Rauenthalite

Chemical formula: Ca<sub>3</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·10H<sub>2</sub>O

Rauenthalite is a hydrated calcium arsenate, forming colorless to white, minute crystals and coatings, found in old mine dumps.

## Characteristics Rauenthalite is a hydrated calcium arsenate, typically occurring as very small, acicular or bladed crystals, rarely exceeding 1 mm in length. These crystals form radial aggregates, rosettes, as well as coatings and efflorescences on rock surfaces. Due to the small size of the crystals, its macroscopic features are difficult to observe with the naked eye. ## Physical Properties This mineral is characterized by low hardness, approximately 2 on the Mohs scale, which means it is very soft and can be scratched with a fingernail. It has a vitreous luster. It is transparent to translucent. Its density is low, which is typical for hydrated minerals. ## Colors and Varieties Rauenthalite is usually colorless or white. No colored or commercial varieties have been distinguished. ## History and Name The mineral's name comes from its discovery locality – the dumps of the former mine in the Rauenthal valley, near Sainte-Marie-aux-Mines in the Vosges (France). It was described and approved as a new mineral species in 1965 by Pierrot. ## Uses Rauenthalite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or minerals from specific localities (so-called "systematists").

Properties

Mohs hardness
2
Luster
Vitreous
Streak
White
Density
2.45
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Rauenthalite is difficult to identify without specialized equipment. It is recognized by its characteristic, minute, acicular or bladed crystals forming radial aggregates. The locality of occurrence (specific sites) and co-occurrence with other arsenates are key. ## Distinguishing from Similar Minerals It can be confused with other secondary, hydrated arsenates, such as pharmacolite, sainfeldite, or gypsum. Pharmacolite forms similar aggregates but has different crystallographic parameters. Gypsum is much more common and has a different genesis. Final distinction requires chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal Forms It forms elongated, acicular or bladed crystals, often flattened. These crystals combine into radial, stellate, or fan-shaped aggregates, and also form thin crusts and coatings.

Geological environment

## Genesis Rauenthalite is a secondary mineral, forming in the oxidation zones (weathering) of ore deposits rich in arsenic, especially in the presence of calcium-rich rocks (e.g., limestones, dolomites). It forms as a result of reactions of percolating waters with primary arsenic minerals, such as arsenopyrite or löllingite. ## Mineral Associations It co-occurs with other secondary arsenic minerals, such as pharmacolite, picropharmakolite, sainfeldite, fluckite, gypsum, as well as with calcite and iron oxides. ## Localities The most important localities for this rare mineral include: - France: Rauenthal valley near Sainte-Marie-aux-Mines, Vosges (type locality). - Germany: St. Andreasberg mine in the Harz mountains; Schneeberg and Johanngeorgenstadt areas in Saxony. - Czech Republic: Jáchymov (St. Joachimsthal). - USA: Sterling Hill mine, Ogdensburg, New Jersey.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a rauenthalite specimen primarily depends on the abundance and development of its crystals. The most prized specimens are those with visible, well-formed, radial aggregates on a contrasting matrix. Due to the microscopic size of the crystals, the aesthetics of the entire aggregate are more important than individual crystals. Association with other rare arsenates is also important. ## Popular Localities The most classic and sought-after specimens by collectors come from the type locality in Sainte-Marie-aux-Mines, France, and from historical localities in Germany (Schneeberg, St. Andreasberg).

Care and storage

## Cleaning Rauenthalite specimens should be cleaned with utmost care, using only a soft brush to remove dust. The use of water is absolutely inadvisable, as the mineral is soluble in it. Compressed air can damage delicate crystals. ## What to Avoid Avoid contact with water and any chemicals. The mineral is sensitive to changes in humidity – in dry air, it can dehydrate and transform into phaunouxite, and then into haidingerite. It should be protected from high temperatures and direct sunlight. ## Storage Specimens should be stored in airtight, sealed containers (e.g., "membrane box" or "perky box") to maintain stable ambient humidity. Storage in dry conditions leads to its irreversible decomposition.

Sources

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