Fibroferrite
Chemical formula: Fe<sup>3+</sup>S<sup>6+</sup>O<sub>4</sub>(OH)·5H<sub>2</sub>O
Hydrous iron sulfate, forming characteristic, silky fibrous aggregates of pale yellow or whitish color.
Description
## Characteristics Fibroferrite is a hydrated iron sulfate, named for its typical mode of occurrence. It forms delicate, fibrous or acicular crystals, often gathered into radial, spherical, or tangled aggregates resembling felt or cotton. Specimens have a characteristic silky luster, and their brittleness makes them very delicate to the touch. ## Physical Properties This mineral is very soft, with a Mohs hardness of about 2.5, meaning it can be scratched with a fingernail. It is also very light, with a density of only 1.9-2.0 g/cm³. It exhibits perfect cleavage in one direction. It is translucent, and its luster is described as silky, which is directly related to its fibrous structure. ## Colors and Varieties Fibroferrite most often ranges in color from pale yellow and greenish-yellow to grayish-white or almost colorless. Its color is often related to the degree of hydration and the presence of impurities. There are no named commercial or colored varieties. ## History and Name The mineral's name, given in 1870 by Hermann Credner, comes from the Latin words *fibra* (fiber) and *ferrum* (iron), which accurately describes its fibrous appearance and chemical composition. It is a mineral long known from historical mining regions. ## Uses Fibroferrite has no industrial significance. It is solely an object of interest for collectors specializing in secondary minerals and scientists studying the weathering processes of the oxidation zones of ore deposits.
Diagnostic features
## Identification The most important diagnostic feature of fibroferrite is its form – aggregates of thin, silky fibers. Also characteristic are its pale yellow color, low hardness (approx. 2.5), and occurrence in the oxidation zones of sulfide deposits. It is soluble in water. ## Distinguishing from Similar Minerals Fibroferrite is sometimes confused with other secondary iron sulfates: - **Copiapite**: Usually has a more intense, sulfur-yellow color and more often forms microcrystalline coatings and crusts, rather than exclusively fibrous aggregates. - **Jarosite**: Most often occurs as earthy masses or very fine, distinct crystals; it is harder and has a yellow to brownish hue. - **Melanterite**: Typically has a green or blue-green color and is a mineral from which fibroferrite often forms through dehydration. ## Crystal Forms Fibroferrite forms exclusively capillary, acicular, or fibrous crystals. They occur as radial or chaotically tangled aggregates, as well as coatings and crusts on other minerals or rocks.
Geological environment
## Genesis It is a typical secondary mineral, formed by the oxidation of iron sulfides (mainly pyrite and marcasite) under conditions of low humidity. It forms as a weathering product in the oxidation zones (iron caps) of ore deposits. It often occurs as efflorescences on the walls of old mine workings and on dumps, especially in arid and semi-arid climates. ## Mineral Associations Fibroferrite often co-occurs with other secondary sulfates, such as melanterite, copiapite, halotrichite, alunogen, epsomite, as well as gypsum, limonite, and native sulfur. ## Localities It is a widely distributed mineral worldwide. Known localities include the mine in Recsk, Hungary; the copper deposits in Chuquicamata, Chile; the historical mining region of Rio Tinto, Spain; and numerous mines in Arizona, California, and Nevada in the USA.
Rarity
Not very common
Collector aspects
## Quality Criteria Most valued by collectors are specimens with rich, well-formed, and undamaged fibrous aggregates. The intensity and purity of the color, as well as the aesthetic arrangement on the matrix, are important. Co-occurrence with other, colorfully contrasting secondary minerals adds to its attractiveness. ## Popular Localities Specimens from classic, historical localities, such as Recsk (Hungary) or deposits in the Atacama Desert (Chile), are particularly sought after by collectors specializing in mineral systematics.
Care and storage
## Cleaning Fibroferrite specimens are extremely sensitive to water, in which they readily dissolve. Cleaning should only be done dry, using a very soft brush to remove dust or applying a gentle puff of compressed air from a safe distance. ## What to Avoid Contact with water and any moisture must be strictly avoided. Fibroferrite is unstable in humid environments – it can absorb or lose water, leading to its chemical and physical degradation. It should also be protected from mechanical damage due to its high brittleness and softness. ## Storage The best way to store fibroferrite is to place it in a tightly sealed container (e.g., a plastic box) along with a desiccant, such as silica gel. This will ensure a stable, dry environment and protect the specimen from degradation.