Evseevite

Chemical formula: Na₂Mg(As₅+O₄)F

Evseevite is an extremely rare sodium and magnesium arsenate, forming colorless, microscopic crystals in volcanic exhalation products.

## Characteristics Evseevite is a mineral from the arsenate group, with the chemical formula Na₂Mg(AsO₄)F. It occurs as very small, colorless, and transparent prismatic crystals, reaching sizes up to 0.2 x 0.5 mm. It also forms granular aggregates. Due to the microscopic size of the specimens, its visual characteristics are difficult to observe without specialized equipment. ## Physical Properties The mineral's hardness on the Mohs scale is 4. It is characterized by a vitreous luster and is transparent. The density calculated based on the formula and unit cell parameters is 3.61 g/cm³. ## Colors and Varieties To date, only colorless evseevite crystals have been found. No colored varieties or trade names are known. ## History and Name The mineral is named in honor of Anatoly Vasilievich Evseev (1954-2016), a Russian mineralogist, collector, and mineral specialist from Moscow State University. It was officially recognized by the International Mineralogical Association (IMA) in 2018. The type locality, or first place of discovery, is the Arsenatnaya fumarole on Tolbachik volcano in Kamchatka, Russia. ## Applications Evseevite, as an extremely rare mineral occurring in microscopic quantities, has no practical application. Its significance is purely scientific and collectible, being an object of interest for specialized research institutions and collectors of rare minerals.

Properties

Mohs hardness: 4
Luster: Vitreous
Streak: White
Density: 3.61
Transparency: Transparent
Crystal system: Orthorhombic

Diagnostic features

## Identification Identification of evseevite is impossible without advanced analytical techniques such as X-ray diffraction (XRD) or X-ray microanalysis (EDS/WDS). A key clue is its unique occurrence environment – high-temperature volcanic fumaroles. Visually, it is indistinguishable from many other colorless minerals. ## Distinguishing from Similar Minerals At its occurrence site, evseevite can be confused with many other rare arsenates and sulfates, which also form colorless, microscopic crystals. Definitive differentiation is only possible based on chemical composition and crystal structure analysis. ## Crystal Forms Evseevite forms short, prismatic crystals that often grow together into granular or radial aggregates.

Geological environment

## Genesis This mineral forms as a result of exhalative processes, crystallizing directly from hot volcanic gases (sublimate) in a fumarolic environment. It is a typical mineral for areas of high volcanic activity, rich in arsenic and fluorine. ## Mineral Associations Evseevite co-occurs with other rare fumarolic minerals such as berzeliite, svabite, anapaite, fluorapatite, and hematite. ## Localities The only confirmed locality of evseevite in the world is its type locality: Arsenatnaya fumarole, located on the second cinder cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, on Tolbachik volcano (Kamchatka, Russia).

Rarity

Extremely rare

For collectors

## Quality Criteria For a mineral of this rarity, the primary criterion of value is its mere presence and confirmed identification. Specimens with well-formed, sharp crystals, as large as possible for this species, are most highly prized. Association with other rare, well-defined minerals on the same rock matrix is also of great importance. ## Popular Localities There are no "popular" localities for this mineral. The only source of specimens is the very inaccessible type locality on an active volcano in Kamchatka, making the material extremely difficult to obtain.

Care and storage

## Cleaning Evseevite specimens are typically micromounts that should not be cleaned in a traditional manner. Any attempts at mechanical cleaning or using liquids can irreversibly damage the delicate crystals. If necessary, dust can be carefully removed with a stream of compressed air from a distance. ## What to Avoid Avoid contact with water, acids, and other chemicals. The mineral is relatively soft (hardness 4), so it is susceptible to scratches and mechanical damage. As an arsenate, its dust can be toxic if inhaled or ingested, so caution should be exercised. It should also be protected from high temperatures and shocks. ## Storage The safest storage method is to keep the specimen in its original, tightly sealed micromount box. This protects the mineral from dust, moisture, mechanical damage, and reduces the risk of accidental contact.

Sources

IMA Mineral List (CNMNC)