Brassite

Chemical formula: Mg(As⁵⁺O₃OH)·₄H₂O

Brassite is a very rare, hydrated magnesium arsenate, forming colorless or white, fine coatings and aggregates.

## Characteristics Brassite is a hydrated magnesium arsenate, occurring extremely rarely in nature. It typically forms very fine, microcrystalline coatings, crusts, or earthy aggregates on host rocks. Individual crystals are rarely visible to the naked eye and most often appear as small, bladed or acicular forms, which can create rosette-like aggregates. ## Physical Properties Due to its mode of occurrence, determining its physical properties is difficult. This mineral is very soft, with a Mohs hardness of approximately 2. It has a luster described as dull, earthy, or silky in the case of fine fibrous aggregates. It is transparent to translucent. ## Colors and Varieties Brassite is usually colorless or white. No colored or commercial varieties are distinguished. ## History and Name The mineral's name comes from the Hungarian mineralogist and botanist, Sámuel Brassai (1797-1897). It was first described in 1973 based on material from the Gabe Gottes mine in Haut-Rhin, France, and from Jáchymov, Czech Republic. ## Uses Brassite has no industrial application. Its significance is purely scientific and as a collector's item, being a very rare arsenate.

Properties

Mohs hardness: 2
Color: White
Luster: Dull
Streak: White
Density: 2.28
Cleavage: {001} perfect.
Fracture: Irregular/Uneven,Splintery
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Identifying brassite is very difficult without advanced analytical methods. It can be preliminarily recognized by its mode of occurrence – it forms fine, white or colorless, bladed crystals in acicular or rosette-like aggregates, as well as earthy coatings. It occurs in paragenesis with other rare arsenates. ## Distinguishing from Similar Minerals It can be confused with many other secondary arsenates, such as pharmacolite, picropharmacolite, or rösslerite, with which it often co-occurs. It differs from them in chemical composition and crystal structure, which requires confirmation by methods such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal Forms It forms small, bladed crystals, often grouped into rosettes, fan-like aggregates, or irregular aggregates. It also occurs as earthy aggregates and powdery coatings.

Geological environment

## Genesis Brassite is a secondary mineral, formed in the oxidation (weathering) zones of ore deposits rich in arsenic, in a magnesium-rich environment. It forms as a result of reactions between percolating waters and pre-existing arsenical minerals. ## Mineral Associations It often co-occurs with other arsenates, such as pharmacolite, picropharmacolite, rösslerite, sainfeldite, as well as with gypsum and other secondary minerals. ## Localities The most important localities for this very rare mineral are Jáchymov in the Czech Republic, the Gabe Gottes mine in Sainte-Marie-aux-Mines (Haut-Rhin, France), and the Sterling Hill mine in Ogdensburg (New Jersey, USA).

Rarity

Very rare

For collectors

## Quality Criteria The most prized brassite specimens are those with well-formed, though still microscopic, crystals forming distinct aggregates, such as rosettes. Contrast with the host rock color and the richness of associations with other rare minerals are important. Due to the nature of the mineral, specimens are almost exclusively micromounts. ## Popular Localities Specimens from classic localities such as Jáchymov or Sainte-Marie-aux-Mines are most sought after by collectors specializing in rare minerals or systematic collections.

Care and storage

## Cleaning Brassite specimens are extremely delicate and sensitive. Mechanical cleaning and contact with water, which can dissolve or damage the fine crystals, should be avoided. If cleaning is absolutely necessary, compressed air can be used very carefully from a safe distance to remove dust. ## What to Avoid Contact with water and all chemicals must be strictly avoided. The mineral is sensitive to changes in humidity and temperature. It should not be heated or exposed to direct sunlight. Due to its low hardness, it is very susceptible to mechanical damage. ## Storage Brassite specimens should be stored in stable conditions, preferably in sealed containers (such as "micromount boxes") that protect against dust, moisture, and physical damage. Storage in a dry place is recommended.