorine. ## Crystal Forms Crystals are prismatic, often elongated and acicular. They usually occur as inclusions in the host rock, sometimes forming fibrous or radial aggregates.

Geological environment

## Genesis Ferro-ferri-fluoro-leakeite forms under specific conditions of contact metamorphism. It develops in fenites – metasomatic rocks that form at the contact of alkaline intrusions (e.g., nepheline syenites) with quartz-rich sedimentary rocks, such as sandstones. ## Mineral Associations This mineral coexists with other minerals typical of alkaline environments and fenites. The most common associations include aegirine, nepheline, microcline, albite, quartz, fluorite, zircon, and other rare minerals like eudialyte or lorenzenite. ## Localities The only confirmed occurrence (type locality) of this mineral in the world is the Dara-i-Pioz massif in the Alai Mountains of Tajikistan. This is a famous locality for many rare alkaline minerals.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of a specimen primarily depends on its richness – the more visible, well-formed crystals on the rock matrix, the more valuable the specimen. Contrast with surrounding minerals (e.g., black needles on light albite or nepheline) and the presence of rare associated minerals are also important. The size of individual crystals rarely exceeds 1 mm, so any larger specimen is exceptional. ## Popular Localities The only source of specimens is the Dara-i-Pioz massif in Tajikistan. Specimens from this locality are the only authentic representatives of this mineral available on the collector's market.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. Due to the possible presence of inclusions in a delicate rock matrix, washing with water, and especially ultrasonics, should be avoided as it could damage the specimen. ## What to Avoid Avoid contact with acids and other chemicals that could react with the mineral. Do not heat specimens or expose them to sudden temperature changes. Prolonged exposure to strong sunlight is not recommended. ## Storage Ferro-ferri-fluoro-leakeite specimens, being rare and usually small, are best stored in sealed membrane boxes or mineralogical display cases, which protect them from dust and mechanical damage. They should be kept separate from harder minerals to prevent scratching.

Sources

IMA Mineral List (CNMNC)