Eylettersite

Chemical formula: Th⁴⁺₀.₇₅Al₃(PO₄)₂(OH)₆

Eylettersite is a rare phosphate mineral, a member of the alunite supergroup, distinguished by its thorium content and occurrence as microscopic crystals.

## Characteristics Eylettersite is a rare mineral belonging to the alunite supergroup, specifically to the plumbogummite group. Chemically, it is a hydrated aluminum and thorium phosphate. It most commonly occurs as very fine, spheroidal aggregates or earthy masses. Individual crystals are microscopic, typically rhombohedral in shape, and do not exceed 10 micrometers, making them invisible to the naked eye. ## Physical Properties Due to the microscopic size of the crystals, most physical properties are difficult to precisely determine for a single crystal. Hardness and density have been determined for aggregates. The mineral exhibits strong radioactivity due to the presence of thorium. ## Colors and Varieties Eylettersite is observed in colors ranging from pale yellow, through yellow, to yellowish-brown. No named varieties of this mineral have been distinguished. ## History and Name The mineral's name honors Léopold Eyletters (1923-1993), a Belgian mining engineer who worked at the Kobokobo mine in the Democratic Republic of Congo, where this mineral was first discovered. Eylettersite was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 1969. ## Uses Eylettersite has no industrial application. Its significance is purely scientific and as a collector's item, as a rare member of the alunite supergroup.

Properties

Mohs hardness
3-4
Color
Cream white
Luster
Dull
Streak
White
Density
3.34
Cleavage
Good on {0001}
Fracture
Uneven
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of eylettersite with the naked eye is impossible. It requires advanced laboratory techniques, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS) to confirm its structure and composition, especially the presence of thorium and aluminum. Strong radioactivity is a key diagnostic indicator. ## Distinguishing from Similar Minerals It can be confused with other secondary yellow phosphate minerals, such as crandallite, plumbogummite, or florencite. Differentiation requires chemical analysis – eylettersite is distinguished by its dominant thorium content. Other minerals in this group mainly contain calcium, lead, or rare earth elements. ## Crystal Forms It forms microscopic, well-formed crystals with a rhombohedral habit. Most often, it occurs in the form of spherulitic or spherical aggregates, as well as coatings and earthy masses.

Geological environment

## Genesis Eylettersite is a secondary mineral, forming in the weathering zones of phosphate-rich granitic pegmatites under tropical conditions. It forms as a result of hydrothermal alteration or weathering of primary thorium-bearing minerals, such as monazite, in the presence of solutions rich in phosphorus and aluminum. ## Mineral Associations It most commonly co-occurs with crandallite, wavellite, strengite, cyrilite, apatite, monazite, beryl, columbite, and quartz. ## Localities The most important and type locality is the Kobokobo mine in South Kivu Province, Democratic Republic of Congo. This is the only locality from which well-characterized specimens of this mineral originate.

Rarity

Very rare

For collectors

## Quality Criteria The quality of an eylettersite specimen primarily depends on the richness and visibility of aggregates on the rock matrix. Since individual crystals are microscopic, the intensity of color and the aesthetic arrangement of yellow coatings and spherules against a contrasting quartz or other parent mineral are assessed. Specimens with clearly defined spherulitic aggregates are more valued than earthy coatings. ## Popular Localities The only source of collector specimens is the historical type locality – the Kobokobo mine in the Democratic Republic of Congo. Material from this location is extremely rarely available on the market.

Care and storage

## Cleaning Eylettersite specimens are generally not cleaned due to their microscopic nature and fragility. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water and chemicals. ## What to Avoid The mineral is strongly radioactive – limit exposure and store it away from other radiation-sensitive minerals. Avoid contact with acids and any cleaning fluids. Protect from abrasion and shock. ## Storage It is recommended to store in specialized, lead-lined containers designed for radioactive minerals, in a dry place. Specimens should be clearly labeled as radioactive. Store away from areas of permanent human habitation.

External references

Sources

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