Quenstedtite

Chemical formula: Fe<sup>3+</sup><sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>3</sub>·11H<sub>2</sub>O

Quenstedtite is a hydrated iron(III) sulfate, forming characteristic, purplish-red, tabular crystals that readily lose water in dry air.

## Characteristics Quenstedtite is a sulfate mineral that, in its fresh form, exhibits an intense, purplish-red or reddish color. Its crystals are usually small, tabular or short-prismatic in shape, often grouped into radial or fibrous aggregates, as well as granular masses and crusts. It is an unstable mineral at room conditions – when exposed to dry air, it quickly loses water from its structure, becoming lighter, yellowish, and dull, transforming into coquimbite. ## Physical Properties Quenstedtite is a relatively soft mineral, with a hardness of 2.5 on the Mohs scale. It has a vitreous luster, and sometimes a pearly luster on fracture surfaces. It is transparent to translucent. Its density is approximately 2.1 g/cm³. ## Colors and Varieties The typical color of quenstedtite ranges from reddish-purple to peach-red. No named varieties are distinguished, and color changes to yellow or whitish are a result of its dehydration and transformation into other, less hydrated iron sulfates. ## History and Name The mineral was first described in 1888 by G. Linck. Its name honors the German mineralogist and paleontologist, Friedrich August von Quenstedt (1809–1889), a professor at the University of Tübingen. ## Uses Quenstedtite has no industrial significance. Due to its instability and small crystal size, it is primarily a mineral of interest to specialized collectors and scientists studying sulfide weathering processes.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
White to pale reddish
Density
2.1
Cleavage
Perfect on {010}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification The key diagnostic feature of quenstedtite is its characteristic reddish-purple color combined with instability in dry air. A fresh specimen quickly dulls and lightens, which is typical for this mineral. It is also soluble in water. ## Distinguishing from Similar Minerals Quenstedtite can be confused with other secondary iron sulfates, such as coquimbite or kornelite. Coquimbite, being a product of its dehydration, has a similar composition but is usually purple or colorless and crystallizes in the trigonal system. Kornelite has a similar color but is even more unstable and hygroscopic. Differentiation often requires chemical or crystallographic analysis. ## Crystal Forms Quenstedtite crystals are usually tabular, flattened parallel to {010}, or short-prismatic. They often form radial aggregates, rosettes, as well as fibrous or granular masses and coatings.

Geological environment

## Genesis Quenstedtite is a secondary mineral, formed as a result of the weathering (oxidation) of sulfide minerals, mainly pyrite and marcasite, in dry and arid climates. It forms as an evaporation product of iron sulfate-rich solutions in the oxidation zones of ore deposits. ## Mineral Associations It most commonly co-occurs with other iron sulfates that form under similar conditions. Typical associated minerals include: coquimbite (often as a product of its alteration), kornelite, voltaite, copiapite, rozenite, and melanterite. ## Localities The most important and classic localities for quenstedtite are found in the Atacama Desert in Chile, particularly in the Tierra Amarilla area (Copiapó mine) and Sierra Gorda. It is also known from the Rammelsberg mine in the Harz Mountains in Germany, from Spain (Rio Tinto mines), Italy (Vulcano island), and from some localities in the United States (e.g., Utah).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, transparent crystals of intense, reddish-purple color. The size of the crystals and their attractive arrangement in radial aggregates or rosettes are important. Due to the mineral's instability, specimens that have not undergone dehydration and have retained their original color and luster are much more highly prized. ## Popular Localities Classic and most sought-after quenstedtite specimens come from the desert regions of Chile, especially from Tierra Amarilla in the Atacama region. Specimens from this locality are considered type specimens for this species.

Care and storage

## Cleaning The mineral is very sensitive to water and moisture, so it should not be cleaned wet. To remove dust, a very soft, dry brush or compressed air from a safe distance can be used. ## What to Avoid Contact with water, alcohol, and other liquids that can dissolve it must be strictly avoided. The mineral is unstable in dry air – it loses water, changing its color and structure. It should be protected from direct sunlight and heat sources, which accelerate the dehydration process. ## Storage Quenstedtite specimens must be stored in airtight, hermetic containers (e.g., "perky box" type or boxes with membranes) to maintain stable ambient humidity and prevent water loss. Storage in a dry place will cause its irreversible transformation into coquimbite.

External references

Sources

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