Nioboixiolite-(Fe^3+^)

Chemical formula: (Nb₅+₀.₅Fe₃+₀.₅)O₂

Nioboixiolite-(Fe³⁺) is a rare niobium and iron oxide, occurring as black, submetallic grains in granitic pegmatites.

## Characteristics Nioboixiolite-(Fe³⁺) is an oxide mineral belonging to the ixiolite group. It typically forms small, anhedral (irregularly shaped) grains or massive aggregates. Its crystals, when present, are poorly formed and have a platy or short-prismatic habit. It is opaque and black in color. ## Physical Properties This mineral is characterized by a hardness of 6-6.5 on the Mohs scale. It has a submetallic to pitchy luster. It is a dense mineral, with a calculated density of 5.81 g/cm³. ## Colors and Varieties Nioboixiolite-(Fe³⁺) is black, and in thin fragments, it may be translucent brown. No distinct color varieties or commercial varieties are recognized. ## History and Name The name "nioboixiolite" refers to the dominant niobium in its chemical composition and its relationship to the structurally similar mineral, ixiolite. The suffix "-(Fe³⁺)" specifies that trivalent iron is the dominant cation in one of the structural sites. The mineral was formally described as a new species relatively recently, following the redefinition of the ixiolite group by the International Mineralogical Association (IMA). ## Uses Due to its rarity and occurrence in small quantities, nioboixiolite-(Fe³⁺) has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and scientists studying pegmatites.

Properties

Mohs hardness: 6-6.5
Luster: Submetallic to resinous
Streak: Black
Density: 5.81
Cleavage: Imperfect on {010}
Fracture: Subconchoidal to uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Identifying nioboixiolite-(Fe³⁺) in the field is practically impossible without specialized equipment. In a collection, it can be identified based on its black color, submetallic luster, high density, and, most importantly, confirmed occurrence in granitic pegmatites. Definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to determine its chemical composition. ## Distinguishing from Similar Minerals Nioboixiolite-(Fe³⁺) is visually indistinguishable from other black, heavy oxide minerals found in pegmatites, such as columbite, tantalite, wodginite, wolframoixiolite, or ixiolite itself. All these minerals form similar black, submetallic masses or poorly formed crystals. The differences between them lie in the proportions of niobium, tantalum, tungsten, iron, manganese, and tin, which can only be determined by chemical analysis. ## Crystal Forms This mineral most often occurs as anhedral grains intergrown within the rock. Rarely, it forms poorly developed, platy or prismatic crystals, usually of small size.

Geological environment

## Genesis Nioboixiolite-(Fe³⁺) is an accessory mineral formed in the late stages of crystallization of granitic pegmatites, especially those of lithium-rich character (LCT type - lithium, cesium, tantalum). It crystallizes in an environment rich in niobium, iron, and other rare elements. ## Mineral Associations It typically co-occurs with other minerals characteristic of pegmatites, such as quartz, albite (especially the cleavelandite variety), microcline, muscovite, lepidolite, as well as other niobium and tantalum oxides, e.g., columbite-(Fe), columbite-(Mn), and microlite. ## Localities Confirmed occurrences of this mineral are few. It has been identified, among others, in pegmatites in the Viitaniemi area in Finland (type locality) and in the Mops No. 1 pegmatite in the Orange River area in the Northern Cape Province, Republic of South Africa.

Rarity

Very rare

For collectors

## Quality Criteria The collector appeal of nioboixiolite-(Fe³⁺) specimens is low from an aesthetic perspective, as this mineral does not form well-developed or colorful crystals. Its value lies almost exclusively in its rarity. For specialized collectors (so-called "systematists"), the most desirable specimens are those where the presence of nioboixiolite-(Fe³⁺) has been confirmed by chemical analysis. The value of a specimen is enhanced by a rich association with other rare pegmatite minerals and a precisely determined locality. ## Popular Localities The most prized specimens by specialist collectors are those from the type locality in Viitaniemi, Finland, as they serve as reference material for this mineral.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. The use of water or any chemical agents is not recommended, as they may react with the mineral's surface or with associated minerals. ## What to Avoid Avoid contact with acids and other chemicals. The mineral is stable, but it should not be heated or subjected to sudden temperature changes. Prolonged exposure to moisture can be harmful to some co-occurring minerals. ## Storage Nioboixiolite-(Fe³⁺) specimens are best stored under stable conditions, in closed collector boxes or display cases, to protect them from dust and mechanical damage. Due to its rarity, it is advisable to include a label with the precise locality for the specimen.