Microsommite

Chemical formula: [(Na,K)₆(S₆+O₄)][Ca₂Cl₂][(Si₆Al₆O₂₄)]

Microsommite is a rare tectosilicate from the cancrinite group, forming small, hexagonal crystals with a vitreous luster, found in the lavas of Vesuvius.

## Characteristics Microsommite is a mineral belonging to the cancrinite group, classified as a tectosilicate. Its name refers to the small size of the crystals in which it usually occurs. It forms hexagonal, prismatic or tabular crystals, often terminated by pyramids. They are typically very small, reaching up to 2 mm in length. It occurs as single crystals or their radial aggregates. It is a transparent to translucent mineral with a vitreous luster. ## Physical Properties This mineral is characterized by a hardness of approximately 6 on the Mohs scale. Its density ranges from 2.42 to 2.53 g/cm³. The luster is vitreous, and on fracture surfaces, it can be greasy. It does not exhibit cleavage, and its fracture is conchoidal to uneven. ## Colors and Varieties Microsommite is most often colorless or white. It can also take on yellowish or greenish hues. No named varieties are distinguished. ## History and Name The mineral was first described in 1872 by the Italian mineralogist Arcangelo Scacchi. The name "microsommite" comes from the Greek words *μικρός* (mikrós - small) and *σῶμα* (sōma - body), alluding to the small size of its crystals. The historical lavas of Mount Vesuvius in Italy were designated as the type locality. ## Uses Microsommite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or minerals from specific localities, such as the Vesuvius-Somma complex.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: White
Density: 2.42-2.53
Cleavage: None
Fracture: Conchoidal to uneven
Transparency: Transparent to translucent
Crystal system: Hexagonal

Diagnostic features

## Identification A characteristic feature of microsommite is its hexagonal crystal habit (hexagonal prisms) and its occurrence in a specific geological environment – volcanic limestone blocks altered by lava. The small size of the crystals is also a key indicator. Vitreous luster and lack of cleavage aid in identification. ## Distinguishing from Similar Minerals Microsommite can be confused with other minerals from the cancrinite or sodalite group, such as afghanite, which also form hexagonal crystals and occur under similar conditions. Differentiation often requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). It is distinguished from quartz by its lower hardness and specific mineral paragenesis. ## Crystal Forms Microsommite crystallizes as small, elongated prismatic crystals with a hexagonal cross-section. Crystals may be terminated by flat bases or hexagonal pyramid faces. It often forms radial aggregates of tiny crystals.

Geological environment

## Genesis Microsommite is a mineral of metamorphic origin. It forms as a result of contact metamorphism of limestone blocks or other calcium-rich rocks that have been entrained and altered by hot phonolitic or trachytic lava. It is a typical skarn mineral formed under conditions of high temperature and low pressure. ## Mineral Associations This mineral often co-occurs with other rare minerals typical of the Vesuvius-Somma volcanic complex. The most common associations include: biotite, sanidine, augite, nepheline, leucite, sodalite, davyn, and other minerals from the cancrinite group. ## Localities The most important and classic locality for microsommite is the Somma-Vesuvius volcanic complex in Campania, Italy, from which the best specimens originate. It has also been reported in the Laacher See volcanic complex in the Eifel mountains, Germany. These are the only well-confirmed localities for this mineral.

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the quality and development of the crystals are paramount. Specimens with sharp, well-defined, transparent crystals with a vitreous luster are most highly valued. Contrast with the rock matrix and the presence of interesting associated minerals are also important. Due to their small size, specimens are usually evaluated under magnification. The size of the crystals, even if only a few millimeters, significantly increases the value of the specimen. ## Popular Localities By far the most valued and sought-after microsommite specimens come from its type locality – historical lavas from the Somma-Vesuvius complex in Italy. Specimens from this location are considered classic and serve as a benchmark for this mineral.

Care and storage

## Cleaning Microsommite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Always ensure the specimen is completely dry before storing it in a collection. ## What to Avoid Avoid contact with acids and other aggressive chemicals, which can damage the mineral. Microsommite is relatively hard but brittle, so it should be protected from impacts and falls. It should not be heated or subjected to sudden temperature changes. ## Storage Due to their small size and fragility, microsommite specimens are best stored in enclosed "micromount" boxes, which protect them from mechanical damage and dust. Exposure to humid environments should be avoided.

Sources

IMA Mineral List (CNMNC)