Ammoniovoltaite

Chemical formula: (N<sup>3-</sup>H<sub>4</sub>)<sub>2</sub>Fe<sup>2+</sup><sub>5</sub>Fe<sup>3+</sup><sub>3</sub>Al(S<sup>6+</sup>O<sub>4</sub>)<sub>12</sub>(H<sub>2</sub>O)<sub>18</sub>

Ammoniovoltaite is a rare sulfate from the voltaite group, distinguished by the presence of ammonium ions in its structure and the formation of small, vitreous crystals.

## Characteristics Ammoniovoltaite is a complex hydrated sulfate of iron, aluminum, and ammonium, belonging to the voltaite group. This mineral forms small, usually not exceeding 1 mm, cubic or octahedral crystals. It typically occurs as granular aggregates, coatings, and crusts on host rocks. Its appearance is characteristic of evaporation or weathering products. ## Physical Properties Ammoniovoltaite crystals are characterized by a vitreous luster. The mineral is relatively soft, with a Mohs hardness of about 2.5. It is also quite light, with a density close to 2.71 g/cm³. It is brittle and has an uneven fracture. ## Colors and Varieties Ammoniovoltaite most often ranges in color from dark green to almost black. The color depends on the ratio of iron ions in the ferrous and ferric oxidation states. No commercial or color varieties are distinguished. ## History and Name The mineral's name, given in 2010 by Igor V. Pekov and collaborators, refers to its chemical composition – the presence of ammonium ions (ammonio-) and its structural relationship with voltaite (-voltaite), with which it forms a series. It was discovered in a burning coal mine dump in Kopeysk, Chelyabinsk Oblast, Southern Urals, Russia. ## Uses Ammoniovoltaite has no industrial significance. Its occurrence is limited to specific geological environments, and specimens have only scientific and collector's value for rare mineral and micromineral collectors.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
Greenish gray
Density
2.71
Cleavage
None
Fracture
Uneven
Transparency
Translucent to Opaque
Crystal system
Cubic

Diagnostic features

## Identification Ammoniovoltaite is recognized by its dark green to black color, vitreous luster, and characteristic form of small, regular crystals. The key is its occurrence environment – products of fumarolic activity, coal mine dump fires, or sulfide weathering. ## Distinguishing from Similar Minerals It can be confused with other minerals from the voltaite group, such as voltaite or ammoniojarosite. Definitive differentiation requires advanced chemical analyses (EDS/WDS) to confirm the presence and proportions of ammonium, iron, and aluminum ions. Visually, it is difficult to distinguish from other dark secondary sulfates. ## Crystal Forms It forms well-developed, isometric crystals with cubic, octahedral habits, and their combinations. It also occurs as granular aggregates and powdery coatings.

Geological environment

## Genesis Ammoniovoltaite is a secondary mineral, forming under specific conditions. It forms as a sublimation product in volcanic exhalations (fumaroles) or, more commonly, as a result of fires in coal mine dumps. It can also crystallize in the oxidation zones of sulfide deposits, where acidic waters rich in sulfates react with organic matter providing ammonia. ## Mineral Associations It most often co-occurs with other secondary sulfates, such as voltaite, tschermigite, mascagnite, boussingaultite, as well as native sulfur and sal ammoniac. ## Localities The most important and well-documented occurrences of this mineral are: - Burning dump of coal mine No. 45 in Kopeysk, Chelyabinsk basin, Russia (type locality). - Fumaroles of Vesuvius and La Fossa volcano on Vulcano Island, Italy. - Anna Mine in Alsdorf, North Rhine-Westphalia, Germany.

Rarity

Very rare

For collectors

## Quality Criteria The quality of ammoniovoltaite specimens is primarily assessed based on the size and perfection of crystal development. The most desirable are sharp, lustrous, single crystals or groups of crystals distinctly set on a contrasting matrix. Due to the microscopic nature of most occurrences, even millimeter-sized crystals are considered valuable. The dark green color is typical and not a primary evaluation criterion. ## Popular Localities Specimens from the type locality in Kopeysk (Russia) and from Italian volcanoes (Vesuvius, Vulcano) are the most classic and valued by collectors specializing in rare minerals or fumarolic minerals.

Care and storage

## Cleaning Due to its water solubility, the mineral must not be cleaned with water. Only gentle dust removal with a soft brush or compressed air from a safe distance is permissible. ## What to Avoid Contact with water, water vapor, and any liquids that could dissolve it must be strictly avoided. The mineral is sensitive to high humidity, which can lead to its slow degradation. It should be protected from temperature changes and direct sunlight. ## Storage Ammoniovoltaite specimens must be stored in dry conditions. The best solution is a sealed, airtight container (a "perky box") with a moisture-absorbing material (e.g., silica gel). Display should be in enclosed display cases, away from sources of moisture.

Sources

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