Ferrinatrite

Chemical formula: Na₃Fe₃+(S₆+O₄)₃·₃H₂O

Ferrinatrite is a rare, water-soluble sodium and iron sulfate, forming characteristic bright yellow efflorescences and acicular crystals.

## Characteristics Ferrinatrite is a hydrated sodium and iron sulfate, typically occurring as fine-grained or fibrous aggregates, as well as efflorescences and crusts. Less commonly, it forms small, acicular or capillary crystals grouped into radial aggregates. Due to its water solubility, it is an unstable mineral, found exclusively in dry, desert climates. ## Physical Properties Ferrinatrite is a relatively soft mineral, with a hardness of approximately 2.5 on the Mohs scale. It has a silky or vitreous luster. It is light, with a density of about 2.55 g/cm³. Its crystals, though rare, exhibit perfect cleavage in one direction. ## Colors and Varieties This mineral is characterized by a bright yellow, greenish-yellow, or grayish-white color. It does not form color varieties or commercial types. ## History and Name The name ferrinatrite comes from the Latin words *ferrum* (iron) and *natrium* (sodium), referring to its chemical composition. The mineral was first described in 1889 by Arturo A. Issel based on samples from the Sierra Gorda mine in the Antofagasta region of Chile. ## Uses Ferrinatrite has no industrial applications. It is solely of interest to collectors specializing in rare minerals and scientists studying sulfide weathering processes in desert conditions.

Properties

Mohs hardness: 2.5
Luster: Vitreous
Streak: Pale yellow
Density: 2.53-2.60
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Trigonal

Diagnostic features

## Identification A key diagnostic feature of ferrinatrite is its complete solubility in water, which can be easily tested on a small fragment. Its characteristic bright yellow color, occurrence as efflorescences and fibrous aggregates in dry, desert environments, often in association with other soluble sulfates, are also distinctive. ## Distinguishing from Similar Minerals Ferrinatrite can be confused with other secondary iron sulfates, such as jarosite or copiapite. - **Jarosite** (KFe₃(SO₄)₂(OH)₆) is insoluble in water and harder. - **Copiapite** (Fe²⁺Fe³⁺₄(SO₄)₆(OH)₂·20H₂O) has a similar appearance, but often a more intense, sulfur-yellow color and is also water-soluble, making differentiation difficult without chemical analysis. Ferrinatrite often forms more acicular crystals. - **Native sulfur** has a similar color but is much lighter, more brittle, and has a characteristic odor when rubbed. ## Crystal Forms Ferrinatrite crystallizes in the trigonal system. It is most commonly found in fine-grained, powdery, or fibrous masses. Rarely, it forms well-developed, small crystals with an acicular or capillary habit, often grouped into radial or chaotic aggregates.

Geological environment

## Genesis Ferrinatrite is a secondary mineral, forming in the oxidation zones (gossans) of ore deposits rich in iron sulfides (mainly pyrite). It forms as a result of the weathering of these minerals in extremely dry, desert climatic conditions. It can also form as a sublimation product from volcanic gases (fumaroles) or as an evaporite in drying, sulfate-rich lakes. ## Mineral Associations It most commonly co-occurs with other soluble sulfates, such as metavoltine, copiapite, coquimbite, tamarugite, as well as gypsum, halite, and native sulfur. ## Localities The most important and classic localities for ferrinatrite are in the Atacama Desert in Chile, particularly in the Antofagasta (e.g., Sierra Gorda, Chuquicamata mines) and Tarapacá regions. It also occurs in dry regions of Argentina, Peru, Bolivia, and in the state of Utah in the USA. It is also known from volcanic fumaroles, e.g., on Vesuvius in Italy.

Rarity

Rare

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, acicular crystals forming attractive, radial aggregates. An intense, pure, lemon-yellow color is highly prized. The aesthetic composition on the matrix and the absence of mechanical damage are also important, which is difficult to achieve due to the mineral's fragility. Specimens from classic localities in Chile are particularly sought after. ## Popular Localities The best collector specimens, with distinct crystals, come from the copper mines in the Atacama Desert in Chile, such as Chuquicamata and Sierra Gorda. Most of the material available on the market originates from there.

Care and storage

## Cleaning This mineral **must not be cleaned wet**. Any contact with water, even a damp cloth, will cause it to dissolve and be destroyed. Only very careful dust removal with a soft, dry brush or a stream of compressed air from a distance is permissible. ## What to Avoid Water, high humidity, water vapor, alcohol, and all other liquids must be strictly avoided. Storage in a damp room (e.g., a basement) or fluctuations in humidity can lead to deliquescence (melting) and destruction of the specimen. Direct sunlight and high temperatures should also be avoided. ## Storage Ferrinatrite specimens must be stored in hermetically sealed containers (e.g., membrane boxes, airtight display cases) with a desiccant (silica gel). This is absolutely crucial for preserving the mineral. Display should only take place in a sealed cabinet with controlled humidity.

Sources

IMA Mineral List (CNMNC)