Thomsonite-Sr

Chemical formula: NaSr₂Al₅Si₅O₂₀·₆-₇H₂O

A rare zeolite from the thomsonite group, distinguished by strontium dominance in its chemical composition, forming acicular or radial aggregates.

## Characteristics Thomsonite-Sr is a rare mineral from the zeolite group, being the strontium analog of thomsonite-Ca. It belongs to the thomsonite series, where strontium (Sr) replaces calcium (Ca). It most often forms small, acicular or hair-like crystals gathered in radial, spherulitic, or fibrous aggregates. Individual crystals are usually too small to be observed with the naked eye. Aggregates typically have a white or colorless hue and a silky luster. ## Physical Properties Thomsonite-Sr is characterized by a hardness in the range of 5-5.5 on the Mohs scale. Its crystals exhibit perfect cleavage in two directions. The luster is vitreous on crystal faces and silky on the surfaces of fibrous aggregates. It is a relatively light mineral, with a density of approximately 2.5 g/cm³. ## Colors and Varieties This mineral is usually colorless or white. Due to its rarity, no color or commercial varieties are distinguished. ## History and Name The name "thomsonite-Sr" refers to its chemical composition – it is a member of the thomsonite series with dominant strontium. It was recognized by the International Mineralogical Association (IMA) in 1999. Its discovery is linked to research on zeolites from alkaline complexes. ## Applications Thomsonite-Sr has no industrial applications. Its significance is purely scientific and collectible, as a rare representative of the zeolite group.

Properties

Mohs hardness: 5-5.5
Luster: Vitreous, Silky
Streak: White
Density: 2.5
Cleavage: Perfect on {010}, good on {100}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Thomsonite-Sr is difficult to identify visually without specialized tests. Its characteristic features are radial or spherulitic aggregates composed of very fine, acicular crystals with a silky luster. It occurs in gas cavities and fissures in volcanic rocks. ## Distinguishing from Similar Minerals It can be confused with other fibrous zeolites, such as natrolite, mesolite, scolecite, or thomsonite-Ca. Final differentiation from thomsonite-Ca and other zeolites requires advanced chemical analyses (e.g., EDS) to confirm strontium dominance. ## Crystal Forms It forms elongated, bladed, or acicular crystals, which almost always occur as radial, spherical (spherulitic), or fibrous aggregates. Single, well-formed crystals are extremely rare.

Geological environment

## Genesis Thomsonite-Sr is a low-temperature hydrothermal mineral. It forms in the late stages of crystallization in gas cavities and fissures in alkaline volcanic rocks, such as phonolites and nephelinites. It crystallizes from strontium-rich solutions. ## Mineral Associations It often co-occurs with other zeolites, such as analcime, natrolite, thomsonite-Ca, as well as with calcite, nepheline, and microcline. ## Localities The most important localities for this mineral are in Russia, on the Kola Peninsula (Khibiny and Lovozero massifs), where it was discovered. It has also been reported in the Mont Saint-Hilaire quarry in Quebec (Canada) and in the Los Archipelago volcanic complex (Guinea).

Rarity

Very rare

For collectors

## Quality Criteria The most prized thomsonite-Sr specimens are those that form well-defined, spherical aggregates (spherulites) with a distinct silky luster, set on a contrasting rock matrix. The size of the aggregates and the absence of mechanical damage are key factors influencing value. Due to its rarity, any well-documented specimen is valuable. ## Popular Localities Specimens from the classic localities on the Kola Peninsula in Russia, which provided the type material for this mineral, are by far the most sought after by collectors.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. Due to the delicate, fibrous structure of the aggregates, avoid washing in water, which could damage them. Ultrasonic cleaning is prohibited. ## What to Avoid Avoid contact with water and all chemicals, including acids and bases. The mineral is sensitive to high temperatures, which can cause water loss from its structure (dehydration) and destroy the specimen. Protect it from impacts and vibrations. ## Storage Thomsonite-Sr specimens, due to their fragility and delicacy, are best stored in enclosed, padded "membrane boxes" or in stable containers that protect against dust and mechanical damage. Avoid exposure to direct sunlight and changes in humidity.

Sources

IMA Mineral List (CNMNC)