Oxynatromicrolite

Chemical formula: (Na,Ca,U₄+)₂(Ta₅+,Nb₅+)₂O₆(O,F)

Oxy-natromicrolite is a rare mineral from the microlite group, a sodium and tantalum oxide, found as small, resinous grains in granitic pegmatites.

## Characteristics Oxy-natromicrolite is a complex oxide from the pyrochlore group, specifically from the microlite series. It occurs extremely rarely, usually as irregular or rounded grains not exceeding 1-2 mm in size. Well-formed crystals are exceptional and most often take the form of octahedra. It is typically intergrown with other minerals, mainly quartz or albite. ## Physical Properties This mineral is characterized by a hardness in the range of 5-6 on the Mohs scale and a significant density of approximately 6.45 g/cm³, which is typical for tantalum-bearing minerals. It has a strong, resinous to vitreous luster. It is brittle and exhibits an uneven or conchoidal fracture. No cleavage has been observed. ## Colors and Varieties Oxy-natromicrolite most often ranges in color from yellow, through orange-yellow, reddish-brown, to almost black. Its color is associated with impurities, mainly uranium. No named commercial or color varieties are distinguished. ## History and Name The mineral's name, approved in 2011, refers to its chemical composition – the dominance of oxygen (oxy), sodium (natro), and its belonging to the microlite group. It is the oxygen analogue of natromicrolite. It was described based on material from the Pilat pegmatite, in the Auvergne-Rhône-Alpes region of France. ## Uses Due to its extreme rarity, oxy-natromicrolite has no industrial applications. It is solely an object of scientific interest and is valued by specialized collectors of rare minerals.

Properties

Mohs hardness: 5-6
Luster: Resinous, Vitreous
Streak: Light brown
Density: 6.45
Cleavage: None
Fracture: Uneven, Conchoidal
Transparency: Translucent to opaque
Crystal system: Cubic

Diagnostic features

## Identification Identifying oxy-natromicrolite under amateur conditions is practically impossible. Features such as high density, resinous luster, color (from yellow to brown), and occurrence in granitic pegmatites can only suggest a mineral from the pyrochlore group. Certain identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other minerals from the pyrochlore group, such as microlite, pyrochlore, uranmicrolite, or betafite. All these minerals have a similar appearance, hardness, luster, and occur in similar geological environments. Distinguishing them from each other is only possible based on chemical composition analysis. ## Crystal Forms It most often forms anhedral (unformed) or subhedral (poorly formed), rounded grains. Small, octahedral crystals are rarely found.

Geological environment

## Genesis Oxy-natromicrolite is a mineral of magmatic origin, crystallizing in the late stages of the evolution of granitic pegmatites, especially those enriched in rare earth elements, niobium, and tantalum (LCT-type pegmatites - lithium, cesium, tantalum). ## Mineral Associations It usually co-occurs with quartz, albite, microcline, muscovite, as well as other rare pegmatite minerals such as cassiterite, tantalite, columbite, uraninite, or zircon. ## Localities The most important and confirmed localities for this mineral are: - Pilat pegmatite, Auvergne-Rhône-Alpes, France (type locality). - Quixaba pegmatite, Paraíba, Brazil. - Specimens have also been found in Russia and potentially in other highly differentiated pegmatites worldwide.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of oxy-natromicrolite is primarily defined by the presence of well-formed, even microscopic, octahedral crystals. Specimens where the mineral is clearly visible against a contrasting matrix (e.g., white albite) are also valued. The intensity and transparency of the color, though rare, also increase the specimen's value. However, the certainty of identification from a specific, verified locality is key. ## Popular Localities The most sought-after specimens by collectors are those from the type locality in Pilat, France, and from the Quixaba pegmatite in Brazil, from which some of the best-studied specimens originate.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. The use of water or any chemical agents is not recommended, especially since this mineral often occurs in association with soluble or sensitive minerals. ## What to Avoid Avoid contact with acids and other chemicals. As a uranium-bearing mineral, it can be metamict (have a damaged crystal lattice due to radiation), which makes it more susceptible to chemical changes. Avoid ultrasonics and drastic temperature changes. ## Storage Store in stable room conditions, away from direct sunlight and humidity. It is best to keep it in a closed display box to limit dust accumulation and protect it from mechanical damage.

Sources

IMA Mineral List (CNMNC)