Picropharmacolite

Chemical formula: Ca₄Mg(As⁵⁺O₃OH)₂(As⁵⁺O₄)₂·₁₁H₂O

Picropharmacolite is a hydrated calcium and magnesium arsenate, forming characteristic, silky aggregates of acicular or hair-like crystals.

## Characteristics Picropharmacolite is a hydrated calcium and magnesium arsenate, belonging to the pharmacolite group. It most commonly occurs as delicate, flexible aggregates with a radial, spherulitic, or fibrous structure. It also forms small, acicular or hair-like crystals, which can form spherical aggregates or coatings and efflorescences on other minerals. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1-2, meaning it can be scratched with a fingernail. It is light, with a density of approximately 2.55 g/cm³. Crystals and fibers are flexible. The luster is usually silky, and on fracture surfaces, it can be pearly. It is transparent to translucent. ## Colors and Varieties Picropharmacolite is usually white, less commonly colorless. No colored or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1819 by D.L.G. Karsten, refers to its chemical composition and appearance. The Greek word "pikros" (πικρός) means "bitter," referring to the presence of magnesium (which was part of bitter-tasting salts), and "pharmakon" (φάρμακον) means "poison," an allusion to its arsenic content. The suffix "-lite" comes from the word "lithos" (λίθος), meaning "stone." It was discovered in the dumps of the former smelter in Richelsdorf, Hesse (Germany). ## Uses Due to its rarity, small size, and low hardness, picropharmacolite has no industrial applications. It is solely an object of interest for collectors of rare minerals.

Properties

Mohs hardness: 1-2
Luster: Pearly, Silky
Streak: White
Density: 2.55
Cleavage: On {100} and {010}, perfect.
Fracture: Fibrous,Micaceous
Transparency: Transparent,Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Picropharmacolite is most easily recognized by its characteristic, silky aggregates of small, white, acicular or hair-like crystals, often forming spherical aggregates. Its extremely low hardness (1-2 on the Mohs scale) and the flexibility of its fibers are key diagnostic features. It occurs in the oxidation zones of arsenic-rich ore deposits. ## Distinguishing from Similar Minerals Picropharmacolite is sometimes confused with other secondary arsenates, such as pharmacolite (which does not contain magnesium) or gypsum (which has a different crystal form and is a sulfate). It is distinguished from gypsum by its silky luster and often radial aggregate structure. Accurate differentiation from other rare arsenates with similar appearances (e.g., haidingerite) requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms Crystals are very small, acicular or hair-like, elongated along the [001] axis. They typically form radial, spherical, or fibrous aggregates, as well as delicate coatings and crusts.

Geological environment

## Genesis Picropharmacolite is a secondary mineral, forming in the oxidation (weathering) zones of arsenic-rich ore deposits, especially in carbonate rocks (limestones, dolomites). It forms as a result of the reaction of arsenic-containing solutions with calcium and magnesium minerals. ## Mineral Associations It co-occurs with other arsenic minerals, such as pharmacolite, erythrite, annabergite, roselite, as well as with calcite, dolomite, and gypsum. ## Localities The most important occurrences of picropharmacolite include the historical type locality in Richelsdorf (Hesse, Germany) and mines in the Bou Azzer region in Morocco. It is also known from Jáchymov and Příbram in the Czech Republic, the Sterling Hill mine in New Jersey (USA), as well as from Saxony in Germany and the Attica region in Greece.

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, spherical aggregates of radial crystals, set on a contrasting rock matrix. The size and aesthetics of these aggregates, as well as the absence of damage to the delicate fibers, are important. Specimens from classic, historical localities, such as Richelsdorf, are particularly sought after. ## Popular Localities The best quality specimens, forming striking white "balls" on the substrate, come from mines in the Bou Azzer region in Morocco. These are the standard for this mineral in the collector's market.

Care and storage

## Cleaning Picropharmacolite specimens are extremely delicate and fragile. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove loose dust, or the specimen can be carefully blown with compressed air from a safe distance. Avoid contact with water, which can damage fibrous aggregates. ## What to Avoid The mineral is sensitive to acids and other chemicals. It should be protected from moisture, high temperatures, and direct sunlight. Due to its extreme softness, any physical contact that could crush or damage the delicate crystals should be avoided. ## Storage It is recommended to store specimens in closed, padded collector boxes (e.g., "micromounts") to protect them from dust, mechanical damage, and changes in humidity. Display should only take place in sealed display cases.