Amarantite

Chemical formula: Fe<sup>3+</sup><sub>2</sub>O(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·7H<sub>2</sub>O

Amarantite is a rare, hydrated iron sulfate, distinguished by its characteristic, intense amaranth-red color and acicular crystals.

## Characteristics Amarantite is a secondary iron sulfate that forms from the oxidation of sulfide minerals, primarily pyrite, in dry and acidic conditions. Its most characteristic feature is its intense, amaranth to brick-red color, from which its name is derived. It typically occurs as delicate, acicular or fibrous crystals forming brittle aggregates, coatings, and crusts on host rocks. The crystals are often so fine that they form felted or radial aggregates. ## Physical Properties Amarantite crystals are very brittle and delicate. This mineral is characterized by a vitreous luster and is transparent to translucent. Its Mohs hardness is 2.5, making it a soft mineral. The density ranges from 2.15 to 2.29 g/cm³. ## Colors and Varieties The dominant and defining color of amarantite ranges from amaranth to orange-red to brick-red. Some specimens may exhibit slight pleochroism, meaning a change in hue depending on the observation direction. No varieties are distinguished. ## History and Name The mineral's name, given in 1888 by Gideon E. Moore and George A. Koenig, comes from the Greek word "amarantos" (ἀμάραντος), meaning "unfading" or "everlasting flower," referring to its permanent, intense color. It was discovered in the Caracoles mine in the Antofagasta region of Chile. ## Uses Due to its rarity, brittleness, and instability in humid environments, amarantite has no industrial applications. It is exclusively a valued and sought-after collector's mineral.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
Pale yellow
Density
2.15-2.29
Cleavage
Perfect on {010}, good on {100}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification The key diagnostic feature of amarantite is its unique amaranth-red color, combined with its occurrence as delicate, acicular crystals. Its water solubility is also an important, though destructive, test characteristic. Its occurrence in dry, desert environments in iron sulfide deposits is a strong indicator. ## Distinguishing from Similar Minerals Amarantite is sometimes confused with other secondary iron sulfates, such as hohmannite or butlerite, which often co-occur with it. Hohmannite, however, has a more orange-brown color and often forms at the expense of amarantite in humid environments. Butlerite has a more orange-yellow hue. Accurate differentiation of these minerals often requires chemical or crystallographic analysis. ## Crystal Forms Amarantite crystals are elongated, acicular, or fibrous. They usually form radial, stellate, or haphazardly tangled aggregates. Less commonly, single, well-formed tabular or prismatic crystals are found.

Geological environment

## Genesis Amarantite is a secondary mineral, forming in the oxidation zones (weathered iron caps, so-called "gossans") of iron sulfide deposits, especially pyrite and marcasite. Its formation requires specific conditions: a strongly acidic environment and very low humidity (desert or dry climate). It forms as a result of the slow evaporation of iron sulfate-rich solutions. ## Mineral Associations It most often co-occurs with other secondary iron sulfates, such as hohmannite, butlerite, copiapite, szomolnokite, fibroferrite, as well as with pyrite, chalcopyrite, and hematite. ## Localities The most important and classic localities for amarantite are in the Atacama Desert in Chile, particularly in the area of the Caracoles mine (type locality), Chuquicamata, and Sierra Gorda. It is also known from the Iron Monarch mine in South Australia, from the Calamita mine on Elba Island (Italy), from the Rammelsberg mine in the Harz Mountains (Germany), and from several localities in arid regions of the USA (Utah, California, Arizona).

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with large, well-formed, acicular crystals forming rich, radial aggregates of intense amaranth color. High contrast between the red crystals and the light host rock is important. Specimens with visible, individual crystals are rarer and more desirable than felted coatings. The stability of the specimen (lack of transformation into hohmannite) also increases its value. ## Popular Localities Classic, most sought-after specimens come from the Atacama Desert in Chile (Caracoles, Chuquicamata). High-quality amarantites have also been found in Iron Monarch, Australia. Specimens from these localities are considered exemplary for this species.

Care and storage

## Cleaning Amarantite is water-soluble, so it must absolutely not be cleaned with water. To remove dust, a very soft brush or compressed air from a distance and with low force can be used very carefully. ## What to Avoid Avoid contact with water, water vapor, and any liquids that could dissolve it. The mineral is unstable in humid air – under the influence of moisture, it can transform into hohmannite, losing its structure and color. Protect it from high temperatures, which can cause dehydration. It is very brittle, so avoid shocks and impacts. ## Storage Amarantite specimens require storage in dry conditions. It is best to keep them in airtight, sealed containers (e.g., "membrane boxes" or boxes with a desiccant, such as silica gel). Exposure should be away from direct sunlight and sources of moisture.

External references

Sources

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