Tristramite

Chemical formula: (Ca,U₄+,Fe₃+)(PO₄,S₆+O₄)·₂H₂O

Tristramite is a rare, secondary uranyl mineral of the rhabdophane group, forming pale yellow, earthy or fibrous coatings and aggregates.

## Characteristics Tristramite is a hydrated calcium, uranium, and iron phosphate, belonging to the rhabdophane group. It occurs as very fine, fibrous crystals forming aggregates, coatings, and efflorescences with an earthy or chalky appearance. Individual crystals are usually too small to be seen with the naked eye. Due to its chemical composition, this mineral is radioactive. ## Physical Properties Tristramite crystals are extremely small and delicate. The mineral is characterized by low hardness, approximately 2-3 on the Mohs scale, and low density. It is translucent to opaque, and its luster is described as earthy or dull. ## Colors and Varieties Tristramite occurs in shades from pale yellow to bright yellow, which is typical for many secondary uranium minerals. No varieties have been distinguished. ## History and Name The mineral was first described in 1982 by J.E.T. Horn, M.K.R. Fuchs, E.E. Horn, and D.D. Hogarth. Its name comes from its discovery location (type locality) – Geevor Mine in Cornwall, England. This mine was also historically known as Wheal an Tristram. ## Uses Tristramite has no industrial application. Its significance is purely scientific and as a collector's item, as a rare secondary mineral from the oxidation zones of uranium deposits.

Properties

Mohs hardness: 2-3
Luster: Earthy
Streak: Light yellow
Density: 3.2
Cleavage: None
Fracture: Earthy
Transparency: Translucent to opaque
Crystal system: Hexagonal

Diagnostic features

## Identification Tristramite is identified by its characteristic mode of occurrence – as pale yellow, earthy or chalk-like coatings and crusts. A key diagnostic feature is its radioactivity, which can be detected with a Geiger counter. It occurs in the oxidation zones of uranium deposits. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals of similar color and form, such as ningyoite, with which it forms a series, or other phosphates from the rhabdophane group. Definitive differentiation from similar minerals (e.g., gypsum, other sulfates) requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS), especially to confirm the presence of phosphorus, uranium, and iron. ## Crystal Forms It forms very fine, acicular or fibrous crystals that combine into dense, felted aggregates. It also occurs as earthy, powdery, or chalk-like coatings and crusts on host rocks.

Geological environment

## Genesis Tristramite is a secondary mineral, formed in the oxidation zones of hydrothermal uranium-bearing deposits. It forms as a result of weathering and alteration of primary uranium minerals, such as uraninite, in the presence of phosphate and sulfate-rich solutions. ## Mineral Associations It often co-occurs with other minerals from the oxidation zone. The most common associations include uraninite, chalcopyrite, sphalerite, galena, fluorite, quartz, and other secondary uranium minerals. ## Localities The most important and confirmed occurrences of this rare mineral are in Cornwall (England), especially in the Geevor Mine (type locality) and South Terras. It is also known from several locations in Germany (Saxony and Thuringia) and the Czech Republic.

Rarity

Rare

For collectors

## Quality Criteria The collector's appeal of tristramite primarily depends on the richness and intensity of the coating's color on the host rock. The most prized specimens are those where the mineral forms a well-visible, thick, and uniform layer of vibrant yellow color. The aesthetics of the host rock and the presence of associated minerals that create an interesting contrast are also important. ## Popular Localities By far, the most classic and sought-after specimens by collectors come from the historical localities in Cornwall, England, which are its discovery site. Specimens from German and Czech localities are also present on the collector's market.

Care and storage

## Cleaning Tristramite specimens should generally not be wet cleaned. Due to its delicate, earthy or fibrous nature, it is best to remove dust with a soft brush or compressed air from a safe distance. Contact with water can damage or dissolve delicate aggregates. ## What to Avoid Contact with water, chemicals, acids, and ultrasonics should be strictly avoided. The mineral is brittle and sensitive to mechanical shock. As a radioactive mineral, it requires careful handling – hands should be washed after each contact, and dust inhalation should be avoided. ## Storage Specimens should be stored in a dry place, in closed containers (e.g., "perky boxes"), to protect them from dust, moisture, and mechanical damage. They should be kept away from radiation-sensitive minerals and stored in a designated area for radioactive minerals, with appropriate labeling.

Sources

IMA Mineral List (CNMNC)