Bílinite

Chemical formula: Fe²⁺Fe³⁺₂(S⁶⁺O₄)₄·₂₂H₂O

Bílinite is a rare, hydrated iron(II) and iron(III) sulfate, forming delicate, acicular crystals of a pale yellow color.

## Characteristics Bílinite is a sulfate mineral belonging to the halotrichite group. It occurs as delicate, fibrous or acicular aggregates, which can form efflorescences and crusts. Individual crystals are very small, often microscopic, and tend to form radial or tangled clusters. Due to its delicate nature, specimens are fragile and rarely found as well-formed crystals. ## Physical Properties This mineral is characterized by low hardness, approximately 2 on the Mohs scale, meaning it is very soft. It has a silky luster, resulting from its fibrous structure. It is transparent to translucent. Its density is low, around 1.88 g/cm³. ## Colors and Varieties Bílinite occurs in shades from pale yellow to yellowish-white. Its color is characteristic and associated with the presence of iron ions. No named varieties of this mineral are distinguished. ## History and Name The mineral's name comes from the city of Bílina in the Czech Republic, near which its type locality is situated – the Maxim Gorki lignite mine (now Bílina-Světice). It was first described in 1913 by the Czech mineralogist Josef Janeček. ## Uses Bílinite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and scientists studying sulfide weathering processes.

Properties

Mohs hardness: 2
Color: White to yellowish
Luster: Silky
Streak: White
Density: 1.875
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Bílinite is identified by its characteristic form – pale yellow, silky, acicular or fibrous aggregates. A key diagnostic feature is its water solubility. Its occurrence in post-mining environments, as a product of sulfide weathering, is also an important clue. ## Distinguishing from Similar Minerals It can be confused with other secondary iron sulfates, such as halotrichite, melanterite, or jarosite. Halotrichite forms similar “hairs” but is usually white or colorless. Melanterite is greenish or bluish and also water-soluble. Jarosite is yellow but insoluble in water and harder. Final distinction often requires chemical analysis (XRD). ## Crystal Forms Crystals are acicular, elongated along the c-axis, and very small. They usually form fibrous, radial, tangled aggregates, as well as efflorescences and crusts on host rocks.

Geological environment

## Genesis Bílinite is a secondary mineral, formed by the oxidation (weathering) of iron sulfides, mainly pyrite and marcasite, in a sulfur-rich environment. It forms as an evaporation product of mine waters under conditions of low temperature and humidity, often in old mine workings, on dumps, and in sulfide rock outcrops. ## Mineral Associations It often co-occurs with other sulfates formed under similar conditions, such as melanterite, halotrichite, epsomite, gypsum, and also native sulfur. ## Localities The most important localities worldwide include primarily the type locality in Bílina, Czech Republic. It is also known from mines in the Dubník area in Slovakia, from Pécs in Hungary, from Vulcano Island (Sicily, Italy), and from several places in Germany (e.g., Saxony). Occurrences have also been reported in the USA, including Utah.

Rarity

Rare

For collectors

## Quality Criteria The most prized by collectors are specimens with abundant, well-formed acicular aggregates of intense, pale yellow color. Contrast with the rock matrix and the absence of damage to the delicate fibers are important. Due to the rarity of the mineral, even small but well-defined samples are considered valuable. ## Popular Localities Specimens from the type locality in the Czech Republic (Bílina) and from Dubník, Slovakia, are the most classic and sought after by collectors specializing in mineral systematics or secondary minerals.

Care and storage

## Cleaning Bílinite specimens should generally not be wet cleaned. The mineral is water-soluble, which leads to its permanent damage or complete destruction. To remove dust, a very soft brush or compressed air from a safe distance can be used very carefully. ## What to Avoid Contact with water and other liquids must be strictly avoided. Bílinite is also sensitive to changes in air humidity – in too dry an environment, it may dehydrate and disintegrate, and in too humid an environment, it may absorb water and deliquesce. It should be protected from high temperatures and direct sunlight. ## Storage The best way to store specimens is to place them in a sealed, airtight container (a “perky box”) or a display case with controlled humidity. Isolation from ambient humidity fluctuations is crucial for preserving the integrity of the sample. Due to its fragility, vibrations and touching should be avoided.