Ammonioalunite
Chemical formula: N<sup>3-</sup>H<sub>4</sub>Al<sub>3</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>
A rare aluminum and ammonium sulfate from the alunite group, most often forming white, earthy aggregates in areas of volcanic activity.
Properties
- Mohs hardness
- 2.5-3
- Luster
- Vitreous to Earthy
- Streak
- White
- Density
- 2.81
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven to Conchoidal
- Transparency
- Translucent to Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Identifying ammonioalunite based on visual characteristics is difficult. Its environment of occurrence (fumaroles, burning coal dumps) is key. It is recognized by its white color, low hardness, and often earthy form. Definitive identification requires advanced research methods, such as X-ray diffraction (XRD) or chemical analysis to confirm the presence of the ammonium ion. ## Distinguishing from similar minerals It can be confused with other white sulfate or clay minerals. The most similar is alunite, from which it is practically impossible to distinguish in field conditions – this requires chemical analysis (detection of potassium in alunite or ammonia in ammonioalunite). It differs from gypsum by its slightly higher hardness. Kaolinite and other clay minerals have a different genesis and often a different aggregate structure. ## Crystal forms Crystals are rare and microscopic, with a rhombohedral or tabular habit. Most often, it forms compact, earthy, porous aggregates or crusts.
Geological environment
## Genesis Ammonioalunite is a low-temperature hydrothermal mineral. It forms as a result of fumarolic and solfataric activity, where acidic, sulfur- and ammonia-rich volcanic gases react with aluminum-rich surrounding rocks. It is also found in the oxidation zones of sulfide deposits and in burning coal dumps, where ammonia is produced from the decomposition of organic matter. ## Mineral associations It often co-occurs with other sulfate minerals and volcanic exhalation products, such as native sulfur, alunogen, tschermigite, mascagnite, gypsum, and clay minerals. ## Localities The most important localities worldwide include The Geysers in California (USA), where it was discovered. It also occurs in the Vesuvius volcano area and in the Phlegraean Fields (Italy), in the Alcaparrosa mine (Chile), in burning coal dumps in Pennsylvania (USA), in Pecs (Hungary), and in Čadca (Slovakia).
Rarity
Rare
For collectors
## Quality criteria As a rare mineral, ammonioalunite is primarily valued by systematic collectors and specialists. Specimens from a well-defined locality, especially from the type locality, are most highly prized. Although well-formed crystals are extremely rare and highly desirable, in collecting practice, rich, clean aggregates with a characteristic form for a given locality, often in association with other rare fumarolic minerals, are valued. ## Popular localities Specimens from The Geysers in California are classics and have historical value. Samples from burning coal dumps in Pennsylvania and from European localities, such as the Naples area in Italy, are also sought after.
Care and storage
## Cleaning Ammonioalunite specimens, especially those with an earthy structure, are delicate and potentially soluble. They should only be dry-cleaned, using a soft brush to remove dust. Avoid contact with water, which can damage or dissolve the mineral. ## What to avoid Absolutely avoid water, ultrasonic cleaners, and all chemical agents, including acids. The mineral is soft, so it must be protected from scratching by harder minerals. It is sensitive to high temperatures. ## Storage It is recommended to store specimens in a dry place, in closed containers or display cases, to protect them from moisture and dust. It should be separated from other minerals to avoid mechanical damage.