Guérinite

Chemical formula: Ca₆(HAs⁵⁺O₄)₃(As⁵⁺O₄)₂·₁₀.₅H₂O

Guérinite is a rare, hydrated calcium arsenate, forming delicate, colorless crystals and aggregates with a pearly luster.

## Characteristics Guérinite is a rare mineral from the arsenate group, chemically classified as a hydrated calcium arsenate. It occurs as small, bladed or acicular crystals, which often form radial, spherulitic, or fibrous aggregates. Specimens are typically colorless and transparent to translucent, and their delicate nature makes them brittle. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1.5. It exhibits a pearly luster on cleavage surfaces and a vitreous luster on other surfaces. Its density is approximately 2.68 g/cm³. It is a very brittle mineral. ## Colors and Varieties Guérinite is usually colorless. No colored or commercial varieties are distinguished. ## History and Name The mineral's name honors René Guérin (1894-1956), a French mineral collector who first found this mineral. It was described as a new mineral species in 1961 by Hubert Struk and Eva-Marie Walitzi. The type locality, from which the first studied specimens originated, is the Daniel mine in Schneeberg, Saxony (Germany). ## Uses Due to its rarity, small crystal size, and brittleness, guérinite has no industrial applications. It is of purely scientific interest and is sought after by specialized collectors of rare minerals.

Properties

Mohs hardness: 1.5
Luster: Pearly, Vitreous
Streak: White
Density: 2.68
Cleavage: Three directions of cleavage mentioned, but not described.
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Characteristic features of guérinite include its very low hardness (1.5 on the Mohs scale), pearly luster, occurrence in delicate, radial aggregates, and paragenesis with other arsenates in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals It can be confused with other secondary, white arsenates or sulfates, such as pharmacolite or gypsum. It differs from pharmacolite in crystal habit (guérinite forms blades, pharmacolite forms needles and coatings). It is distinguished from gypsum by its significantly higher density and co-occurrence with arsenic minerals. ## Crystal Forms Guérinite forms small, bladed or acicular crystals, most commonly occurring as radial or spherulitic aggregates. Fibrous or rosette-like groupings are less common.

Geological environment

## Genesis Guérinite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, especially in the presence of calcium-rich rocks (e.g., calcite, dolomite). It forms as a result of reactions between percolating waters and primary arsenic minerals. ## Mineral Associations It most commonly co-occurs with other calcium arsenates, such as pharmacolite, picropharmakolite, and sainfeldite. Other minerals from the oxidation zone, such as erythrite, annabergite, as well as calcite and gypsum, may also accompany it. ## Localities The most important occurrences of guérinite are in Europe. Besides the type locality in Schneeberg, Germany, it is also known from Jáchymov in the Czech Republic and from the Gabe Gottes mine in Sainte-Marie-aux-Mines, France. It also occurs in several locations in Japan.

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the size and richness of crystal aggregates are most important. The most valued specimens are those where the radial or spherulitic groupings of guérinite are well-formed, undamaged, and clearly stand out from the host rock. Due to the mineral's colorless nature, contrast with a darker matrix enhances the specimen's appeal. ## Popular Localities The most classic and highly prized guérinite specimens by collectors come from historical localities in Europe, such as Schneeberg in Germany and Jáchymov in the Czech Republic. Specimens from these locations have historical value and serve as a reference point for this mineral.

Care and storage

## Cleaning Due to its extreme brittleness and softness, guérinite is practically unsuitable for mechanical cleaning. Contact with water, which can damage it, should be avoided. The safest way to remove dust is with a soft brush or by carefully blowing air with a photographic air blower. ## What to Avoid Absolutely avoid contact with water and other liquids, ultrasound, chemicals, and sudden temperature changes. The mineral is very soft and brittle, susceptible to scratches and mechanical damage even with the lightest touch. As an arsenate, mineral dust is toxic – inhalation should be avoided. ## Storage Guérinite specimens should be stored in stable conditions, in closed, padded collector boxes, away from light, humidity, and vibrations. It should be isolated from other minerals to prevent abrasion and damage.

External references

https://www.mindat.org/min-1767.html