Pharmacosiderite

Chemical formula: KFe³⁺₄(As⁵⁺O₄)₃(OH)₄·₆-₇H₂O

Pharmacosiderite is a hydrated iron and potassium arsenate, forming characteristic cubic crystals with intense green or yellowish-brown colors.

## Characteristics Pharmacosiderite is a hydrated iron and potassium arsenate, belonging to the pharmacosiderite group. This mineral is prized by collectors for its well-formed, cubic crystals, which often occur as small, lustrous cubes. Crystals can be single, but more often form druses and crusts on the host rock. Its appearance is very characteristic, and the intensity of its color depends on impurities and the degree of hydration. ## Physical Properties Pharmacosiderite is a relatively soft mineral, with a Mohs hardness of 2.5. Its luster is typically vitreous, and on some surfaces, it can be resinous or greasy. It is a brittle mineral, with a density of approximately 2.9 g/cm³. Crystals are most often translucent, less commonly transparent or almost opaque. ## Colors and Varieties Typical colors of pharmacosiderite include a wide range of green shades – from emerald green, through olive green, to grass green. Yellow, honey-yellow, brown, and even reddish-brown specimens are also common. Some varieties may show a color change depending on the degree of hydration. No named commercial varieties are distinguished, and classification is primarily based on color and crystal habit. ## History and Name The name pharmacosiderite comes from the Greek words *pharmakon* (φάρμακον) meaning "poison" and *sideros* (σίδηρος) meaning "iron", which is a direct reference to its chemical composition – it is a poisonous iron arsenate. The mineral was first described by the German mineralogist Johann Friedrich Ludwig Hausmann in 1813. ## Uses Pharmacosiderite has no industrial application due to its rarity, small crystal size, and toxicity (arsenic content). Its significance is purely scientific and collectible.

Properties

Mohs hardness: 2.5
Color: Pale green, green, emerald-green, dark brown, honey-yellow to yellow-brown, brownish red, hyacinth-red; green, yellow, light brown in transmitted light (biaxially sectored).
Luster: Vitreous
Streak: white
Density: 2.797
Cleavage: Imperfect to good on {001}.
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Isometric

Diagnostic features

## Identification The most important diagnostic feature of pharmacosiderite is its crystal habit – it almost always forms distinct, small cubes. Its characteristic color (intense green, yellow, or brown) and occurrence in the oxidation zones of arsenic-rich ore deposits are also distinctive. ## Distinguishing from Similar Minerals Pharmacosiderite is sometimes confused with scorodite, which often co-occurs with it. However, scorodite forms orthorhombic crystals (often dipyramidal or tabular in habit), not isometric. It is distinguished from other green secondary minerals (e.g., mimetite, pyromorphite) primarily by its cubic crystal form. ## Crystal Forms Pharmacosiderite crystallizes in the isometric system, forming almost perfect cubes. Crystals are usually small, reaching several millimeters. They often occur as druses, which are dense aggregates of small crystals covering the rock surface. Granular or massive aggregates are less common.

Geological environment

## Genesis Pharmacosiderite is a secondary mineral, formed in the oxidation (weathering) zones of ore deposits that are rich in arsenic, especially arsenopyrite. It forms as a result of reactions of surface waters with primary minerals containing arsenic and iron. It is a typical mineral of the iron cap (gossan) zone. ## Mineral Associations This mineral often co-occurs with other arsenates and sulfates formed under similar conditions. The most common associated minerals include: scorodite, arsenopyrite, pyrite, pharmacoalumite, jarosite, goethite, and quartz. ## Localities Significant pharmacosiderite specimens come from many places around the world. In Europe, it is known from Cornwall (Great Britain), mines in the Sainte-Marie-aux-Mines region (France), and Saxony (Germany). Beautiful crystals have also been found in mines in the Atacama region in Chile, in Utah (USA), and in Tsumeb in Namibia.

Rarity

Not very common

For collectors

## Quality Criteria The most valued specimens by collectors are those with sharp, well-formed, lustrous crystals of intense, emerald-green color. Druses where crystals are densely packed and contrast with the color of the host rock are also highly regarded. Crystal size is a key factor – specimens with cubes larger than a few millimeters are rare and sought after. The absence of mechanical damage is also important. ## Popular Localities Classic, historical specimens come from tin mines in Cornwall (Great Britain). Currently, beautiful green crystals are supplied from the Majuba Hill mine in Nevada (USA). Specimens from the Clara mine in the Black Forest (Germany) are also valued for their crystal quality.

Care and storage

## Cleaning Pharmacosiderite specimens should be cleaned with the utmost care. It is best to use a soft brush to remove dust. If wet cleaning is necessary, use a small amount of distilled water and a very soft brush, then immediately dry the specimen with cool air (e.g., from a hairdryer on the lowest setting). Always wash hands after contact with the mineral. ## What to Avoid Pharmacosiderite is sensitive to chemicals, especially acids and bases. Ultrasonic cleaners and steam cleaning should be avoided. The mineral is sensitive to heat, which can cause dehydration and color change. It should not be exposed to prolonged strong sunlight. ## Storage Specimens should be stored in stable conditions, away from sources of heat and moisture. It is best to keep them in closed boxes or display cases to protect them from dust and mechanical damage. Due to its arsenic content, it should be stored out of reach of children and pets.