Cuatrocapaite-(NH_4_)

Chemical formula: (N<sup>3-</sup>H<sub>4</sub>)<sub>3</sub>(NaMg&#9744;)(As<sup>3+</sup><sub>2</sub>O<sub>3</sub>)<sub>6</sub>Cl<sub>6</sub>·16H<sub>2</sub>O

Cuatrocapaite-(NH₄) is an extremely rare ammonium, sodium, and magnesium arsenite, forming colorless, hexagonal crystals in the Torrecillas mine in Chile.

## Characteristics Cuatrocapaite-(NH₄) is a complex, hydrated arsenite with chlorine, ammonium, sodium, and magnesium. It occurs as small, colorless, and transparent crystals with a hexagonal outline. It typically forms thin, tabular or short, prismatic habits. Due to the microscopic size of the crystals, this mineral is almost exclusively available as micromounts. ## Physical Properties The crystals are characterized by a vitreous luster and are fully transparent. The hardness of the mineral has not been determined due to the small size and rarity of samples. The density calculated based on chemical composition and crystal structure is approximately 3.05 g/cm³. ## Colors and Varieties The mineral is colorless. No colored varieties or trade names are known. ## History and Name The mineral's name comes from the mining claim "Cuatro Capas" (Spanish for "Four Layers"), where its type locality is situated. The -(NH₄) suffix in the name indicates the dominant role of the ammonium ion in its structure. The mineral was officially approved by the International Mineralogical Association (IMA) in 2019. Its type locality is the Torrecillas mine in the Tarapacá Region, Chile. ## Uses Cuatrocapaite-(NH₄) has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals.

Properties

Luster
Vitreous
Streak
White
Density
3.05
Cleavage
Perfect on {0001}
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of cuatrocapaite-(NH₄) is extremely difficult and requires advanced analytical methods, such as X-ray diffraction (XRD) and X-ray microanalysis (EDS/WDS). A preliminary indication for a specialist may be the hexagonal shape of the crystals, their colorless nature, and co-occurrence with other rare arsenites from the type locality. ## Distinguishing from Similar Minerals It can be confused with other colorless secondary minerals occurring in the same paragenesis, such as torrecillasite. Definitive differentiation is impossible without specialized laboratory tests. ## Crystal Forms It forms tabular or short prismatic crystals with a hexagonal outline, occurring individually or in small groups on the rock matrix.

Geological environment

## Genesis This is a secondary mineral, formed in the oxidation zone of polymetallic hydrothermal deposits under extremely arid climatic conditions. It is believed that the source of ammonium ions (NH₄) necessary for its crystallization was nearby guano deposits. ## Mineral Associations Cuatrocapaite-(NH₄) co-occurs with other rare minerals, such as torrecillasite, magnesiocoritnigite, native arsenic, as well as annabergite and gypsum. ## Localities The only confirmed occurrence of this mineral in the world is its type locality – the Torrecillas mine, located in the Salar Grande area, in the Iquique Province (Tarapacá Region) in northern Chile.

Rarity

Extremely rare

For collectors

## Quality Criteria For a micromount mineral like cuatrocapaite-(NH₄), collector value is primarily determined by the quality and development of the crystals. Specimens with sharp, undamaged, transparent hexagonal crystals, preferably set on a contrasting matrix and accompanied by other rare minerals, are most highly prized. ## Popular Localities As the mineral is known from only one location in the world, all collector specimens originate from the Torrecillas mine in Chile.

Care and storage

## Cleaning The mineral is likely water-soluble, so all contact with liquids should be avoided. Cleaning is highly inadvisable. If necessary, a stream of compressed air or a very soft, dry brush can be carefully used. ## What to Avoid Contact with water, chemical agents, and high humidity must be strictly avoided. The mineral is sensitive to temperature changes, which can lead to its dehydration. As an arsenic compound, it is toxic – avoid inhaling dust and direct skin contact. ## Storage Specimens should be stored in a sealed, dry container, preferably in a micromount box, away from light sources and temperature fluctuations. The use of a desiccant is recommended.

Sources

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