Rösslerite

Chemical formula: Mg(As₅+O₃OH)·₇H₂O

Rösslerite is a rare, hydrated magnesium arsenate, forming colorless, acicular crystals or white, earthy coatings and crusts.

## Characteristics Rösslerite is a hydrated magnesium arsenate, most commonly occurring as delicate, acicular or fibrous crystals aggregated into radial clusters. It also forms earthy masses, loose coatings, and crusts. This mineral is colorless or white. Due to its delicate nature and water solubility, it is unstable in high humidity conditions and easily dehydrates into a less hydrated form. ## Physical Properties Rösslerite crystals are very soft, with a Mohs hardness of 2-3. They exhibit a vitreous or pearly luster on crystal surfaces, while earthy aggregates are dull. It is a light mineral, with a density of approximately 2.1 g/cm³. It is transparent to translucent. ## Colors and Varieties Rösslerite is typically colorless or white. No colored varieties or trade names are distinguished. ## History and Name The mineral was first described in 1861 by the German mineralogist Carl Friedrich Rammelsberg. The name comes from Karl Rössler (1788-1863), director of the mine in Bieber, Hesse (Germany), where the mineral was discovered. ## Uses Rösslerite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals and a subject of scientific research.

Properties

Mohs hardness: 2-3
Luster: Vitreous
Streak: White
Density: 2.1
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Rösslerite is most easily recognized by its typical forms of occurrence – delicate, acicular crystals forming radial aggregates or white, earthy coatings. A key diagnostic feature is its solubility in water and instability in dry air. It reacts with acids. ## Distinguishing from Similar Minerals It can be confused with other secondary arsenates or sulfates, such as pharmacolite or gypsum. From pharmacolite (Ca(AsO₃OH)·2H₂O), it differs in chemical composition (magnesium content instead of calcium) and often a more delicate structure. From gypsum, it is distinguished by its significantly lower hardness and reaction with acids. Precise identification requires advanced analytical methods (e.g., XRD, EDS). ## Crystal Forms Rösslerite crystals are typically elongated, acicular, or fibrous. They form characteristic radial or stellate aggregates. It also occurs as brittle crusts, coatings, and earthy masses.

Geological environment

## Genesis Rösslerite is a secondary mineral, formed in the oxidation zones (weathering) of ore deposits rich in arsenic, especially in the presence of magnesium-rich rocks (e.g., serpentinites, dolomites). It forms as a result of the reaction of arsenic acid-containing waters with magnesium minerals. ## Mineral Associations It co-occurs with other arsenates, such as pharmacolite, picropharmakolite, annabergite, erythrite, as well as with calcite, dolomite, and ore minerals (e.g., löllingite, skutterudite). ## Localities Important localities worldwide include the Bieber mine in Hesse and the Schneeberg area in Saxony (Germany), where it was discovered. It also occurs in Jáchymov (Czech Republic), Bou Azzer (Morocco), Cobalt in Ontario (Canada), and the Sterling Hill mine in New Jersey (USA).

Rarity

Rare

For collectors

## Quality Criteria The most prized rösslerite specimens are those with well-formed, long, acicular crystals forming aesthetic, radial aggregates on a contrasting rock matrix. The size and "freshness" of the aggregate are also important, as this mineral easily dehydrates and disintegrates. Specimens preserved in their original, unaltered form are rare and sought after. ## Popular Localities Classic and most valued specimens come from historical localities in Germany (Bieber, Schneeberg). Currently, specimens from Bou Azzer in Morocco also appear on the collector's market.

Care and storage

## Cleaning Rösslerite specimens should not be wet cleaned. The mineral is soluble in water and acids. To remove dust, a very soft brush or compressed air from a safe distance can be used with extreme caution. ## What to Avoid Contact with water, water vapor, and all chemicals must be strictly avoided. Rösslerite is unstable and easily loses water in dry air, transforming into less hydrated magnesium arsenates, which leads to crystal disintegration. It must be protected from changes in humidity and temperature. ## Storage Specimens should be stored in sealed, transparent containers (e.g., "membrane box" or "perky box") that protect them from mechanical damage and changes in ambient humidity. Storage in stable room conditions is crucial for preserving the integrity of the specimen.

Sources

IMA Mineral List (CNMNC)