Masaitisite
Chemical formula: KCu<sup>2+</sup><sub>5</sub>O<sub>2</sub>(Se<sup>4+</sup>O<sub>3</sub>)<sub>2</sub>Cl<sub>3</sub>
Masaitisite is an extremely rare copper and potassium selenite, forming microscopic, dark green crystals in volcanic exhalations.
Description
## Characteristics Masaitisite is a selenite mineral, occurring as microscopic, tabular or prismatic crystals. It typically forms radial or chaotically arranged aggregates. Its color ranges from dark green to almost black. It exhibits a vitreous luster. Due to the size of the crystals, which rarely exceed 0.2 mm, it is a typical micromount mineral, impossible to observe with the naked eye. ## Physical Properties Masaitisite crystals are brittle, and their thin fragments can be translucent. The fracture is uneven. Hardness on the Mohs scale and density have not been experimentally measured due to the small size and rarity of the material. The density calculated from crystallographic data is approximately 4.55 g/cm³. ## Colors and Varieties This mineral is color-uniform, occurring in shades from dark green to black. No color varieties or commercial varieties have been distinguished. ## History and Name Masaitisite was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2018. Its name honors Vladimir Logvinovich Masaitis (1927–2018), a Russian geologist and petrologist, specializing in the petrology of impact craters (astroblemes). ## Applications As a mineral occurring in microscopic quantities and unstable under ambient conditions, masaitisite has no practical applications. It is solely an object of scientific research and is valued by specialized collectors of micromount minerals.
Diagnostic features
## Identification Identification of masaitisite is impossible without advanced analytical techniques. It requires chemical analysis (EDS/WDS) to confirm its unique composition (presence of potassium, copper, selenium, and chlorine) and X-ray diffraction (XRD) to determine its crystal structure. A key clue is its origin from a specific fumarole of Tolbachik volcano. ## Distinguishing from Similar Minerals Many dark green and black copper minerals, such as tenorite, as well as other rare selenites and chlorides (e.g., belloite, avdoninite), occur in the fumarole environment of Tolbachik. Distinguishing masaitisite from these minerals based on visual characteristics is unfeasible and requires specialized research equipment. ## Crystal Forms Masaitisite forms elongated, tabular or prismatic crystals, which often combine into small, radial or disordered aggregates on the surface of basaltic lava.
Geological environment
## Genesis Masaitisite is a secondary mineral, formed in the process of volcanic exhalations. It crystallizes directly from hot gases (as a sublimate) in an oxidizing environment, at a temperature of approximately 450–500°C. It forms as a result of the reaction of volcanic gases with the basaltic lava crust. ## Mineral Associations This mineral occurs in association with other rare fumarolic minerals. The most common associations include kotunite, belloite, avdoninite, eriochalcite, sylvite, halite, tenorite, and hematite. ## Localities The only confirmed locality of masaitisite in the world is its type locality: Arsenatnaya fumarole on the second cinder cone of the Northern Breakthrough of the Great Fissure Eruption of Tolbachik, Tolbachik volcano, Kamchatka Peninsula, Russia.
Rarity
Extremely rare
Collector aspects
## Quality Criteria As a micromount mineral, the collector's value of a masaitisite specimen depends on several factors. The most important is the presence of well-formed, sharp crystals forming rich, photogenic aggregates. Specimens where masaitisite co-occurs with other rare, well-identified minerals from this locality are highly prized. The size of the aggregates, although still microscopic, is also important. ## Popular Localities All known specimens come from a single location in the world – the fumarole of Tolbachik volcano in Kamchatka. This location is inaccessible to typical tourism or amateur collecting, and material acquisition takes place within the framework of scientific expeditions.
Care and storage
## Cleaning Masaitisite is water-soluble and very sensitive to changes in humidity. **It must absolutely not be cleaned wet or with any chemical agents.** Any attempt at cleaning risks its complete destruction. ## What to Avoid Avoid contact with water, water vapor, and chemicals. The mineral is unstable under atmospheric conditions, so it must be protected from moisture and sudden temperature changes. It should not be exposed to direct sunlight. ## Storage Masaitisite specimens must be stored in tightly sealed containers, preferably in specialized "micromount" boxes that provide insulation from the environment. This is the only way to preserve the specimen in an intact state.