Hisingerite

Chemical formula: Fe<sup>3+</sup><sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>·2H<sub>2</sub>O

Hisingerite is a hydrated iron(III) silicate, forming black, vitreous masses with a conchoidal fracture, often mistaken for obsidian or volcanic glass.

## Characteristics Hisingerite is a mineral from the phyllosilicate group, chemically classified as a hydrated iron(III) silicate. It typically occurs as amorphous or very poorly crystallized, compact masses, crusts, and coatings. Its appearance is distinctive – it forms black, lustrous aggregates resembling tar or volcanic glass (obsidian). Fibrous or earthy forms are rarely observed. Due to its structure, hisingerite does not form well-developed crystals visible to the naked eye. ## Physical Properties This mineral is characterized by a Mohs hardness of approximately 3, meaning it is relatively soft. Its luster is variable, ranging from vitreous on fresh fractures to greasy or dull on weathered surfaces. It is opaque. Density varies depending on the degree of hydration, usually falling within the range of 2.5-3.3 g/cm³. ## Colors and Varieties The predominant color of hisingerite is black, sometimes with a brownish or greenish tint. The streak, which is the color of the powdered mineral, is yellowish-brown or greenish-gray, serving as an important diagnostic feature. No named varieties are distinguished. ## History and Name The mineral's name, given in 1828 by Jöns Jacob Berzelius, honors Wilhelm Hisinger (1766-1852), a Swedish chemist and mineralogist who first performed a chemical analysis of this mineral. The type locality, from which the first described specimens originated, includes mines in Riddarhyttan and Gillinge, Sweden. ## Uses Due to its rarity and lack of significant concentrations, hisingerite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or skarn minerals.

Properties

Mohs hardness
3
Luster
Vitreous
Streak
Yellow-brown
Density
2.5 - 3.3
Cleavage
None
Fracture
Conchoidal
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification The most important features for identifying hisingerite are its black color, vitreous luster, and characteristic conchoidal fracture, resembling broken glass. The yellowish-brown streak is also key, distinguishing it from many other black minerals. Its relatively low hardness (approximately 3 on the Mohs scale) is also helpful in identification. ## Distinguishing from Similar Minerals - **Obsidian (volcanic glass):** Looks very similar but has a lighter, usually white or gray streak and is much harder (5-5.5). - **Anthracite (coal):** Is lighter, often smudges fingers, and has a black streak. - **Dark varieties of sphalerite:** Usually have higher hardness (3.5-4) and perfect cleavage, which hisingerite lacks. Sphalerite's streak is also lighter, often pale yellow to brown. - **Goethite (in compact form):** May have a similar appearance, but its streak is distinctly yellowish-brown to orange-brown, and the mineral itself is harder (5-5.5). ## Crystal Forms Hisingerite is an amorphous or poorly crystalline (cryptocrystalline) mineral, therefore it does not form macroscopic crystals. It occurs as compact, reniform, or botryoidal masses, as well as coatings, crusts, and fissure fillings. Concentric or radially fibrous aggregates are sometimes observed.

Geological environment

## Genesis Hisingerite is a secondary mineral, forming in the oxidation (weathering) zones of iron ore deposits, especially in skarns and magnetite deposits. It forms as a result of hydrothermal alteration or weathering of iron-rich minerals such as olivines, pyroxenes, or iron sulfides (pyrite, pyrrhotite). ## Mineral Associations It often co-occurs with minerals typical of oxidation zones and skarn rocks. Its most common associations include magnetite, pyrite, chalcopyrite, quartz, calcite, minerals from the garnet group (andradite), epidote, and various minerals from the serpentine group. ## Localities The most important and historical occurrences of hisingerite are found in Sweden (Riddarhyttan, Gillinge, Tunaberg). It is also known from Finland (Orijärvi mine), Germany (Schwarzenberg), Norway, Great Britain (Scotland), and also from the USA, especially from the states of Arizona, Pennsylvania, and Michigan (iron ore deposits).

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with an intensely black color and a strong, vitreous luster. Well-formed botryoidal or reniform shapes with a smooth, shiny surface enhance attractiveness. Specimens showing a distinct, conchoidal fracture are also valuable. Contrasting combinations with other minerals, such as white quartz or calcite, also increase the visual and collectible value of the specimen. ## Popular Localities Classic, historical specimens come from Swedish mines in the Västmanland region. Currently, collectible material is also obtained from some locations in the USA, but hisingerite remains a rare mineral on the market.

Care and storage

## Cleaning Hisingerite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, a damp (not wet) cloth can be used. Avoid washing under running water, as the mineral may be sensitive to excessive moisture. ## What to Avoid Avoid contact with chemicals, especially acids, which can damage it. The mineral is relatively soft, so it must be protected from scratching by harder minerals. It should not be exposed to prolonged water exposure or sudden temperature changes. ## Storage It is recommended to store specimens in separate, padded boxes or on a soft surface in display cases, away from harder specimens. Protection from dust and moisture is crucial to preserve its natural appearance.

External references

Sources

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