Siderotil

Chemical formula: Fe²⁺S⁶⁺O₄·₅H₂O

Siderotile is a hydrated iron(II) sulfate that forms fibrous or acicular coatings and crusts in mines and on mine dumps.

## Characteristics Siderotile is a sulfate mineral belonging to the chalcanthite group. It occurs as a weathering product of sulfide minerals, mainly pyrite and marcasite. It forms characteristic, delicate efflorescences, coatings, and crusts with a fibrous, acicular, or capillary structure. It is less commonly found in granular or powdery aggregates. Due to its nature, it is an unstable mineral, sensitive to changes in air humidity. ## Physical Properties It is a very soft mineral, with a hardness of 2.5 on the Mohs scale. It has a silky luster, and in the case of granular aggregates, an earthy luster. It is translucent. Its crystals and fibers are flexible. ## Colors and Varieties Siderotile ranges in color from whitish, through yellowish, to light green. In transmitted light, it is colorless. Its color depends on the degree of hydration and the presence of impurities. No named varieties are distinguished. ## History and Name The mineral's name comes from the Greek words *sideros* (iron) and *tilos* (fiber), referring to its chemical composition and typical fibrous form. It was first described in 1891. The type locality (locus typicus) was indicated as the Idrija mine in Slovenia. ## Uses Siderotile has no industrial significance. It is solely an object of interest for collectors specializing in secondary minerals and mineralogical systematics.

Properties

Mohs hardness: 2.5
Luster: Silky
Streak: White
Density: 0
Transparency: Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Siderotile is identified by its characteristic fibrous or capillary form, silky luster, and occurrence as a weathering product of iron sulfides. A key feature is its water solubility and instability under varying humidity conditions. It has a bitter, metallic taste, but taste testing is highly discouraged due to toxicity. ## Distinguishing from Similar Minerals It can be confused with other secondary iron sulfates, such as melanterite or rozenite. Melanterite (FeSO₄·7H₂O) contains more water, often forms larger crystals, and has a more greenish color. Rozenite (FeSO₄·4H₂O) is a dehydration product of siderotile and is usually white and powdery. Gypsum, which has a similar appearance, is much harder and does not dissolve as easily in water. Accurate differentiation of these sulfates often requires chemical analysis (XRD). ## Crystal Forms Siderotile crystals are very rare, microscopic in size, and have a tabular or prismatic habit. It typically forms fibrous, acicular, capillary aggregates, as well as crusts, coatings, and earthy masses.

Geological environment

## Genesis Siderotile is a secondary mineral, formed in oxidation (weathering) zones of ore deposits rich in iron sulfides, such as pyrite, marcasite, or pyrrhotite. It forms under low humidity conditions, as a result of the dehydration of more hydrated melanterite. It is a typical mineral for mine workings, tunnels, and mine dumps. ## Mineral Associations It most often co-occurs with other iron sulfates, such as melanterite, rozenite, szomolnokite, and copiapite. It is also accompanied by the primary minerals from which it forms: pyrite and marcasite, as well as gypsum, epsomite, and native sulfur. ## Localities It is known from many localities worldwide where sulfide deposits have been exploited. Besides the type locality in Idrija (Slovenia), it occurs, among others, in Rammelsberg and Goslar in Germany; in Falun in Sweden; on Elba Island in Italy; in Bisbee, Arizona (USA); and in mines of the Sudbury region in Canada.

Rarity

Not very common

For collectors

## Quality Criteria The collector's appeal of siderotile primarily depends on the abundance and form of its aggregates. Most valued are specimens with well-developed, dense fibrous or acicular aggregates, forming impressive "beards" or "carpets" on the host rock. Color contrast (e.g., light green or yellowish) with a dark matrix enhances visual value. Due to its instability, specimens preserved in their original form, without signs of dehydration, are more highly rated. ## Popular Localities Although it is a widely distributed mineral, specimens of particular collector's value are not common. Historically important are finds from classic European localities, such as mines in the Harz Mountains in Germany. Good quality specimens also come from some mines in the USA, e.g., in Arizona.

Care and storage

## Cleaning Siderotile is water-soluble, so it must absolutely not be cleaned wet. Specimens can only be cleaned mechanically, with the utmost care, using a soft brush or compressed air from a safe distance to remove dust. ## What to Avoid Avoid contact with water and any moisture, which leads to its dissolution. It is sensitive to changes in humidity – in dry air, it can lose water and transform into rozenite (FeSO₄·4H₂O), and in humid air, it can absorb water, changing into melanterite (FeSO₄·7H₂O). It should be protected from direct sunlight and high temperatures. ## Storage Siderotile specimens require storage under stable conditions. It is best to place them in a sealed, closed container (a "perky box") or in a display case with controlled humidity. Due to its fragility and delicacy, vibrations and contact with other minerals should be avoided.