Natrojarosite

Chemical formula: NaFe₃+₃(S₆+O₄)₂(OH)₆

Natrojarosite is a sodium iron hydroxy-sulfate of the alunite group, most commonly forming earthy aggregates and coatings of yellowish-brown color in the oxidation zones of ore deposits.

## Characteristics Natrojarosite is a secondary mineral belonging to the alunite supergroup, being the sodium analogue of the more widespread jarosite. It most often occurs as earthy, powdery, or compact masses, as well as crusts and concretions. It rarely forms crystals visible to the naked eye, which are usually microscopic, platy or rhombohedral forms with trigonal symmetry. Its appearance is usually inconspicuous, resembling argillaceous or ocherous coatings on rocks. ## Physical Properties This mineral is characterized by low hardness, ranging from 2.5 to 3.5 on the Mohs scale, making it brittle and easily scratched. The luster varies from vitreous on crystal faces to dull or earthy in the case of aggregates. It is a translucent mineral, becoming opaque in compact aggregates. Its density ranges from 3.16-3.28 g/cm³. ## Colors and Varieties Natrojarosite ranges in color from light yellow, through yellowish-brown, to dark brownish. The coloration depends on the degree of crystallinity and the presence of any impurities. There are no named commercial or color varieties. ## History and Name The mineral's name refers to its chemical composition and structural similarity to jarosite. The prefix "natro-" comes from the Latin word *natrium* (sodium), indicating the dominance of this element. The mineral was first described in 1902 by Waldemar Hillebrand and Samuel Penfield based on samples found in Soda Springs Valley in Mineral County, Nevada (USA). ## Applications Natrojarosite has no significant industrial application. It is mainly of interest to collectors specializing in secondary and systematic minerals, as well as to scientists. Its presence can be an indicator of specific geochemical conditions in the weathering zones of sulfide deposits.

Properties

Mohs hardness: 2.5-3.5
Luster: Vitreous to dull or earthy
Streak: Pale yellow
Density: 3.16-3.28
Cleavage: Distinct on {0001}
Fracture: Uneven to conchoidal
Transparency: Translucent to opaque
Crystal system: Trigonal

Diagnostic features

## Identification Characteristic features of natrojarosite include its yellowish-brown color, most often earthy or powdery habit, and occurrence in oxidation zones (iron caps) of ore deposits. It is soft and leaves a pale yellow streak. However, certain visual identification is very difficult. ## Differentiation from Similar Minerals Macroscopic differentiation of natrojarosite from other minerals of the jarosite group (e.g., potassium jarosite) is practically impossible without advanced chemical (e.g., EDS) or X-ray (XRD) analyses. It can also be confused with other secondary iron minerals, such as goethite or "limonite," which, however, usually have a darker, brownish-yellow streak. Beudantite, which often occurs in the same environments, can also be similar. ## Crystal Forms Natrojarosite crystals are usually microscopic and have a platy habit, often with trigonal or hexagonal outlines, as well as rhombohedral. However, it most often forms compact, earthy or powdery aggregates, crusts, and fillings of small rock fissures.

Geological environment

## Genesis Natrojarosite is a secondary mineral, formed by the oxidation of sulfide iron minerals (mainly pyrite) in the presence of sodium-rich solutions. It forms under strongly acidic and oxidizing conditions, typical of weathering zones (gossans, iron caps) of ore deposits, especially in dry and semi-arid climates. It can also crystallize from volcanic exhalations (fumaroles) and as a product of post-mining activity in old mines. ## Mineral Associations This mineral often co-occurs with other sulfates and oxyhydroxides formed under similar conditions. Its most common associations include: jarosite, goethite, hematite, alunite, beudantite, anglesite, cerussite, and quartz. ## Localities Important localities worldwide include the type locality in Soda Springs Valley (Nevada, USA) and the Chuquicamata deposit in Chile, from which large quantities of this mineral originate. It is also known from mines in Arizona and Utah (USA), from the Laurion complex in Greece, and from many other oxidation zones of deposits worldwide.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most valuable specimens are those with well-formed, even if microscopic, crystals with distinct luster and intense color. The attractiveness is enhanced by the placement of crystals on a contrasting rock matrix. In the case of massive aggregates, purity and intensity of color are important. Since the mineral often requires analysis for certain identification, specimens with analytically confirmed composition are more highly valued by systematic collectors. ## Popular Localities Specimens with sharp microcrystals come, among others, from the Apex mine in Utah (USA) and from the historic mines in Laurion, Greece. Massive, rich aggregates are known from the Chuquicamata deposit in Chile. The type locality in Nevada also provided material for research that ended up in collections worldwide.

Care and storage

## Cleaning Natrojarosite specimens, especially those with an earthy and porous structure, are very delicate. For dust removal, it is best to use a soft brush or compressed air from a safe distance. Contact with water should be minimized, and if necessary, distilled water should be used. After wet cleaning, the specimen must be thoroughly dried at room temperature. ## What to Avoid The mineral is sensitive to acids, which can decompose it. Ultrasonic cleaners, strong detergents, and sudden temperature changes should be avoided. As a relatively soft mineral, it is susceptible to scratches and mechanical damage. Prolonged exposure to moisture can lead to its slow degradation. ## Storage It is recommended to store specimens in a dry place, preferably in sealed boxes or display cases, which protects them from dust, moisture, and damage. Direct contact with harder minerals should be avoided. Well-formed microcrystals are best displayed in "micromount" boxes.