Phaunouxite

Chemical formula: Ca₃(As₅+O₄)₂·₁₁H₂O

Phaunouxite is a hydrated calcium arsenate, forming colorless or white, silky aggregates of acicular crystals, found in the oxidation zones of arsenic deposits.

## Characteristics Phaunouxite is a hydrated calcium arsenate that visually presents as delicate, silky, or dull aggregates. It most often forms aggregates composed of very fine, acicular or hair-like crystals, which can form radial rosettes, coatings, or efflorescences on the host rock. Individual crystals are usually too small to be seen with the naked eye, and the material in mass has a chalky or fibrous appearance. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. The crystals are flexible. It has a low density, around 2.16 g/cm³. The luster of phaunouxite is described as silky for fibrous aggregates or dull and earthy for fine-grained aggregates. It is translucent to opaque. ## Colors and Varieties Phaunouxite is typically colorless or white. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Gabe Gottes mine in the Phaunoux valley, in the town of Sainte-Marie-aux-Mines (Markirch) in Alsace (France). It was described and approved as a new mineral species in 1980 by Nelly Bariand and François Pillard. ## Uses Phaunouxite has no industrial application. It is solely a mineral of scientific and collector's interest, sought after by specialists collecting rare arsenates or minerals from specific localities.

Properties

Mohs hardness: 2
Luster: Silky
Streak: White
Density: 2.16
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Triclinic

Diagnostic features

## Identification Phaunouxite is most easily recognized by its characteristic mode of occurrence – delicate, white, silky, or chalky aggregates of acicular crystals. Its low hardness, flexibility of fibers, and occurrence in paragenesis with other arsenates are key diagnostic features. ## Distinguishing from Similar Minerals Phaunouxite can be confused with other secondary calcium arsenates, such as **rauenthalite** (Ca₃(AsO₄)₂·10H₂O) or **haidingerite** (Ca(AsO₃OH)·H₂O). Rauenthalite is a dehydration product of phaunouxite and has a very similar appearance. Distinguishing these minerals without advanced analytical methods (like XRD) is practically impossible. Haidingerite usually forms better-developed, vitreous tabular crystals, rather than acicular aggregates. ## Crystal Forms Phaunouxite crystals are elongated, acicular, or hair-like, often with a bladed cross-section. They typically occur as tangled fibrous aggregates, radial aggregates, rosettes, or as fine efflorescences and coatings.

Geological environment

## Genesis Phaunouxite is a secondary mineral, formed in the oxidation (weathering) zones of polymetallic ore deposits rich in arsenic. It forms as a result of the reaction of calcium-rich waters with primary arsenic minerals, such as arsenopyrite or löllingite, under low temperature and pressure conditions. ## Mineral Associations This mineral co-occurs with other secondary arsenates. At the type locality (Sainte-Marie-aux-Mines), it was found in association with pharmacolite, picropharmacolite, rauenthalite, haidingerite, native arsenic, and gypsum. ## Localities The most important and classic locality for phaunouxite is the Gabe Gottes mine in Sainte-Marie-aux-Mines in the Vosges (Alsace, France). This is its type locality. Beyond France, its occurrence has also been reported in Jáchymov in the Czech Republic and in the Sterling Hill mine in Ogdensburg (New Jersey, USA).

Rarity

Rare

For collectors

## Quality Criteria For collectors, the most valuable specimens are those with well-defined, radial aggregates or rich, silky coatings on a contrasting rock matrix. Specimens from the type locality, Sainte-Marie-aux-Mines, are also highly prized. Due to the microscopic size of the crystals, their individual form is less important than the overall aesthetics and richness of the entire aggregate. ## Popular Localities Specimens from the historic type locality – the mine in Sainte-Marie-aux-Mines, France – are by far the most valued and sought after by collectors. Material from this location has the greatest historical and scientific value.

Care and storage

## Cleaning Phaunouxite specimens are extremely delicate and sensitive to water. Wet cleaning should be avoided, as water can dissolve or damage the fine crystals. The safest method is to gently remove dust with a soft brush or carefully use a photographic blower (air bulb). ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is sensitive to changes in humidity – in dry air, it can dehydrate and transform into rauenthalite, losing its original structure. It should be protected from high temperatures and direct sunlight. ## Storage It is recommended to store specimens in airtight, sealed containers (e.g., "membrane boxes" or boxes with cotton) to stabilize humidity and protect against mechanical damage. Due to its arsenic content, hands should be washed after handling the mineral.

Sources

IMA Mineral List (CNMNC)