Ammoniojarosite

Chemical formula: N₃-H₄Fe₃+₃(S₆+O₄)₂(OH)₆

Ammoniojarosite is a rare mineral of the alunite group, an ammonium analogue of jarosite, most often forming yellow, earthy coatings and aggregates.

## Characteristics Ammoniojarosite is a hydrated iron and ammonium sulfate, belonging to the alunite supergroup. It is the ammonium analogue of the much more common jarosite. It most often occurs as earthy, powdery, or compact masses, as well as crusts and coatings. Well-formed crystals are rare and microscopic in size, taking the form of tablets or rhombohedra. Its color is characteristic and ranges from pale yellow to yellowish-brown. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 3.5. The luster of earthy aggregates is dull or earthy, while on the faces of rare crystals it can be vitreous. It is brittle, and its density is about 3.02 g/cm³, which is lower than that of most metal sulfates. It is usually opaque, although very small and thin crystals may be translucent. ## Colors and Varieties Ammoniojarosite occurs in various shades of yellow, from pale yellow and straw-yellow, through intense yellow, to dark yellowish-brown. There are no named color or commercial varieties. ## History and Name The mineral's name refers to its chemical composition - the presence of ammonium ions (Latin: *ammonium*) - and its structural similarity to jarosite. It was first described in 1902 by William F. Hillebrand and Samuel L. Penfield. The type locality (first finding) is given as an unspecified location in Utah, USA. ## Uses Ammoniojarosite has no industrial significance. It is solely an object of scientific and collecting interest, being a valuable acquisition for systematic collections.

Properties

Mohs hardness: 3.5
Luster: Earthy
Streak: Pale yellow
Density: 3.02
Cleavage: Distinct on {0001}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Trigonal

Diagnostic features

## Identification Characteristic features of ammoniojarosite include its intense yellow color, earthy or powdery habit, and specific occurrence environment, such as burning coal mine dumps. It is relatively soft and leaves a pale yellow streak. ## Distinguishing from Similar Minerals Ammoniojarosite is visually indistinguishable from **jarosite**. Definitive identification requires advanced chemical analyses (e.g., EDS) to confirm the presence of ammonium instead of potassium. It can be confused with **native sulfur**, which has a similar color but is much softer (1.5-2.5 on the Mohs scale) and occurs under different conditions. Fields of **limonite** are usually more brownish and have a darker, yellowish-brown streak. ## Crystal Forms Crystals, if present, are microscopic and belong to the trigonal system. They usually take the form of hexagonal tablets or small rhombohedra. Most often, it is found in the form of earthy, powdery, or compact aggregates, as well as crusts and coatings.

Geological environment

## Genesis Ammoniojarosite is a secondary mineral, forming in acidic and sulfate-rich environments, with the simultaneous presence of an ammonia source. Typical places of its formation are oxidation zones of sulfide deposits (especially pyrite) in shales containing organic matter. It often forms on and around burning coal dumps, where ammonia is produced by the thermal decomposition of organic matter. It is also found in volcanic exhalation products. ## Mineral Associations It often co-occurs with other sulfates, such as gypsum, melanterite, jarosite, and alunite. It is also accompanied by native sulfur, pyrite, marcasite, and clay minerals. ## Localities Important global localities include areas associated with coal mining. Significant specimens come from burning dumps in the Kladno region in the Czech Republic and from the vicinity of Wuda City in Inner Mongolia (China). In the USA, it occurs, among other places, in coal mines in Pennsylvania and in shales in Utah and Wyoming.

Rarity

Rare

For collectors

## Quality Criteria The quality of an ammoniojarosite specimen is primarily assessed based on the intensity and purity of its yellow color and the richness of the aggregate on the rock matrix. Specimens where the mineral forms thick, uniform crusts are most valued. Samples with macroscopic, well-formed crystals would be exceptionally sought after, though extremely rare. Association with other rare secondary minerals also increases its value. ## Popular Localities The most known and valued specimens by collectors come from classic localities associated with burning coal, such as Kladno in the Czech Republic and Wuda in China. These localities have provided rich, intensely colored samples that set the standard for this mineral.

Care and storage

## Cleaning Ammoniojarosite specimens are very delicate and should be handled with care. For dust removal, it is best to use a soft brush or a stream of compressed air from a safe distance. Wet cleaning should be avoided, as water can damage or dissolve delicate, earthy aggregates. ## What to Avoid This mineral is sensitive to chemicals, especially acids and bases. Ultrasonic cleaners should not be used, as they could destroy its brittle structure. It should be protected from moisture, prolonged contact with water, and sudden temperature changes. ## Storage It is recommended to store specimens in a dry place, in closed containers or display cases, to protect them from dust, moisture, and mechanical damage. "Perky boxes" are an ideal solution for smaller or fragile specimens.

Sources

IMA Mineral List (CNMNC)