Hilarionite

Chemical formula: Fe³⁺₂(S⁶⁺O₄)(As⁵⁺O₄)(OH)·6H₂O

Hilarionite is a rare, secondary iron sulfate-arsenate, forming aggregates of tiny, yellowish-green crystals.

## Characteristics Hilarionite is a hydrated iron sulfate-arsenate, occurring as tiny, acicular or tabular crystals. It most often forms radial aggregates, rosettes, or spherulitic clusters up to 1 mm in diameter. Individual crystals are very small, usually not exceeding 0.2 mm in length. ## Physical Properties Hilarionite crystals have a hardness of approximately 3 on the Mohs scale. They are brittle and have an uneven fracture. The luster is vitreous to silky, particularly noticeable in aggregates. The calculated density is 2.99 g/cm³. ## Colors and Varieties This mineral is characterized by a vivid, yellowish-green color. The streak is pale yellow. It is translucent. No varieties have been distinguished. ## History and Name The mineral's name comes from the ancient Hilarion mine, part of the Lavrion mining complex in Greece, where it was first discovered. Hilarionite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2011 (IMA number 2011-027). It was described by a team of mineralogists, including Nikita V. Chukanov. ## Uses Due to its rarity and microscopic crystal size, hilarionite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or minerals from specific localities (so-called "systematics" and "locality" collectors).

Properties

Mohs hardness
2
Color
Light green (with typically olive or greyish tint) to light yellowish-green, grey-green
Luster
Vitreous to silky
Streak
Light yellow
Density
2.40
Cleavage
None
Fracture
Irregular/Uneven,Splintery
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of hilarionite is based on its characteristic yellowish-green color, its occurrence as radial or spherulitic aggregates, and its geological environment. Due to the microscopic size of the crystals, definitive confirmation requires advanced analytical methods such as Raman spectroscopy or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other secondary arsenates and sulfates of iron, such as kaatialaite, bukovskýite, or jarosite. Distinguishing it "by hand" is practically impossible. Hilarionite is differentiated by its unique X-ray diffraction pattern and specific Raman spectrum. ## Crystal Forms Crystals are very fine, acicular or tabular, up to 0.2 mm long. They typically form radial aggregates, rosettes, and spherical clusters (spherulites) up to 1 mm in diameter, growing on the host rock.

Geological environment

## Genesis Hilarionite is a secondary mineral, forming in oxidation zones (so-called iron caps or gossans) of polymetallic deposits rich in arsenic and sulfides. It forms as a result of the weathering of primary ores (mainly arsenopyrite and pyrite) under low pH (acidic) conditions. ## Mineral Associations It co-occurs with other secondary minerals of the oxidation zone. In its type locality (Lavrion, Greece), it has been found in association with goethite, jarosite, bukovskýite, gypsum, and also rare arsenates such as kaatialaite and annabergite. ## Localities Confirmed occurrences of hilarionite are very few. The most important and type locality is the ancient dumps and mines in the Lavrion district, Attica, Greece. Outside Greece, its presence has been noted in the Maria Josefa mine in Rodalquilar (Almería, Spain), also in the oxidation zone of a gold deposit.

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the most important criteria are the richness and aesthetics of the aggregates. The most prized specimens are those with well-formed, spherical or radial clusters of hilarionite, clearly contrasting in color with the host rock (matrix). Association with other rare secondary minerals is also important. ## Popular Localities By far the most known and valued specimens come from the type locality – the Lavrion district in Greece. Specimens from Spain are much rarer on the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using only compressed air to remove dust. Due to its solubility in acids and potential sensitivity to water, all wet cleaning should be avoided. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is brittle, so it should be protected from impacts and vibrations. It should not be heated or exposed to prolonged moisture. ## Storage It is recommended to store specimens in closed, stable containers (e.g., "micromount boxes") to protect them from dust, moisture, and mechanical damage. They should ideally be kept in a dry place at a constant temperature.

External references

Sources

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