Fluoro-leakeite
Chemical formula: NaNa<sub>2</sub>(Mg<sub>2</sub>Al<sub>2</sub>Li)Si<sub>8</sub>O<sub>22</sub>F<sub>2</sub>
A very rare mineral from the amphibole group, characterized by a dark green to black color and occurrence in alkaline pegmatites.
Description
## Characteristics Fluoro-leakeite is a silicate belonging to the sodium amphibole group. It forms elongated, prismatic crystals, often with longitudinal striations on the faces. It usually occurs as needles embedded in rock or as granular aggregates. Its color is most often dark green, blue-green to almost black, which makes it difficult to distinguish from other dark amphiboles at first glance. ## Physical Properties This mineral is characterized by a hardness of about 6 on the Mohs scale and a vitreous luster. It is translucent in thin fragments, and larger crystals or aggregates are opaque. It has typical amphibole, bidirectional cleavage, intersecting at angles of approximately 56° and 124°. Its density is about 3.16 g/cm³. ## Colors and Varieties The dominant color is dark green, often with a bluish tint, transitioning to black. No named color varieties or commercial varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of fluorine (Fluoro-) and to the surname of Professor Bernard Elgey Leake (1932-2018), a British geologist and expert in amphiboles. The mineral was officially recognized by the International Mineralogical Association (IMA) in 2009. The type locality (locus typicus) is Zagi Mountain in Pakistan. ## Applications Due to its extreme rarity, fluoro-leakeite has no industrial applications. It is solely an object of scientific interest and a valued acquisition for advanced systematic and regional mineral collections.
Diagnostic features
## Identification Field identification of fluoro-leakeite is very difficult. A key indicator is its environment of occurrence – alkaline pegmatites. Dark color, prismatic habit, and vitreous luster are characteristic features, but common to many amphiboles. Certain identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS), to confirm its unique chemical composition with high sodium, lithium, and fluorine content. ## Distinguishing from similar minerals Fluoro-leakeite can be easily confused with other dark sodium amphiboles, such as arfvedsonite or riebeckite, which often occur in the same environment. Distinguishing it from them is almost exclusively possible based on chemical analysis. It is distinguished from black tourmaline (schorl) by the presence of distinct, bidirectional cleavage, which tourmaline does not possess. ## Crystal Forms It most often forms well-developed, elongated prismatic crystals. It also occurs in the form of radial aggregates, subparallel aggregates, or as grains embedded in rock.
Geological environment
## Genesis Fluoro-leakeite is an igneous mineral, crystallizing in the late stages of alkaline magma evolution, under conditions of high concentration of sodium, lithium, and volatile elements, especially fluorine. Its formation is closely related to peralkaline (alkali-oversaturated) pegmatites and syenites. ## Mineral Associations This mineral coexists with other minerals typical of alkaline pegmatites. In the type locality in Pakistan, these include aegirine, arfvedsonite, riebeckite, quartz, microcline, and rare earth element minerals such as bastnäsite-(Ce). ## Localities The most important and well-known worldwide occurrence of fluoro-leakeite is the Zagi Mountain massif, Hameed Abad, Khyber Pakhtunkhwa, in Pakistan. This is the type locality, from which the best specimens originate. It has also been reported in the Norra Kärr alkaline complex in Sweden.
Rarity
Very rare
Collector aspects
## Quality Criteria The collector's value of fluoro-leakeite specimens primarily depends on the quality and size of the crystals. Well-formed, sharply terminated, single crystals with an intense, dark green color and strong luster are most highly prized. Specimens where fluoro-leakeite contrasts with a light rock matrix (e.g., with quartz or microcline) or is associated with other rare minerals also have high value. ## Popular Localities The vast majority of specimens available on the collector's market come from a single location – Zagi Mountain in Pakistan. This locality is famous for providing the world's best crystals of this mineral.
Care and storage
## Cleaning Specimens should only be cleaned mechanically, using a soft brush. In case of heavier soiling, distilled water can be used. Due to its excellent cleavage, ultrasonic cleaners should be avoided, as they can cause crystals to crack along cleavage planes. ## What to avoid The mineral is sensitive to strong acids. Avoid sudden temperature changes. Although not particularly light-sensitive, prolonged exposure to direct sunlight is not recommended for any collector's mineral. ## Storage It is recommended to store specimens in closed boxes or display cases to protect them from dust and mechanical damage. Due to its cleavage, avoid placing it in direct contact with harder minerals.