Calciojohillerite

Chemical formula: NaCaMgMg<sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>3</sub>

Calciojohilleryite is a very rare sodium, calcium, and magnesium arsenate, forming small, prismatic crystals in volcanic exhalation products.

## Characteristics Calciojohilleryite is a mineral of the alluaudite group, an hydrated arsenate. It occurs as small, prismatic crystals up to 0.3 mm long, which often form radial or granular aggregates and clusters. The most commonly found specimens are in the form of thin coatings and crusts on volcanic scoria. ## Physical Properties This mineral is characterized by a vitreous luster and is transparent to translucent. Its Mohs hardness is approximately 3, making it relatively soft. The calculated density is 3.75 g/cm³. ## Colors and Varieties Calciojohilleryite is usually light green, greenish-yellow, or colorless. No named varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominant calcium (Latin: *calx* – lime) – and its structural relationship to johilleryite, of which it is a calcium analogue. It was approved as a new mineral species by the IMA in 2013. The type locality is Tolbachik volcano in Kamchatka, Russia. ## Uses Due to its extreme rarity, calciojohilleryite has no industrial applications. It is of purely scientific and collecting interest.

Properties

Mohs hardness
3
Luster
Vitreous
Streak
White
Density
3.75
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of calciojohilleryite is based on its specific occurrence environment (volcanic fumaroles), light green color, radial aggregates of small crystals, and co-occurrence with other rare arsenates. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other minerals from the alluaudite group, especially johilleryite and bradaczekite, which occur in the same environment and have a similar appearance. Distinguishing them is almost exclusively possible based on chemical analysis – calciojohilleryite is characterized by the dominance of calcium in a specific structural position. ## Crystal Forms It forms short, prismatic crystals, usually gathered in radial, stellar, or granular aggregates. It also occurs as thin crusts and coatings.

Geological environment

## Genesis Calciojohilleryite is a fumarolic mineral. It forms by sublimation from volcanic gases in high-temperature exhalation zones (400-600°C), crystallizing directly on porous, basaltic scoria. ## Mineral Associations It co-occurs with other rare arsenates formed under similar conditions, such as johilleryite, bradaczekite, nickenichite, dmitryivanovite, anhydrokainite, arsmirandite, as well as with hematite, tenorite, and aphthitalite. ## Localities The only confirmed locality of calciojohilleryite in the world is its type locality – the fumarole field formed after the Great Tolbachik Fissure Eruption in 1975-1976 on Tolbachik volcano (Kamchatka, Far Eastern Federal District, Russia).

Rarity

Very rare

For collectors

## Quality Criteria The quality of collectible specimens, which are almost exclusively micromounts, is assessed based on several factors. The most important are the sharpness and size of the crystals, their transparency, and the intensity of their color. Rich crystalline aggregates and specimens with well-defined associations with other rare minerals from this locality are highly valued. ## Popular Localities Specimens come exclusively from one location in the world – Tolbachik volcano in Kamchatka. Material obtained from historical eruptions is very limited and rarely appears on the market.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush or compressed air to remove dust. Due to its rarity and potential reactivity, contact with water and chemical agents should be avoided. ## What to Avoid As an arsenate, this mineral is potentially toxic – avoid inhaling dust and wash hands after contact. It is soft and brittle, so it must be protected from impacts and scratches. Avoid contact with acids and other chemicals. Store away from moisture. ## Storage The safest way to store specimens is to place them in a sealed "micromount" box, which protects against dust, moisture, and mechanical damage. A label with the name and location should be included.

Sources

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