Fluornatromicrolite

Chemical formula: (Na₁.₅Bi₃+₀.₅)Ta₅+₂O₆F

An extremely rare mineral from the pyrochlore supergroup, an oxide of tantalum, sodium, and bismuth, found in granite pegmatites.

## Characteristics Fluornatromicrolite is a very rare mineral belonging to the pyrochlore supergroup. It forms small, most often octahedral crystals, which may exhibit additional faces modifying their basic shape. It also occurs as irregular, anhedral grains embedded in the rock. Its color ranges from yellowish-brown to reddish-brown. It is characterized by high density and strong luster. ## Physical Properties This mineral is relatively hard, with a Mohs hardness of 5. It exhibits a resinous to adamantine luster and a high density of approximately 7.18 g/cm³. It is translucent to opaque. It shows no cleavage, and its fracture is conchoidal or subconchoidal. ## Colors and Varieties Fluornatromicrolite occurs in shades of brown – from yellowish to reddish. No distinct color varieties or commercial forms have been identified. ## History and Name The mineral's name refers to its chemical composition – the presence of fluorine (Fluor), sodium (Natro), and its belonging to the microlite group. It was officially recognized by the International Mineralogical Association (IMA) in 2011. It was first described in 2013 by M. B. Andrade and co-authors based on specimens found in the Quixaba pegmatite in Brazil. ## Uses Due to its extreme rarity, fluornatromicrolite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for specialized collectors of rare minerals.

Properties

Mohs hardness: 5
Luster: Resinous to adamantine
Streak: Pale yellowish brown
Density: 7.18
Cleavage: None
Fracture: Conchoidal to subconchoidal
Transparency: Translucent to opaque
Crystal system: Cubic

Diagnostic features

## Identification Helpful features for identifying fluornatromicrolite include its high density, resinous or adamantine luster, brown color, and form of small, octahedral crystals. A key indicator is also its occurrence environment – complex granite pegmatites. ## Distinguishing from Similar Minerals Macroscopic differentiation of fluornatromicrolite from other minerals of the microlite or pyrochlore group is practically impossible without advanced chemical analyses (e.g., EDS). It can be confused with other heavy, brown pegmatitic minerals, such as zircon, cassiterite, or some garnets. It differs from zircon by its lower hardness and usually different crystal habit. ## Crystal Forms It most often forms well-developed, isometric crystals with an octahedral habit, sometimes with additional faces. It is also found as irregular grains embedded in other minerals.

Geological environment

## Genesis Fluornatromicrolite is a primary mineral crystallizing in the late stages of the evolution of complex granite pegmatites, especially those enriched in rare elements such as lithium, tantalum, and bismuth. ## Mineral Associations This mineral co-occurs with other minerals typical of pegmatites, such as albite, quartz, lepidolite, elbaite, bismutotantalite, and other rare oxides and phosphates. ## Localities The type locality and main source of specimens is the Quixaba pegmatite, Frei Martinho municipality, Paraíba state, Brazil. This is one of the few confirmed occurrences of this mineral worldwide.

Rarity

Extremely rare

For collectors

## Quality Criteria The most highly valued specimens are those with sharp, well-formed crystals of intense, reddish-brown color. The placement of the crystal on the rock matrix, contrasting with lighter associated minerals like albite or quartz, is also of great importance. Due to its rarity, even microscopic but well-formed crystals are desirable. ## Popular Localities The vast majority, if not all, specimens available on the collector's market come from the type locality – the Quixaba pegmatite in Brazil. Specimens from any other confirmed locality would be extremely valuable.

Care and storage

## Cleaning Fluornatromicrolite specimens should be cleaned very carefully, preferably dry with a soft brush. If necessary, distilled water can be used. Ultrasonic cleaners should be strictly avoided, as they can cause cracks in fragile crystals. ## What to Avoid The mineral should be protected from contact with strong acids and other aggressive chemicals. Sudden temperature changes and abrasion should be avoided, as despite its hardness, it can be scratched by harder minerals. ## Storage It is recommended to store specimens in separate, padded display boxes to avoid contact with other minerals and potential mechanical damage. Protection from dust is also advisable.

Sources

IMA Mineral List (CNMNC)