Caesiumpharmacosiderite

Chemical formula: CsFe₃+₄[(As₅+O₄)₃(OH)₄]·₄H₂O

Cespharmacosiderite is a rare iron and cesium arsenate, distinguished by its intense yellow color and occurrence as small, regular crystals.

## Characteristics Cespharmacosiderite is a hydrated iron and cesium arsenate, belonging to the pharmacosiderite group. This mineral forms characteristic, small crystals with a regular, cubic shape, rarely exceeding 1 mm in size. They usually occur as single, sharply defined cubes or their aggregates, growing on the surface of the host rock (matrix). Its intense, lemon-yellow to yellowish-green color makes it easily noticeable despite its small size. ## Physical Properties Cespharmacosiderite crystals exhibit a luster similar to vitreous or resinous. They are typically translucent, and their Mohs hardness is about 2.5, making them a relatively soft and brittle mineral. The density calculated based on the chemical formula and unit cell parameters is approximately 3.13 g/cm³. ## Colors and Varieties This mineral occurs in shades from bright yellow and lemon-yellow to yellowish-green. No distinct color or commercial varieties have been identified. ## History and Name Cespharmacosiderite was first described in 1999 by D.I. Belakovskiy, M.D. Dokuchaeva, I.A. Bryzgalov, and Y.K. Kabalov. Its name refers to the dominant cesium (Latin: *caesium*) in its composition and its structural and chemical relationship to pharmacosiderite, forming its cesium analogue. It was approved as a new mineral species by the International Mineralogical Association (IMA) under number IMA1996-055. The type locality, from which the first studied specimens originated, is the Ortau granite intrusion in Kazakhstan. ## Applications Cespharmacosiderite has no industrial application. Its significance is purely scientific and collectible, being valued for its rarity, chemical composition, and well-formed microcrystals.

Properties

Mohs hardness: 2.5
Luster: Resinous to vitreous
Streak: Light yellow
Density: 3.13
Cleavage: None
Fracture: Uneven
Transparency: Translucent
Crystal system: Isometric

Diagnostic features

## Identification Characteristic features of cespharmacosiderite include its intense yellow color, regular, cubic crystal shape, and occurrence as small, overgrown aggregates. The luster is usually resinous. Due to the small size of the crystals, identification most often requires the use of a magnifying glass or stereomicroscope. ## Distinguishing from Similar Minerals Cespharmacosiderite can be confused with other minerals from the pharmacosiderite group, such as pharmacosiderite (potassium-rich) or natropharmacosiderite (sodium-rich). Distinguishing them based on visual features is practically impossible and requires advanced chemical analyses (e.g., EDS) to determine the dominant element (cesium, potassium, sodium). It can also be confused with other secondary yellow arsenate minerals, but the cubic crystal shape is highly diagnostic for it. ## Crystal Forms This mineral crystallizes in the isometric system, forming almost perfectly developed cubic crystals. The crystals are usually small, not exceeding 1 mm in size, and occur as single individuals or groups growing on the host rock.

Geological environment

## Genesis Cespharmacosiderite is a secondary mineral, forming in the oxidation (weathering) zones of arsenic-rich ore deposits, especially in the presence of iron-bearing minerals. Its crystallization occurs in an environment where cesium ions are available, originating from the weathering of primary minerals such as pollucite, or from hydrothermal solutions. ## Mineral Associations This mineral often co-occurs with other secondary arsenates and minerals of the oxidation zone. The most common associations include scorodite, arseniosiderite, pharmacosiderite, beudantite, jarosite, goethite, and quartz. ## Localities The most important and well-documented occurrences of cespharmacosiderite worldwide include: - **Kazakhstan** - Ortau granite intrusion, Betpakdala Desert (type locality). - **Germany** - Clara Mine in the Black Forest (Baden-Württemberg), where it occurs as well-formed microcrystals. - **United Kingdom** - Wheal Gorland Mine in Cornwall, a historic locality known for many rare secondary minerals.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with sharply defined, well-formed crystals of intense, lemon-yellow color are most valued by collectors. Aggregates of numerous, lustrous cubes, contrastingly set on a darker rock matrix, are highly prized. Due to the microscopic nature of the mineral, the aesthetic composition and lack of damage to delicate crystals, visible under magnification, are crucial. ## Popular Localities The most sought-after specimens on the collector's market come from the Clara Mine in Germany, which provides aesthetic microcrystals with excellent development and luster. Specimens from the type locality in Kazakhstan are extremely rare and primarily of scientific significance.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using compressed air to remove dust. Due to the softness and brittleness of the crystals, mechanical contact (e.g., with a brush) is highly inadvisable. The use of water, especially ultrasonics, can lead to damage or dissolution of the delicate crystals. ## What to Avoid Avoid contact with all chemicals, including acids and bases. The mineral is sensitive to high temperatures and sudden changes in temperature. It should not be exposed to direct sunlight, which can cause fading. ## Storage Collectible specimens of cespharmacosiderite should be stored under stable conditions, in closed, padded "micromount" boxes to protect them from mechanical damage, dust, and moisture. Display should be away from sources of vibration and direct light.

Sources

IMA Mineral List (CNMNC)