Mountainite

Chemical formula: KNa<sub>2</sub>Ca<sub>2</sub>[Si<sub>8</sub>O<sub>19</sub>(OH)]·6H<sub>2</sub>O

A rare silicate of complex composition, forming colorless or white, fibrous aggregates, discovered in the Wessels mine in South Africa.

## Characteristics Mountainite is a rare, hydrated potassium, sodium, and calcium silicate, belonging to the gyrolite mineral group. It typically occurs as radial or tangled aggregates composed of very fine, acicular or fibrous crystals. Individual crystals are too small to be seen with the naked eye. These aggregates are usually white, cream, or colorless and often exhibit a silky luster. ## Physical Properties Mountainite crystals are very fine and flexible. The mineral's hardness on the Mohs scale is approximately 3. The luster is most often silky, and on crystal faces, it can be vitreous. It is a translucent to opaque mineral. Its density is approximately 2.55 g/cm³. ## Colors and Varieties Mountainite is usually colorless, white, or takes on grayish and creamy hues. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from Edgar Donald Mountain (1901–1985), a professor of geology at Rhodes University in Grahamstown, Republic of South Africa, in recognition of his contribution to the country's mineralogy. The mineral was first described in 1957 by S.A. Gard, W.H. Taylor, and R.A. Chalmers based on specimens from the Bultfontein mine in Kimberley, South Africa. ## Uses Mountainite has no industrial application. It is solely of interest to collectors of rare minerals and scientists.

Properties

Mohs hardness
3
Luster
Vitreous, Silky
Streak
White
Density
2.55
Cleavage
Perfect on {100}, Good on {010}
Fracture
Splintery
Transparency
Translucent to Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Mountainite can be identified by its characteristic, radial or tangled aggregates with a silky luster. Low hardness and a very delicate, fibrous nature are key features. Its occurrence in paragenesis with other minerals from the Kalahari deposit can also be an indicator. ## Distinguishing from Similar Minerals It can be confused with other white, fibrous zeolites or silicates, such as okenite, natrolite, or mesolite. Okenite forms similar aggregates, but its "cotton balls" consist of more flexible and differently formed fibers. Accurate differentiation from the very similar mineral rhodesite often requires chemical or X-ray diffraction (XRD) analysis. ## Crystal Forms It forms acicular or bladed crystals, elongated along the b-axis, which combine into radial, spherulitic, or tangled, felt-like aggregates. Individual, well-formed crystals are extremely rare and microscopic in size.

Geological environment

## Genesis Mountainite is a low-temperature hydrothermal mineral. It forms in gas vesicles and fractures in volcanic rocks (e.g., kimberlites) or as a result of the alteration of manganese deposits, as in the case of the Kalahari deposit. ## Mineral Associations This mineral often co-occurs with calcite, apophyllite, brucite, tobermorite, and in the Kalahari deposit with manganese minerals such as sugilite, hausmannite, or ettringite. At its discovery site (Bultfontein mine), it was accompanied by apophyllite, natrolite, and thaumasite. ## Localities The most important localities, providing the best specimens of mountainite, are in the Republic of South Africa, particularly in the Wessels and N'Chwaning II mines in Kuruman, in the Kalahari manganese field. This mineral has also been identified in the Bultfontein mine in Kimberley (type locality), as well as in several locations in Russia (Kola Peninsula) and Canada (Mont Saint-Hilaire, Quebec).

Rarity

Rare

For collectors

## Quality Criteria The most prized mountainite specimens come from the Wessels mine and are characterized by well-formed, spherical aggregates with a distinct silky luster. A contrasting association with other minerals, especially with purple sugilite or shiny hausmannite, significantly enhances their attractiveness and value. The size and sphericity of the aggregates and the absence of damage are key factors in assessing quality. ## Popular Localities By far the most sought-after specimens by collectors come from the Kalahari manganese field in South Africa, especially from the Wessels mine. Specimens from Mont Saint-Hilaire in Canada are also valued, although usually smaller and less spectacular.

Care and storage

## Cleaning Mountainite specimens are very delicate and fragile. They should only be cleaned with compressed air to remove dust. Any contact with water or other liquids is inadvisable, as it can damage the fibrous aggregates. ## What to Avoid Avoid contact with water and chemical agents. Due to its low hardness and fibrous structure, the mineral is extremely sensitive to mechanical damage, vibrations, and shocks. It should not be subjected to ultrasonic cleaning. ## Storage Specimens should be stored in closed, padded boxes to protect them from dust, moisture, and mechanical damage. It is best to display them in "micromount" boxes or showcases where they will not be exposed to touch.

Sources

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