Thaumasite

Chemical formula: Ca<sub>3</sub>Si(OH)<sub>6</sub>(S<sup>6+</sup>O<sub>4</sub>)(CO<sub>3</sub>)·12H<sub>2</sub>O

Thaumasite is a complex hydrated calcium sulfate, carbonate, and silicate, forming characteristic white, acicular crystals.

## Characteristics Thaumasite is a mineral from the ettringite group, distinguished by its unique chemical composition, which simultaneously contains silicate, sulfate, and carbonate groups. It most often forms aggregates of small, acicular or fibrous crystals, which can form dense, felted masses resembling glass wool or felt. Less commonly, it occurs as radial or tangled masses. Individual, well-formed crystals are rare and appear as slender, hexagonal prisms. ## Physical Properties This mineral is very soft and brittle, with a hardness of only 3.5 on the Mohs scale. It has a low density, approximately 1.88 g/cm³, which makes specimens relatively light. Crystals exhibit a vitreous to silky luster, especially in the case of fibrous aggregates. It is transparent to translucent. ## Colors and Varieties Thaumasite is usually colorless or white. It does not form colored varieties, and its appearance is relatively uniform regardless of location. ## History and Name The name thaumasite comes from the Greek word "thaumazein" (θαυμάζειν), meaning "to be surprised." It refers to the surprising and unusual chemical composition of the mineral, combining silicate, sulfate, and carbonate anions, which was an unprecedented phenomenon at the time of its discovery. The mineral was first described in 1878 by the Swedish mineralogist Adolf Erik Nordenskiöld based on specimens found in the Bjelke mine in Åre, Jämtland, Sweden. ## Uses Thaumasite has no industrial significance. However, it is an interesting mineral sought after by collectors due to its delicate, acicular forms and unusual chemical composition. In civil engineering, its formation in concrete (known as "thaumasite form of sulfate attack" - TSA) is a destructive phenomenon leading to the degradation of the material's structure.

Properties

Mohs hardness
3.5
Luster
Vitreous
Streak
White
Density
1.88
Cleavage
Good on {1010}, poor on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Thaumasite is most easily identified by its characteristic form – white, felted or radial aggregates of very fine, acicular crystals. It is very soft, light, and brittle. A characteristic feature is its reaction with acids – it dissolves with effervescence (release of carbon dioxide). ## Distinguishing from Similar Minerals Thaumasite is sometimes confused with other fibrous minerals such as calcite, aragonite, natrolite, or mesolite. From fibrous zeolites (natrolite, mesolite), it is distinguished by its lower hardness and reaction with acid. From calcite and aragonite, which also react with acids, it is distinguished by its distinctly lower hardness and the acicular, rather than prismatic or tabular, nature of its crystals. ## Crystal Forms Thaumasite crystals are hexagonal, with a habit of slender, elongated needles and columns. They almost always occur as dense, tangled fibrous aggregates, radial rosettes, or masses resembling cotton wool.

Geological environment

## Genesis Thaumasite is a low-temperature mineral, formed as a result of hydrothermal processes or contact metamorphism. It forms in voids and fissures in calcium-rich rocks, such as skarns, metamorphosed limestones and marbles, as well as in volcanic rocks (basalts, tuffs), where reactions occur between sulfate- and carbonate-rich solutions and silicate minerals. ## Mineral Associations It often co-occurs with minerals from the zeolite group (e.g., analcime, apophyllite, stilbite), calcite, dolomite, gypsum, anhydrite, pyrite, and minerals from the ettringite group. ## Localities The most important localities worldwide, from which classic specimens originate, are the mines in Långban and Åre in Sweden (type locality). In the USA, beautiful, acicular aggregates have been found in Paterson and West Paterson, New Jersey. Other known localities include the Harz Mountains in Germany, the Monte Somma-Vesuvius volcanic complex in Italy, and numerous localities in South Africa, especially in the N'Chwaning and Wessels mines in the Kuruman area, where it occurs in the form of large, white spherical aggregates.

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, radial aggregates of acicular crystals, forming aesthetic "hedgehog" or "ball" formations on a contrasting rock matrix. Purity (snow-white color) and lack of damage to the delicate crystals are important. Large, dense aggregates from Paterson (USA) or spherical aggregates from the Kalahari mines (RSA) are among the most sought after. ## Popular Localities Classic and most desired specimens come from quarries in the Paterson area, New Jersey (USA), where thaumasite formed spectacular, white masses on basalt. Another famous source is the manganese mines in the Kalahari Desert in South Africa (N'Chwaning, Wessels), from which large, spherical aggregates originate. The Swedish localities of Långban and Åre also have historical significance.

Care and storage

## Cleaning Thaumasite specimens are extremely delicate and brittle. They should only be cleaned very carefully, using compressed air to remove dust. Any contact with water, especially warm water, can lead to dissolution or damage to the mineral. Using brushes is highly inadvisable due to the risk of breaking the acicular crystals. ## What to Avoid Avoid contact with water and all chemicals, including acids, which rapidly decompose it with the release of CO₂. The mineral is sensitive to elevated temperatures, which can cause dehydration and destruction. It should be protected from shocks and vibrations. ## Storage Thaumasite specimens are best stored in closed, padded boxes or display cases to protect them from dust, mechanical damage, and sudden changes in humidity. Due to its brittleness, it should not be stored in contact with harder minerals.

External references

Sources

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