Bassetite

Chemical formula: Fe<sup>2+</sup>(U<sup>6+</sup>O<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>10</sub>

Bassetite is a rare, secondary uranium mineral characterized by its distinctive yellow color and platy crystals, occurring in the oxidation zones of uranium deposits.

## Characteristics Bassetite is a hydrated uranyl iron phosphate, belonging to the autunite group. It forms very small, thin, platy crystals with square or octagonal outlines, often with truncated corners. These crystals typically occur as fan-shaped or rosetted aggregates, as well as coatings and efflorescences on other minerals. Due to its chemical composition, bassetite is a radioactive mineral. ## Physical Properties Bassetite crystals are very brittle and soft, with a Mohs hardness of approximately 2.5. They exhibit a vitreous luster, and a pearly luster on cleavage surfaces. They are transparent to translucent. The mineral's density is about 3.1 g/cm³. ## Colors and Varieties Bassetite occurs in various shades of yellow – from sulfur-yellow, through yellowish-brown, to olive-green. Its color may darken due to the oxidation of iron. No named varieties are distinguished. ## History and Name The mineral was first described by Arthur F. Hallimond in 1915. Its name comes from its discovery location – the Basset Mine in Redruth, Cornwall (Great Britain). This is its type locality. ## Uses Bassetite, like other secondary uranium minerals, has no direct industrial application due to its rarity and occurrence in small quantities. It is of purely scientific and collector's interest.

Properties

Mohs hardness
2.5
Luster
Vitreous, Pearly
Streak
Pale yellow
Density
3.1
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Bassetite can be identified by its characteristic yellow color, platy crystal habit forming fan-shaped aggregates, and pearly luster on cleavage surfaces. An important diagnostic feature is its strong, green fluorescence under shortwave ultraviolet (UV-SW) light. It is a radioactive mineral. ## Distinguishing from Similar Minerals Bassetite is visually almost identical to other minerals of the autunite group, such as autunite, meta-autunite, torbernite, or uranocircite. Certain differentiation from these minerals often requires advanced chemical analyses (EDS) or X-ray diffraction (XRD). Analysis of associated minerals and the geological context of occurrence can be helpful. ## Crystal Forms Crystals are thin-platy, with a square or octagonal outline. They usually form fan-shaped, radial, or rosetted aggregates. It also occurs as fine-grained efflorescences and coatings.

Geological environment

## Genesis Bassetite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal uranium deposits. It forms as a result of the alteration of primary uranium minerals, mainly uraninite, in the presence of phosphate-rich waters. ## Mineral Associations It most commonly co-occurs with other secondary uranium minerals, such as torbernite, uraninite, autunite, pyromorphite, as well as quartz and goethite. ## Localities The most important bassetite localities worldwide include, in addition to the type locality in Cornwall (Great Britain), deposits in the Ore Mountains region (Schneeberg, Johanngeorgenstadt) in Germany, Jáchymov in the Czech Republic, and several places in France (e.g., Margnac) and the USA (South Dakota).

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals forming aesthetic, fan-shaped or rosetted aggregates are most valued by collectors. An intense, pure yellow or yellowish-green color is desirable. Contrast with the rock matrix and the presence of associated minerals are also important. Due to the small size of the crystals, specimens suitable for viewing without magnification are rare. ## Popular Localities Classic, historical specimens come from the Basset Mine in Cornwall. Currently, specimens from the German and Czech Ore Mountains, which provide aesthetic micromounts, mainly appear on the collector's market.

Care and storage

## Cleaning Bassetite specimens are extremely delicate and brittle, so mechanical cleaning should be avoided. If absolutely necessary, a stream of compressed air can be used to remove dust. Contact with water, which can damage the crystals, should be avoided. ## What to Avoid The mineral is sensitive to water and chemicals. It should be protected from moisture, temperature changes, and direct sunlight, which can cause dehydration and color change. As a radioactive mineral, it requires careful handling – avoid inhaling dust and wash hands after each contact. ## Storage It is recommended to store specimens in sealed, airtight containers (e.g., "membrane boxes") that protect them from mechanical damage, dust, and humidity changes. Due to radioactivity, specimens should be kept away from areas of constant human presence, in a well-ventilated room.

Sources

Read more