Pharmacolite

Chemical formula: Ca(As⁵⁺O₃OH)·₂H₂O

Pharmacolite is a hydrated calcium arsenate, forming delicate, acicular crystals or silky coatings, often found in the oxidation zones of arsenic deposits.

## Characteristics Pharmacolite is a hydrated calcium arsenate belonging to the gypsum supergroup. It most commonly occurs as delicate, acicular or hair-like crystals, which can form radial aggregates, fibrous masses, as well as crusts and coatings with a silky luster. Well-formed, tabular or columnar crystals are less frequently encountered. It is a secondary mineral, formed as a result of the weathering of arsenic minerals. ## Physical Properties This mineral is very soft, with a Mohs hardness of 2-2.5, meaning it can be scratched with a fingernail. It has a relatively low density, approximately 2.53 g/cm³. Crystals exhibit perfect cleavage in one direction. The luster is usually vitreous, and in the case of fibrous aggregates, silky. It is transparent to translucent. ## Colors and Varieties Pharmacolite is most often colorless or white. It can take on grayish hues, and impurities may impart other, pale colorations. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral's name comes from the Greek word *pharmakon* (φάρμακον), meaning "poison," in reference to its toxic arsenic content. The mineral was described in 1800. The type locality (first finding place) was identified as the Sophia mine in Wittichen, Germany. ## Uses Due to its rarity, softness, and toxicity, pharmacolite has no industrial applications. It is solely an object of interest for mineral collectors.

Properties

Mohs hardness: 2-2.5
Luster: Vitreous, Silky
Streak: White
Density: 2.53
Cleavage: On {010}, perfect.
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Pharmacolite is most easily recognized by its characteristic forms of occurrence – delicate, acicular crystals forming silky aggregates and coatings. Its low hardness (2-2.5 on the Mohs scale), white color, silky or vitreous luster, and occurrence in association with other arsenic minerals are key diagnostic features. ## Distinguishing from Similar Minerals Pharmacolite is sometimes confused with other secondary arsenate or sulfate minerals of similar appearance, such as gypsum or erythrite (in its rare, white varieties). It is distinguished from gypsum by its genesis (arsenic-rich environment) and often more acicular form. Distinguishing it from other arsenates requires advanced chemical analysis for unambiguous identification. ## Crystal Forms It most often forms small, acicular or hair-like crystals gathered in radial or tangled aggregates. It also occurs as crusts, coatings, and earthy masses. Single, well-formed tabular or short-prismatic crystals are rarely observed.

Geological environment

## Genesis Pharmacolite is a secondary mineral, formed in the oxidation (weathering) zones of ore deposits rich in arsenic, especially those containing arsenopyrite, skutterudite, or smaltite. It forms as a result of reactions between waters percolating through rocks and primary arsenic minerals in the presence of calcium-rich rocks (e.g., limestones, dolomites). ## Mineral Associations It often co-occurs with other minerals from the oxidation zone, such as picropharmacolite, annabergite, erythrite, arsenopyrite, smaltite, chloanthite, as well as calcite and gypsum. ## Localities Important worldwide localities include historical sites in Germany (Wittichen, Schneeberg, Annaberg in Saxony), the Czech Republic (Jáchymov), France (Sainte-Marie-aux-Mines), and the USA (Sterling Hill, New Jersey). It is found in many places around the world where arsenical deposits are weathering.

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, macroscopic, acicular crystals forming attractive, radial aggregates on a contrasting rock matrix. Rich, silky coatings covering a larger rock surface are also highly valued. Purity (lack of impurities) and aesthetic arrangement on the matrix significantly increase the specimen's value. ## Popular Localities Classic, historical specimens come from German and Czech silver and cobalt mines, such as those in the vicinity of Schneeberg, Annaberg (Germany), or Jáchymov (Czech Republic). These are considered the most desirable by collectors due to their historical significance and often good quality.

Care and storage

## Cleaning Pharmacolite is a very delicate, soft, and partially water-soluble mineral. Cleaning should be kept to an absolute minimum. For dust removal, it is best to use a soft brush or carefully compressed air from a safe distance. Avoid contact with water, which can damage or dissolve the crystals. ## What to Avoid This mineral is sensitive to moisture, water, and chemicals. Ultrasonic cleaners, chemical agents, acids, and detergents should be strictly avoided. Due to its arsenic content, avoid inhaling dust and wash hands after any contact with the specimen. ## Storage Pharmacolite specimens should be stored in a dry place, preferably in a closed, airtight display box, to protect them from dust, moisture, and mechanical damage. It should be separated from harder minerals that could scratch it. Due to its toxicity, it should be stored out of reach of children and pets.