Ixiolite-(Fe^2+^)

Chemical formula: (Ta₅+₂/₃Fe₂+₁/₃)O₂

A rare tantalum, niobium, and iron oxide, occurring as black, opaque granular aggregates with a metallic luster.

## Characteristics Ixiolite-(Fe²⁺) is a complex oxide belonging to the ixiolite group, characterized by the dominance of tantalum and divalent iron. It typically forms irregular, granular aggregates or occurs as embedded grains within the host rock. It is rarely observed as poorly formed, thick, platy crystals. It is an opaque mineral, black or grayish-black in color, with a metallic to submetallic luster. ## Physical Properties Ixiolite-(Fe²⁺) is characterized by significant hardness, ranging from 6 to 6.5 on the Mohs scale, which makes it resistant to scratching. Its density is high, varying from 7.05 to 7.23 g/cm³, which is typical for tantalum-rich minerals. This mineral does not exhibit cleavage, and its fracture is uneven to subconchoidal. It has a black streak. ## Colors and Varieties The mineral is uniform in color, occurring in shades of black, sometimes with a gray or brownish tone. No distinct color varieties or commercial varieties are recognized. ## History and Name The name "ixiolite" comes from the Greek word *Ixion* (Ιξίων), a figure from Greek mythology who was bound to a perpetually revolving wheel in Tartarus. This name alludes to the complex chemical composition of the mineral and the difficulty in its identification. The mineral was originally described by Adolf Erik Nordenskiöld in 1857 based on material from Skogböle, Finland. The suffix -(Fe²⁺) was added later according to mineral nomenclature rules to indicate the dominant cation in a specific structural site.

Properties

Mohs hardness: 6-6.5
Luster: Metallic to submetallic
Streak: Black
Density: 7.05-7.23
Cleavage: None
Fracture: Uneven to subconchoidal
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Ixiolite-(Fe²⁺) is identified by its black color, metallic luster, high density, and significant hardness. It occurs in a characteristic geological environment – granitic pegmatites. The black streak is also an important diagnostic feature. Identification is often difficult without advanced analytical methods (XRD, EDS) due to its resemblance to other "heavy black minerals." ## Distinguishing from Similar Minerals Ixiolite-(Fe²⁺) is difficult to distinguish from other minerals of the columbite-tantalite group, such as columbite-(Fe), tantalite-(Fe), or tapiolite-(Fe), as well as from wolframite. Wolframite has similar hardness and density but often exhibits one distinct cleavage plane, which ixiolite lacks. Definitive differentiation from tantalite and columbite requires chemical analysis to determine the Ta/Nb ratio and structural analysis, as ixiolite has a different (disordered) crystal structure. ## Crystal Forms Crystals are rare, usually poorly formed, with a thick-platy or short-prismatic habit. Most often, it occurs as irregular grains and granular aggregates embedded in quartz, feldspar, or other pegmatitic minerals.

Geological environment

## Genesis Ixiolite-(Fe²⁺) is an accessory mineral in granitic pegmatites, especially in lithium-rich (LCT type - lithium, cesium, tantalum) pegmatites. It forms in the late stages of pegmatitic magma crystallization, under conditions enriched in rare elements such as tantalum, niobium, tin, and manganese. ## Mineral Associations It most commonly co-occurs with quartz, albite (especially its variety cleavelandite), microcline, muscovite, lepidolite, as well as other tantalum and niobium minerals such as tantalite-(Fe), tapiolite-(Fe), microlite, and wodginite. Cassiterite and beryl may also accompany it. ## Localities Important worldwide localities include: the type locality Skogböle on Kimitoön Island in Finland; the Tanco Mine in Bernic Lake, Manitoba (Canada); numerous pegmatites in the Minas Gerais region in Brazil; and pegmatites in Western Australia (e.g., Greenbushes) and Russia (Urals).

Rarity

Rare

For collectors

## Quality Criteria The collector appeal of ixiolite-(Fe²⁺) is limited due to its unassuming appearance. The most prized specimens are rare, well-formed crystals, especially if they are set on a contrasting matrix, such as white albite (cleavelandite) or quartz. Crystal size and a precisely identified locality also increase the specimen's value. ## Popular Localities Specimens from classic localities, such as Skogböle in Finland or the Tanco Mine in Canada, are more sought after by specialized collectors of rare minerals. Brazilian pegmatites also provide specimens, although these are often massive, without well-formed crystals.

Care and storage

## Cleaning Ixiolite-(Fe²⁺) specimens can be cleaned using a soft, dry brush to remove dust. For more stubborn dirt, distilled water and gentle wiping are permissible. Ultrasonic cleaners should be avoided, as they can cause microfractures. ## What to Avoid The mineral is stable, but contact with strong acids, which could potentially damage its surface, should be avoided. It is not sensitive to light, but extreme temperature changes should be avoided. ## Storage Store in stable conditions, away from chemicals. Due to its hardness, it can be stored with other minerals of similar or lesser hardness, but individual boxes or stands are recommended to prevent mutual scratching of specimens.

Sources

IMA Mineral List (CNMNC)