Argentojarosite

Chemical formula: Ag<sup>1+</sup>Fe<sup>3+</sup><sub>3</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>

Argentojarosite is a rare silver, iron, and potassium sulfate, valued by collectors for its characteristic, fine crystals of intense color.

## Characteristics Argentojarosite is a hydrated silver and iron sulfate, belonging to the alunite group. It forms very small but well-formed crystals with a platy or rhombohedral habit. It typically occurs as crusts, coatings, or earthy aggregates on host rocks. Its crystals rarely exceed 1 mm in size, making magnification necessary for their observation. ## Physical Properties Argentojarosite crystals exhibit a vitreous to slightly dull luster. They are brittle, and their hardness on the Mohs scale ranges from 3.5 to 4.5, which is a relatively low value. The mineral's density ranges from 3.64-3.76 g/cm³. ## Colors and Varieties This mineral is characterized by colors ranging from yellow, through yellowish-brown, to dark brown. The color depends on the size of the crystals and the presence of impurities. No commercial or colored varieties are distinguished. ## History and Name Argentojarosite was first described in 1923 by W. T. Schaller based on samples from the Tintic Standard mine in the Dividend district, Utah County, USA. Its name refers to its chemical composition – the presence of silver (Latin: *argentum*) – and its structural similarity to jarosite. ## Uses Due to its rarity and small crystal size, argentojarosite has no industrial application. However, it is a sought-after and valued collector's mineral, especially specimens with well-formed crystals from the type locality.

Properties

Mohs hardness
3.5-4.5
Luster
Vitreous
Streak
Pale yellow
Density
3.64-3.76
Cleavage
Good on {0001}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Trigonal

Diagnostic features

## Identification Argentojarosite is recognized by its characteristic yellowish-brown color, occurrence as small, platy or rhombohedral crystals, and association with other minerals of the oxidation zone of ore deposits. However, definitive identification requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other minerals from the alunite group, such as jarosite, natrojarosite, or plumbojarosite. Distinguishing them based on visual features is practically impossible and requires specialized tests. Jarosite is much more common and is usually not associated with silver mineralization to the same extent as argentojarosite. ## Crystal Forms Crystals are usually very small, with a platy habit, often hexagonal in outline, or rhombohedral. It forms microcrystalline crusts, coatings, and powdery or earthy aggregates.

Geological environment

## Genesis Argentojarosite is a secondary mineral, forming in the oxidation zones (so-called iron caps) of ore deposits rich in silver and iron sulfides. It crystallizes under strongly acidic conditions (low pH) from solutions rich in sulfate, iron, and silver ions. ## Mineral Associations It most often co-occurs with other secondary minerals, such as jarosite, goethite, hematite, anglesite, cerussite, as well as with sulfides (galena, pyrite) and silver minerals (acanthite, native silver). ## Localities The most known and classic locality, from which the best specimens originate, is the Tintic Standard mine in Utah, USA. Other confirmed localities include Tombstone in Arizona (USA), the East Tintic mine in Utah (USA), as well as individual occurrences in Greece (Laurion), Spain, Germany, and Australia.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued argentojarosite specimens are those that possess well-formed and lustrous crystals of an intense, yellowish-brown color, visible under slight magnification. Contrast with a light matrix rock (e.g., quartz) significantly enhances the specimen's attractiveness. Crystal size is a key factor – the larger they are, the more valuable the specimen. The abundance of crystals on the matrix also matters. ## Popular Localities Specimens from the historical, type locality – the Tintic Standard mine in Utah, USA – are by far the most sought after by collectors. They are considered the global standard for this mineral.

Care and storage

## Cleaning Argentojarosite specimens are very delicate and sensitive to water. Cleaning should be limited to an absolute minimum. Only gentle dusting with a soft brush to remove dust is permissible. Contact with water should be avoided, as it can damage or dissolve the fine crystals. ## What to Avoid Water, ultrasonic cleaners, and all chemical agents should be strictly avoided. The mineral is sensitive to changes in temperature and humidity. It should not be exposed to direct sunlight, which can cause it to fade. ## Storage Specimens should be stored in stable conditions, preferably in closed collector boxes (so-called "membrane boxes") or in display cases away from dust and moisture. Due to its brittleness, vibrations and contact with harder minerals should be avoided.

External references

Sources

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