Hohmannite

Chemical formula: Fe³⁺₂O(S⁶⁺O₄)₂·8H₂O

Hohmannite is a rare, secondary iron sulfate, forming characteristic, orange-red, fibrous aggregates and efflorescences in the oxidation zones of ore deposits.

## Characteristics Hohmannite is a hydrated iron sulfate, occurring as fine, acicular or hair-like crystals, which most often form fibrous aggregates, radial clusters, as well as efflorescences and crusts. Due to its delicate structure, specimens are usually small and fragile. This mineral is a product of the weathering (oxidation) of other iron and sulfur-containing minerals, especially pyrite. ## Physical Properties Hohmannite crystals are very soft, with a hardness of 3 on the Mohs scale. They are characterized by a vitreous or silky luster, particularly visible on the surfaces of fibrous aggregates. They are transparent to translucent. The mineral's density is approximately 2.2 g/cm³, which is a relatively low value. ## Colors and Varieties Hohmannite is valued for its intense, amber-orange to orange-red color. No named varieties are distinguished, and the coloration is relatively uniform, depending on impurities and crystallization conditions. ## History and Name The mineral was first described in 1887 by August Frenzel. The name comes from the Chilean mining engineer, Thomas Hohmann, who discovered the first specimens in a mine in Sierra Gorda, Chile. ## Uses Hohmannite has no industrial application. It is solely an object of interest for collectors of rare minerals and scientists studying the weathering processes of sulfide deposits.

Properties

Mohs hardness
3
Color
Brown to burnt orange, light amaranth-red.
Luster
Vitreous
Streak
Orange-yellow
Density
2.255
Cleavage
On {010}, perfect; on {110} and <mi>{1_10},</mi> less perfect.
Fracture
Uneven
Transparency
Transparent,Translucent
Crystal system
Triclinic

Diagnostic features

## Identification Characteristic features of hohmannite include its intense orange-red color, fibrous or acicular crystal habit, and occurrence as efflorescences and crusts on sulfide minerals. A key diagnostic feature is its solubility in water. ## Distinguishing from Similar Minerals Hohmannite is sometimes confused with other secondary iron sulfates, such as amarantite, botryogen, or fibroferrite. Amarantite has a darker, red color and is more stable. Botryogen forms more vitreous, often radial aggregates with an orange to red color. Fibroferrite usually has a lighter, yellowish or greenish hue and a silky luster. Differentiation often requires chemical or crystallographic analysis, but the water solubility test is a good indicator. ## Crystal Forms Hohmannite crystallizes in the triclinic system, forming elongated, acicular or hair-like crystals. It is most commonly found in the form of tangled, fibrous masses, radial aggregates, and thin crusts and efflorescences.

Geological environment

## Genesis Hohmannite is a secondary mineral, forming in dry and hot climates in the oxidation zones (so-called iron caps) of ore deposits, mainly iron sulfides such as pyrite. It forms as a result of the evaporation of acidic mine waters rich in iron sulfate. ## Mineral Associations It co-occurs with other secondary iron sulfates and sulfato-oxides, such as amarantite, botryogen, fibroferrite, metavoltine, copiapite, as well as gypsum and melanterite. ## Localities The most important hohmannite localities are in the Atacama Desert in Chile, especially in the Sierra Gorda region (type locality), Chuquicamata, and Quetena. It is also known from the Rammelsberg mine in the Harz Mountains in Germany, from Vulcano Island (Italy), and from several localities in the United States (Arizona, California, Utah).

Rarity

Rare

For collectors

## Quality Criteria The most prized hohmannite specimens are those that form rich, dense clusters of intensely colored, orange-red crystals on a contrasting rock matrix. The size and aesthetics of the aggregates, as well as the absence of damage to the delicate fibers, are important. Due to the mineral's instability, well-preserved specimens are rare. ## Popular Localities Chilean localities, especially from the Antofagasta region (Sierra Gorda, Chuquicamata), are considered classic and yield the best specimens. Specimens from these places serve as the standard for the species.

Care and storage

## Cleaning Hohmannite specimens are extremely sensitive to water, in which they dissolve. Cleaning is not recommended. If absolutely necessary, compressed air can be used (from a distance and at low pressure) to remove loose dust. ## What to Avoid Avoid any contact with water, water vapor, and high humidity, which leads to the disintegration of the mineral. Also avoid direct sunlight and high temperatures. The mineral is very soft and brittle, susceptible to mechanical damage. ## Storage Storage requires airtight containers or display cases with a desiccant (e.g., silica gel) placed inside. Exposure should take place under stable conditions, away from sources of heat and light.

External references

Sources

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