Eldfellite

Chemical formula: NaFe<sup>3+</sup>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>

Eldfellite is a rare sodium and iron sulfate, forming tiny, yellow crystals as coatings and aggregates.

## Characteristics Eldfellite is a sodium and iron sulfate, occurring as very fine, tabular crystals up to 20 micrometers in size. These crystals form dense aggregates and coatings on rock surfaces. Due to the small size of the crystals, its macroscopic features are difficult to observe, and identification requires advanced analytical methods. ## Physical Properties Eldfellite crystals exhibit a luster described as vitreous to pearly. It is a transparent mineral. Due to the microscopic size of the crystals, hardness and density have not been precisely measured, but only calculated based on the chemical formula and unit cell parameters. ## Colors and Varieties This mineral has a characteristic, pale yellow color. No color varieties or commercial varieties are known. ## History and Name The name eldfellite comes from the Eldfell volcano on the Icelandic island of Heimaey, where this mineral was first identified. This is its type locality. It was approved as a new mineral species by the International Mineralogical Association (IMA) in 2008.

Properties

Luster
Vitreous to Pearly
Streak
Pale yellow
Density
3.36
Cleavage
Perfect on {0001}
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of eldfellite in field or home conditions is impossible. It requires specialized laboratory techniques, such as X-ray diffraction (XRD) and chemical microanalysis (EDS), due to the microscopic size of the crystals and its occurrence as thin coatings. ## Distinguishing from Similar Minerals It can be confused with other secondary iron sulfates, such as jarosite or metavoltine, which also form yellow coatings. Definitive differentiation is only possible based on chemical and crystallographic analysis. ## Crystal Forms Eldfellite forms very small, hexagonal, tabular crystals, which most often occur as dense, porous aggregates and coatings on volcanic rocks.

Geological environment

## Genesis Eldfellite is a fumarolic mineral. It forms as a result of the reaction of sulfur-rich volcanic gases with the surface of volcanic rocks at high temperatures (above 200°C). It is a product of sublimation directly from the gas phase. ## Mineral Associations At the type locality on Eldfell volcano, it co-occurs with other sulfates, such as anhydrite, bassanite, gypsum, and hematite. ## Locations The only confirmed occurrence of eldfellite in the world is its type locality – Eldfell volcano on Heimaey island in the Vestmannaeyjar archipelago off the southern coast of Iceland.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, eldfellite is not evaluated according to typical collector criteria, such as crystal size or form. The value of a specimen lies in its scientific significance and the confirmed presence of the mineral on a fragment of the host rock. The most valuable specimens are those from a well-documented type locality. ## Popular Localities The only source of specimens is the Eldfell volcano in Iceland. Material from this location is extremely rare in collections.

Care and storage

## Cleaning Due to its extreme fragility, solubility, and microscopic nature, eldfellite specimens should not be cleaned mechanically or chemically. Any attempt at cleaning risks destroying the mineral. ## What to Avoid Contact with water and other liquids that can dissolve the mineral must be strictly avoided. Specimens must be protected from moisture, temperature changes, and vibrations. ## Storage Eldfellite specimens require storage in dry, stable conditions, preferably in tightly sealed containers, such as "micromount" boxes, away from dust and moisture. They are not suitable for open display.

Sources

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