Prachařite
Chemical formula: CaSb<sup>5+</sup><sub>2</sub>(As<sup>3+</sup><sub>2</sub>O<sub>5</sub>)<sub>2</sub>O<sub>2</sub>(H<sub>2</sub>O)<sub>7</sub>·3H<sub>2</sub>O
Prachařite is a very rare, hydrated calcium arsenate-antimonate, forming colorless or white, hexagonal crystals with good luster.
Properties
- Mohs hardness
- 2-2.5
- Luster
- Vitreous to pearly
- Streak
- White
- Density
- 2.838
- Cleavage
- good cleavage parallel to {0001} and a distinct cleavage parallel to <mi>{10_10}</mi>.
- Fracture
- Irregular/Uneven
- Transparency
- Transparent,Translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Identification of prachařite is based on its characteristic crystal form – small, hexagonal tablets forming rosette-like aggregates. Key features also include its white or colorless appearance, low hardness (2-2.5), and occurrence in paragenesis with other rare arsenates and antimonates in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals It can be confused with other white or colorless secondary minerals of similar habit, such as gypsum or some micas. It is distinguished from them by its specific hexagonal crystal symmetry, mineral paragenesis (co-occurrence with arsenic and antimony minerals), and formation environment. Certain identification requires advanced analytical methods, such as X-ray diffraction (XRD) or Raman spectroscopy. ## Crystal Forms Prachařite crystallizes as thin, tabular crystals with a hexagonal outline. These crystals often combine into characteristic, fan-shaped or rosette-like aggregates, which can reach up to 2 mm in size.
Geological environment
## Genesis Prachařite is a secondary mineral, formed in the oxidation zones (so-called iron caps) of polymetallic hydrothermal deposits. It forms as a result of the weathering of primary ore minerals rich in arsenic and antimony, in the presence of calcium-rich solutions. ## Mineral Associations At the type locality (Plaka Mine in Greece), prachařite co-occurs with other rare secondary minerals such as goethite, pitticite, arseniosiderite, kaatialaite, sainfeldite, pharmacolite, gypsum, and calcite. ## Localities The only confirmed and described occurrence of prachařite in the world is its type locality: Plaka Mine No. 80, in the Plaka mining area, Lavreotiki, East Attica, Greece.
Rarity
Very rare
For collectors
## Quality Criteria The quality of prachařite specimens is primarily assessed based on the size and development of crystal aggregates. The most valued specimens are those with clearly formed, sharp rosettes or crystal fans, contrastingly deposited on the rock matrix (most often on goethite). The size of the aggregates, their abundance on the sample, and the absence of mechanical damage significantly increase their collector's value. ## Popular Localities The only source of prachařite specimens is its type locality in Greece – Plaka Mine No. 80. All specimens available on the collector's market originate from this single location.
Care and storage
## Cleaning Prachařite specimens are extremely delicate and sensitive. Cleaning should be limited to an absolute minimum. If necessary, compressed air can be used very carefully from a safe distance to remove loose dust particles. Any contact with water or other liquids should be avoided. ## What to Avoid Contact with water, acids, bases, and all chemicals must be strictly avoided. The mineral is very soft, susceptible to scratches and mechanical damage. It should not be heated or exposed to prolonged sunlight, although its light sensitivity has not been thoroughly investigated. ## Storage Storage requires specialized conditions. It is best to keep the specimen in its original, sealed "micromount" box, away from dust, moisture, and temperature fluctuations. It should be protected from vibrations and impacts.