Tvedalite

Chemical formula: Ca₄Be₃Si₆O₁₇(OH)₄·₃H₂O

Tvedalite is a very rare beryllium calcium silicate, forming characteristic radial aggregates of acicular crystals.

## Characteristics Tvedalite is a mineral from the silicate group, chemically classified as a beryllium calcium silicate, often with an admixture of manganese. Typical specimens form spherical or hemispherical aggregates with a radial structure, composed of very fine, acicular or fibrous crystals. These aggregates reach sizes of up to several centimeters in diameter and exhibit a delicate, silky luster on the fiber surfaces. ## Physical Properties The mineral's hardness is estimated at about 5 on the Mohs scale. It has a density of approximately 2.75 g/cm³. The crystals are translucent and exhibit a vitreous luster on faces and a silky luster on fibrous aggregates. ## Colors and Varieties Tvedalite is most often colorless, white, or pale pink. The pink coloration is likely due to a small admixture of manganese replacing calcium in the crystal structure. No named varieties are distinguished. ## History and Name The mineral was first described in 1991 by Gunnar Raade, Morten B. Raade, and Per K. Egeberg. Its name comes from the discovery locality – the Tvedalen valley near Larvik in Norway, which is its type locality. ## Uses Due to its extreme rarity, tvedalite has no industrial application and is solely a mineral of scientific and collector's interest.

Properties

Mohs hardness: 5
Luster: Vitreous to silky
Streak: White
Density: 2.75
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification The key diagnostic feature of tvedalite is its mode of occurrence – radial, spherical aggregates composed of fine, acicular crystals. ## Distinguishing from similar minerals It can be confused with other zeolites or silicates of similar appearance, such as natrolite, mesolite, or chiavennite. It differs from natrolite and mesolite by its harder structure and often greater cohesion of aggregates. Chiavennite, with which it co-occurs, usually has a more vitreous luster and different physical properties. Final distinction requires advanced research methods (e.g., XRD). ## Crystal forms It forms acicular or fibrous crystals elongated along the crystallographic axis, which almost always occur as radial, spherical, or hemispherical aggregates.

Geological environment

## Genesis Tvedalite forms under hydrothermal conditions in the late stages of crystallization of syenite pegmatites. It fills voids and fractures in the rock. ## Mineral associations It most commonly co-occurs with minerals such as chiavennite, epididymite, eudidymite, apophyllite, quartz, calcite, and fluorite. ## Localities This is an extremely rare mineral. The most important and type locality for its occurrence is the Vevja quarry in the Tvedalen valley near Larvik in Norway. It is also known from several other localities within the same Larvik igneous complex.

Rarity

Very rare

For collectors

## Quality criteria The most prized specimens are well-formed, complete, spherical aggregates with a distinct radial structure, embedded in a contrasting rock matrix. The size of the aggregates and the intensity of any pink coloration also increase the value of the specimen. ## Popular localities The vast majority of the best collector's specimens come from the type locality in Tvedalen, Norway.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, avoiding a strong stream. It should be completely dried before storage. ## What to avoid Tvedalite is brittle and susceptible to mechanical damage. Contact with acids and other chemicals should be avoided. As a beryllium silicate, its dust can be toxic if inhaled, so any activities that could lead to its formation (e.g., cutting, grinding) should be avoided. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from dust and damage. It should be kept away from minerals of greater hardness.

Sources

IMA Mineral List (CNMNC)