Magnesiovoltaite

Chemical formula: K₂Mg₅Fe₃+₃Al(S₆+O₄)₁₂·₁₈H₂O

Hydrated potassium, magnesium, iron, and aluminum sulfate, forming dark, isometric crystals in volcanic environments and oxidation zones.

## Characteristics Magnesiovoltaite is a complex, hydrated sulfate belonging to the voltaite group. It forms isometric crystals, most often in the form of cubes, octahedra, and their combinations. They are usually small and rarely exceed a few millimeters. It also occurs as granular aggregates and crusts. Its crystals range in color from dark green to almost black, which is associated with the presence of iron in the structure. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 2.5. It is brittle, and its fracture is conchoidal. The luster is described as vitreous to resinous, although it can be dull on weathered surfaces. It is a partially water-soluble mineral, which is an important diagnostic feature and crucial for specimen preservation. ## Colors and Varieties Magnesiovoltaite occurs in uniform, dark colors – from dark green, through greenish-black, to black. No color or commercial varieties are distinguished, and classification is strictly based on chemical composition, where magnesium dominates over divalent iron in its structural position. ## History and Name The mineral's name refers to its chemical composition – the dominance of magnesium (magnesio-) and its structural similarity to voltaite. It was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 1990. The type locality, from which this mineral was first described, is in the La Fossa crater on Vulcano Island (Aeolian Islands) in Italy. ## Uses Due to its rarity, small crystal size, and instability in humid environments, magnesiovoltaite has no industrial applications. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness: 2.5
Luster: Vitreous to Resinous
Streak: Light green
Density: 2.69
Cleavage: None
Fracture: Conchoidal
Transparency: Translucent to Opaque
Crystal system: Cubic

Diagnostic features

## Identification Features helpful in identifying magnesiovoltaite include its isometric crystal form (cubes, octahedra), dark green to black color, low hardness (approx. 2.5), and occurrence in a specific environment – as a product of volcanic exhalations (fumaroles) or in oxidation zones of ore deposits. A key feature is its partial water solubility. ## Distinguishing from Similar Minerals Magnesiovoltaite is visually indistinguishable from other dark minerals of the voltaite group (e.g., voltaite itself) without advanced chemical analyses (e.g., EDS) that confirm magnesium dominance. From other dark minerals, such as some garnets or sphalerite, it is distinguished by its significantly lower hardness, characteristic parageneses, and water solubility. ## Crystal Forms It crystallizes in the isometric system, forming well-developed, equant crystals. Cubes and octahedra are most common, as are forms that are a combination of these (cubo-octahedra). It also occurs as granular aggregates and as thin crusts on other minerals or rocks.

Geological environment

## Genesis Magnesiovoltaite is a secondary mineral, forming under specific conditions. Its main formation environments are: 1. **Volcanic fumaroles**: It crystallizes from hot, sulfur-rich volcanic gases, reacting with surrounding rocks that provide potassium, magnesium, aluminum, and iron. 2. **Oxidation zones of sulfide deposits**: It forms as a product of weathering (oxidation) of sulfide minerals, such as pyrite, in the presence of magnesium- and potassium-rich minerals or rocks. This is a typical mineral of desert zones or post-mining formations on mine dumps. ## Mineral Associations It most often co-occurs with other sulfates, such as voltaite, alunogen, pickeringite, metavoltine, copiapite, melanterite, as well as pyrite and native sulfur. ## Localities This is a very rare mineral, known from only a few locations worldwide. The most important include: - La Fossa crater, Vulcano Island, Italy – type locality. - Blue Lizard Mine, San Juan County, Utah, USA – where it occurs in petrified wood. - Pécs-Vasas, Hungary. - Lake Szorkol, Tajikistan.

Rarity

Very rare

For collectors

## Quality Criteria The most prized magnesiovoltaite specimens are those with sharply defined, well-formed crystals with a strong luster. Crystal size is of great importance – even those a few millimeters in size are considered valuable. The attractiveness of a specimen is also enhanced by its placement on a contrasting matrix and its association with other rare sulfate minerals. ## Popular Localities Due to the extreme rarity of the mineral, specimens from any confirmed locality are desired by specialized collectors. The most classic and historically important are those from the type locality on Vulcano Island in Italy and from the Blue Lizard Mine in Utah, USA, from which well-formed microcrystals originate.

Care and storage

## Cleaning Magnesiovoltaite is partially water-soluble, so **it must absolutely not be cleaned with water**. The only safe method is to gently remove dust with a soft, dry brush or carefully use a stream of compressed air from a safe distance. ## What to Avoid Contact with water, water vapor, and any liquid chemicals must be strictly avoided. The mineral is sensitive to high air humidity, which can lead to its slow decomposition. It should also be protected from high temperatures, which can cause the loss of crystallization water and destruction of the structure. ## Storage Magnesiovoltaite specimens must be stored in controlled, low-humidity conditions. The best solution is to place them in a sealed, airtight container (e.g., a perky box) along with a desiccant (silica gel). Avoid exposure to direct sunlight and sudden temperature changes.

Sources

IMA Mineral List (CNMNC)