Feodosiyite

Chemical formula: Cu²⁺₁₁Mg₂Cl₁₈(OH)₈·₁₆H₂O

Feodosiyite is a very rare hydrated copper and magnesium hydroxychloride, forming bright green, acicular crystals in volcanic fumaroles.

## Characteristics Feodosiyite is a mineral from the chloride group, visually presenting as aggregates of fine, acicular or hair-like crystals. Individual crystals are elongated and often form tangled aggregates resembling felt or mineral wool. Due to the small size of the crystals, its features are difficult to observe without magnification. ## Physical Properties Feodosiyite crystals are transparent and exhibit a hardness of approximately 3 on the Mohs scale, making it a relatively soft and delicate mineral. Its density is about 2.57 g/cm³. The fracture is uneven. ## Colors and Varieties This mineral is characterized by a vibrant, bright green color. No color or commercial varieties are known. ## History and Name The mineral's name honors Feodosiy Chernyshov (1856-1914), a prominent Russian geologist and paleontologist, director of the Russian Geological Committee. Feodosiyite was first discovered and described in the products of fumarolic activity of Tolbachik volcano in Kamchatka. ## Uses Feodosiyite has no industrial application. Its significance is purely scientific and collectible, as a unique and rare representative of minerals forming under extreme volcanic conditions.

Properties

Mohs hardness: 3
Density: 2.57
Fracture: Irregular/Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Feodosiyite can be recognized by its characteristic bright green color and mode of occurrence – it forms aggregates of fine, acicular or hair-like crystals. Crucial for identification is its origin – it occurs exclusively in volcanic exhalation products (fumaroles). ## Distinguishing from similar minerals It can be confused with other secondary copper minerals of similar color and form, such as atacamite, paratacamite, or belloite. Definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD), due to visual similarity and co-occurrence in the same environment. ## Crystal forms It forms acicular, hair-like, and bladed crystals, usually grouped into tangled, felt-like, or radial aggregates.

Geological environment

## Genesis Feodosiyite is a mineral of volcanic origin, crystallizing directly from gases (sublimates) in high-temperature fumaroles. It forms as a result of the reaction of hot volcanic vapors rich in chlorine and copper with surrounding rocks. ## Mineral associations This mineral co-occurs with other rare volcanic fumarole minerals, such as tenorite, belloite, avdoninite, eriochalcite, melanothallite, and sylvite. ## Localities The only confirmed locality of feodosiyite in the world is its type locality – Glavnaya Tenoritovaya fumarole on the Tolbachik volcanic field (Kamchatka, Russia).

Rarity

Extremely rare

For collectors

## Quality criteria The quality of feodosiyite specimens is primarily assessed based on the abundance and development of crystals within the aggregate. The most prized specimens are those with distinct, dense clusters of acicular crystals of intense green color, contrasting with the rock matrix. Due to its extreme rarity, any well-preserved specimen holds high value for specialized collectors. ## Popular localities The only source of feodosiyite specimens is Tolbachik volcano in Kamchatka, Russia. Material from this locality is extremely difficult to obtain and rarely appears on the collector's market.

Care and storage

## Cleaning Feodosiyite specimens are extremely delicate and sensitive to water. They should not be cleaned wet. Only very careful dust removal with a soft brush or a stream of compressed air from a distance and with low force is permissible. ## What to avoid Contact with water, which can dissolve or damage the mineral, must be absolutely avoided. It is sensitive to changes in air humidity, high temperatures, and all chemicals. It should be protected from direct sunlight, which can cause fading. ## Storage It is recommended to store specimens in sealed, airtight containers (such as "membrane boxes" or boxes with gaskets) to protect them from moisture and mechanical damage. It is best kept under stable room conditions, away from heat and light sources.