Arsenudinaite

Chemical formula: NaMg₄(As₅+O₄)₃

Arsenudinaite is a rare sodium and magnesium arsenate, forming colorless, hexagonal crystals in volcanic fumaroles.

## Characteristics Arsenudinaite is a sodium and magnesium arsenate belonging to the alluaudite group. This mineral forms elongated, prismatic crystals with a hexagonal cross-section, reaching up to 0.4 x 0.1 mm. It occurs as single, isolated crystals or their radial aggregates. It is colorless and transparent, with a vitreous luster. ## Physical Properties Arsenudinaite crystals are characterized by a hardness of approximately 3.5 on the Mohs scale. They have a vitreous luster and a white streak. The density calculated from crystallographic data is 3.98 g/cm³. ## History and Name The mineral's name refers to its chemical composition – the dominance of arsenic (As) – and its discovery location near the Udina volcano in Kamchatka. It was approved as a new mineral species by the International Mineralogical Association (IMA) in 2018 (IMA2018-009). It was described by a team of mineralogists led by Igor V. Pekov.

Properties

Mohs hardness: 3.5
Luster: Vitreous
Streak: White
Density: 3.98
Cleavage: None
Fracture: Uneven
Transparency: Transparent
Crystal system: Hexagonal

Diagnostic features

## Identification Identification of arsenudinaite is possible almost exclusively using advanced analytical methods, such as Raman spectroscopy or chemical analysis with an electron microprobe (EDS/WDS). Visually, it is characterized by its occurrence as small, colorless, hexagonal crystals in volcanic exhalation products. ## Distinguishing from similar minerals It can be confused with other colorless secondary minerals found in fumaroles, such as arseniopleite or other arsenates. Definitive differentiation requires chemical composition analysis, especially the sodium to magnesium ratio and the presence of arsenic. ## Crystal forms It forms hexagonal, prismatic crystals, often elongated along the crystallographic c-axis. They occur as single crystals or in small, radial aggregates.

Geological environment

## Genesis Arsenudinaite is a mineral of volcanic origin, crystallizing directly from volcanic gases in high-temperature fumarole zones (exhalations). It forms at temperatures of 600-650°C in an oxidizing environment. ## Mineral associations This mineral coexists with other arsenates and sulfates formed under similar conditions. In the type locality, it was found in association with hematite, tenorite, cassiterite, arcanite, johillerite, eriksonite, arseniopleite, svabite, bradaczekite, and anhydrite. ## Localities The only confirmed occurrence of arsenudinaite in the world is its type locality – the Yadovitaya fumarole on the second cinder cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption (GTFE) on Tolbachik volcano, Kamchatka, Russia.

Rarity

Extremely rare

For collectors

## Quality criteria As a "micromount" type mineral, its value is primarily determined by the quality and development of the crystals, rather than their size. The most desirable are well-formed, sharp, transparent, and undamaged hexagonal prisms, preferably embedded in a contrasting rock matrix. The presence of associated minerals, confirming the paragenesis, is also important.

Care and storage

## Cleaning Due to its extreme rarity and delicacy, specimens should not be cleaned mechanically or chemically. It is best to leave any contaminants undisturbed. ## What to avoid Avoid contact with water, acids, and other chemicals. The crystals are brittle and susceptible to mechanical damage. Protect them from vibrations, ultrasound, and sudden temperature changes. ## Storage Arsenudinaite specimens should be stored exclusively in specialized, padded "micromount" boxes. They should be kept in stable conditions, away from dust, humidity, and direct sunlight.

Sources

IMA Mineral List (CNMNC)