Thornasite

Chemical formula: Na<sub>12</sub>Th<sup>4+</sup><sub>3</sub>(Si<sub>8</sub>O<sub>19</sub>)<sub>4</sub>·18H<sub>2</sub>O

A rare sodium and thorium silicate, distinguished by its radioactivity and occurrence as small, hexagonal crystals in nepheline syenites.

## Characteristics Thornasite is a complex, hydrated sodium and thorium silicate, belonging to the very rare minerals. It most often forms small, well-developed crystals with a platy habit and hexagonal outline. They occur individually, in groups, or form attractive, rosette-like aggregates. They are usually too small to be seen with the naked eye, requiring magnification for observation. ## Physical Properties Thornasite crystals are relatively soft, with a Mohs hardness of 4. The luster is vitreous, and on cleavage planes, it can be pearly. The mineral is translucent to opaque. Its density is approximately 2.63 g/cm³. The most important physical characteristic is its radioactivity, resulting from the presence of thorium in its chemical composition. ## Colors and Varieties Thornasite is usually colorless, white, or takes on a pale yellowish hue. No color or commercial varieties are distinguished. ## History and Name The mineral was first described in 1984 by V.E. Ansell, A.C. Roberts, A.G. Plant, and B.D. Sturman. Its name refers to its chemical composition – the elements that build it: thorium, sodium (Natrium), and silicon. The type locality from which the research material originated is the Poudrette quarry at Mont Saint-Hilaire in Quebec, Canada. ## Applications Due to its extreme rarity and occurrence as small crystals, thornasite has no industrial application. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness
4
Luster
Vitreous to Pearly
Streak
White
Density
2.63
Cleavage
Perfect on {0001}
Transparency
Translucent to Opaque
Crystal system
Trigonal

Diagnostic features

## Identification The key diagnostic feature of thornasite is its radioactivity, which can be easily detected with a Geiger counter. In collecting conditions, identification is facilitated by the hexagonal habit of the crystals, often in the form of thin plates, and its co-occurrence with the characteristic mineral assemblage from Mont Saint-Hilaire. Color and luster are auxiliary features. ## Distinguishing from similar minerals Thornasite is sometimes confused with other white or colorless platy minerals occurring in the same paragenesis, such as catapleiite or gaidonnayite. It differs from catapleiite often by its lower transparency and slightly different hue. However, the ultimate and unambiguous distinguishing criterion is the strong radioactivity of thornasite, which similar minerals lack. ## Crystal forms Thornasite crystallizes in the form of thin to thick plates with a hexagonal outline. Crystals often arrange into rosette-like or fan-shaped aggregates, and also form nearly parallel intergrowths.

Geological environment

## Genesis Thornasite is a hydrothermal mineral, crystallizing in the late stages of pegmatite formation within alkaline intrusions, mainly nepheline syenites. It occurs in miarolitic cavities and small rock voids. ## Mineral associations This mineral occurs in association with many other rare alkaline minerals. The most common associations include: aegirine, albite, analcime, ancylite-(Ce), catapleiite, eudialyte, lorenzenite, microcline, nepheline, and sérandite. ## Localities Thornasite is an extremely rare mineral, known from only a few localities worldwide. The most important and well-known locality, which is also the type locality, is the Poudrette quarry at Mont Saint-Hilaire in Quebec (Canada). Other confirmed occurrences include the Khibiny and Lovozero alkaline massifs on the Kola Peninsula in Russia.

Rarity

Extremely rare

For collectors

## Quality criteria The collector's value of thornasite is determined by its extreme rarity. Specimens with well-formed, sharp crystals, especially those forming aesthetic, rosette-like aggregates, are most highly valued. The size of the crystals is of great importance – any specimen with crystals exceeding a few millimeters is considered exceptional. Attractiveness is enhanced by its placement on a contrasting rock matrix and its association with other rare minerals. ## Popular localities By far the most sought-after specimens by collectors come from the discovery site – Mont Saint-Hilaire in Canada. The largest and best-formed crystals of this mineral originate from there.

Care and storage

## Cleaning Thornasite specimens should be cleaned with the utmost care, preferably mechanically, using a soft brush or blowing off contaminants with compressed air. Due to its softness, hard tools should be avoided. Contact with water, especially with detergents, is not recommended. ## What to avoid The most important issue is the radioactivity of the mineral. Prolonged contact and storage of specimens in direct proximity (e.g., on a desk or in a bedroom) should be avoided. Inhaling dust that might be generated if the sample is damaged should be avoided. It is recommended to wash hands after each contact with the mineral. The mineral is also sensitive to shocks and impacts. ## Storage Thornasite specimens must be stored in clearly labeled containers, with information about radioactivity. It is recommended to keep them in well-ventilated areas, away from other minerals that could be damaged (darkened) by radiation (e.g., smoky quartz, topaz). It is best to store them in specialized boxes or in a section of the collection designated for radioactive minerals.

Sources

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