Ichnusaite

Chemical formula: Th<sup>4+</sup>(Mo<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O

An extremely rare, radioactive thorium molybdate, forming colorless, tabular crystals with a pearly luster.

## Characteristics Ichnusaite is a hydrated thorium molybdate, a mineral of exceptional scientific importance due to its rarity and chemical composition. It occurs as extremely small, thin, tabular crystals, rarely exceeding a few tens of micrometers in size. These crystals are colorless and transparent, often with a square or rectangular outline. They typically form characteristic radial aggregates, rosettes, or small, spherical clusters. ## Physical Properties This mineral is characterized by a pearly luster on the perfect cleavage surface {100}, while on other faces the luster is vitreous. It is transparent. Due to the microscopic size and fragility of the crystals, its hardness has not been precisely measured, but it is a very soft mineral. The density calculated based on the chemical composition and unit cell parameters is 4.29 g/cm³. ## Colors and Varieties Ichnusaite is colorless. No colored or commercial varieties are distinguished. ## History and Name The mineral's name comes from the ancient Greek name for Sardinia - *Ichnusa*, where it was discovered. Ichnusaite was first described in 2014 by a group of researchers (Orlandi et al.) based on material collected at the Su Seinargiu locality, in the municipality of Sarroch, Sardinia, Italy. ## Uses Due to its extreme rarity and microscopic size, ichnusaite has no commercial or industrial applications. It is solely an object of scientific research and a valuable acquisition for specialized micromount mineral collectors.

Properties

Luster
Pearly, Vitreous
Streak
White
Density
4.29
Cleavage
Perfect on {100}
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification A key diagnostic feature of ichnusaite is its strong radioactivity, which can be detected with a Geiger counter. Visual identification is very difficult due to the microscopic size of the crystals. Characteristic features include thin, colorless lamellae forming radial aggregates. Final confirmation requires advanced analytical methods such as X-ray diffraction (XRD) and chemical microanalysis (EDS). ## Distinguishing from Similar Minerals Ichnusaite is most often confused with nuragheite - another thorium molybdate discovered in the same locality. These minerals are visually almost identical, and their differentiation is only possible through X-ray analysis. From other colorless platy minerals (e.g., gypsum, muscovite), it is distinguished primarily by its extreme rarity, specific occurrence environment, and strong radioactivity. ## Crystal Forms It forms very thin, tabular crystals with a platy or lamellar habit. These crystals combine into small, radial aggregates, rosettes, and spherulitic clusters.

Geological environment

## Genesis Ichnusaite is a secondary mineral, forming in the oxidation zone of granitic pegmatites. Its crystallization occurs as a result of weathering and alteration of primary minerals containing thorium (likely thorite or uraninite) and molybdenum (molybdenite) under low temperature and pressure conditions. ## Mineral Associations This mineral occurs in close association with nuragheite, muscovite, xenotime-(Y), and remnants of primary molybdenite. ## Localities The only confirmed locality of ichnusaite in the world is its type locality: Su Seinargiu, Sarroch, Cagliari, Sardinia, Italy.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of specimens of this micromineral is assessed based on several criteria. The most important is the abundance and development of crystals - the more well-defined, radial aggregates on the rock matrix, the more valuable the specimen. The aesthetics of the aggregate itself and the presence of associated minerals, especially the equally rare nuragheite, are also important. Due to their microscopic nature, specimens are evaluated under magnification. ## Popular Localities The only source of ichnusaite specimens is the type locality in Sardinia. Material from this location is extremely difficult to obtain and is a rarity in the most advanced systematic and micromount mineral collections.

Care and storage

## Cleaning Ichnusaite specimens are extremely delicate and fragile. Any mechanical cleaning (including brushing) or chemical cleaning is strongly discouraged. This mineral should be treated solely as a specimen for microscopic observation, without attempting to interfere with its structure. ## What to Avoid Ichnusaite is highly radioactive due to its thorium content. Direct contact, inhalation of dust, and storage in areas of permanent human presence must be strictly avoided. The mineral is very soft and has perfect cleavage, making it susceptible to mechanical damage. It should be protected from moisture, temperature changes, and chemicals. ## Storage Specimens must be stored in specialized, lead-lined containers designed for radioactive minerals, with clear radiation warnings. It should be kept away from other minerals that may be sensitive to radiation (e.g., causing color changes in smoky quartz or amethyst).

External references

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