Gjerdingenite-Fe

Chemical formula: K₂Fe₂+(Nb₅+,Ti₄+)₄(Si₄O₁₂)₂(O,OH)₄·₆H₂O

Gjerdingenite-(Fe) is a very rare mineral from the cyclosilicate group, forming small, platy crystals of a brownish color.

## Characteristics Gjerdingenite-(Fe) is a mineral belonging to the gjerdingenite group and the kuzmenkoite subgroup. It occurs as very small, platy or lath-like crystals, rarely exceeding 0.2 mm in length. These crystals often form radial or spherulitic aggregates. It is brittle and has a characteristic vitreous luster. ## Physical Properties This mineral is characterized by a Mohs hardness of 5. Its density is slightly greater than that of quartz, approximately 3.03 g/cm³. The luster is vitreous, and the streak is light brown. It is translucent. ## Colors and Varieties The typical color of gjerdingenite-(Fe) is golden-brown to reddish-brown. No varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of iron (Fe) at the divalent cation position – and its relation to gjerdingenite-(Mn). It was approved as a new mineral by the International Mineralogical Association (IMA) in 2005. It was described based on material from its type locality in Norway. ## Applications Due to its extreme rarity and microscopic crystal sizes, gjerdingenite-(Fe) has no industrial applications. It is solely an object of scientific and collecting interest for specialized micromineral collectors.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: Light brown
Density: 3.03
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of gjerdingenite-(Fe) is impossible without advanced analytical methods, such as energy-dispersive spectroscopy (EDS/EDX) or chemical analysis using an electron microprobe. Visually, at the type locality, it can be suspected based on its golden-brown color, platy crystal habit, and co-occurrence with other rare pegmatite minerals. ## Distinguishing from Similar Minerals It is practically impossible to distinguish from other minerals of the gjerdingenite group (e.g., gjerdingenite-(Mn), gjerdingenite-(Ca)) without chemical analysis. It can also be confused with other fine-grained, brown silicates found in nepheline syenite pegmatites. ## Crystal Forms It forms very small, thin, platy crystals with a lath-like habit, often flattened parallel to {001}. These crystals typically occur as radial or spherulitic (spherical) aggregates.

Geological environment

## Genesis This is a magmatic mineral, crystallizing in the late stages of nepheline syenite pegmatite evolution. It forms under low-temperature conditions in rock cavities and miaroles. ## Mineral Associations This mineral co-occurs with other rare pegmatite minerals. At the type locality (Gjerdingen, Norway), it was found in association with albite, microcline, aegirine, natrolite, catapleiite, elpidite, kuzmenkoite-(Mn), lorenzenite, and tysonite-(Ce) fluoride. ## Localities Confirmed occurrences of gjerdingenite-(Fe) are extremely rare. The most important and also the type locality is the pegmatite in Gjerdingen, in the Nordmarka plutonic complex, near Lunner in Norway. This is the only known locality of this mineral in the world.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of gjerdingenite-(Fe) specimens is primarily assessed based on the richness and aesthetics of the crystal aggregates. Specimens with well-formed, radial or spherulitic clusters of an intense, golden-brown color, contrasting with a light background (e.g., white albite or natrolite), are most highly valued. Good association with other rare minerals is also important. ## Popular Localities The only source of gjerdingenite-(Fe) specimens is its type locality in Gjerdingen, Norway. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to the microscopic size of the crystals and their brittleness, any other method, including wet cleaning, is highly risky and may lead to the destruction of the mineral. ## What to Avoid Avoid contact with all chemicals, including acids and bases. Do not use ultrasonic cleaners. The mineral is brittle, so protect it from impacts and vibrations. ## Storage Micromineral specimens, such as gjerdingenite-(Fe), should be stored in specialized, sealed "micromount" boxes, which protect them from dust, mechanical damage, and sudden changes in humidity.

Sources

IMA Mineral List (CNMNC)