Combeite

Chemical formula: Na₄.₅Ca₃.₅Si₆O₁₇.₅(OH)₀.₅

A rare sodium calcium silicate, forming small, hexagonal crystals, found in alkali-rich lavas.

## Characteristics Combeite is a rare mineral from the silicate group, belonging to the cyclosilicates. It typically occurs as very small, hexagonal, prismatic or tabular crystals, rarely exceeding 1 mm in size. Most often, it forms granular aggregates or occurs as inclusions in other minerals. It is colorless or white and has a vitreous luster. ## Physical Properties This mineral is characterized by a Mohs hardness of 5-6. Its density ranges from 2.84 to 2.87 g/cm³. The crystals exhibit white or pale blue fluorescence under shortwave ultraviolet light. ## Colors and Varieties Combeite is usually colorless to white. No colored varieties or trade names have been distinguished, which is typical for minerals of primarily scientific and collecting interest. ## History and Name The mineral was first described in 1957 by Théodore G. Sahama and Kai Hytönen. The name honors A. D. Combe, a geologist from the Uganda Geological Survey, who collected the first samples of this mineral. The type locality is Nyiragongo volcano in the Democratic Republic of Congo. ## Uses Due to its rarity and small crystal size, combeite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and scientists studying the petrology of alkaline rocks.

Properties

Mohs hardness: 5-6
Color: Colorless
Luster: Vitreous
Streak: White
Density: 2.844
Cleavage: None
Fracture: Uneven
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Field identification of combeite is practically impossible due to the small size of the crystals and its resemblance to many other colorless minerals. Definitive identification requires advanced laboratory methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other silicates found in similar environments, such as nepheline, leucite, or sodalite. Differentiation is based on chemical and crystallographic analysis. Its characteristic, though not always present, pale blue-white fluorescence under UV light is a helpful indicator. ## Crystal Forms It forms hexagonal crystals with a prismatic or tabular habit. It usually occurs as small, granular aggregates or as single, dispersed crystals within the host rock.

Geological environment

## Genesis Combeite is a high-temperature mineral, crystallizing from lavas rich in alkalis (sodium and potassium) and poor in silica. It forms under volcanic conditions, mainly in volcanic bombs, lapilli, and porous fragments of lava ejected during eruptions. ## Mineral Associations It most commonly co-occurs with minerals such as nepheline, melilite, kalsilite, clinopyroxene (diopside, aegirine), magnetite, and perovskite. It is found in the host rock, which is typically melilite nephelinite. ## Localities The most important and classic locality for this mineral is Nyiragongo volcano in the Democratic Republic of Congo. It is also known from the Oldoinyo Lengai volcanic complex in Tanzania and from Bellerberg mountain in the Eifel mountains, Germany. These are the only confirmed, well-documented localities worldwide.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp, and as large as possible combeite crystals are most valued by collectors, which is a rarity. The aesthetic composition with associated minerals is also important, especially if the crystals are embedded in a contrasting matrix. The clarity and transparency of the crystals increase the value of the specimen. ## Popular Localities Specimens from the type locality – Nyiragongo volcano in the Democratic Republic of Congo – are considered the best in the world. Specimens from the German Eifel mountains are also sought after, although they typically feature smaller crystals.

Care and storage

## Cleaning Due to the small size and fragility of the crystals, mechanical cleaning is not recommended. If necessary, compressed air can be used to remove dust. Wet cleaning, especially with acids, is unacceptable due to the mineral's reactivity. ## What to Avoid Contact with water and acids should be strictly avoided, as combeite readily alters into other minerals, especially in a humid environment. It should also be protected from high temperatures and mechanical damage. ## Storage Combeite specimens should be stored in dry conditions, preferably in sealed containers or display cases away from moisture. Storage at stable room temperature is recommended to prevent its degradation.