Autunite: Properties and Occurrence

Autunite (hydrated calcium uranyl phosphate) with the formula: Ca(UO2)2(PO4)2·10-12H2O; is one of the more attractive and popular radioactive minerals. IIt is a yellow-greenish fluorescent mineral with a hardness of 2 – 2 1⁄2.

Autunite was discovered in 1852 near Autun, France. It occurs as an oxidation product of uranium minerals in granite pegmatites and hydrothermal deposits. Associate minerals include meta-autunite, torbernite, phosphuranylite, saleeite, uranophane, and sabugalite.

General Information

  • Category: Phosphate minerals
  • Formula: Ca(UO2)2(PO4)210-12H2O
  • Crystal system: Orthorhombic
  • Crystal class: Dipyramidal (mmm).

Properties

The structure is composed of phosphate tetrahedrons linked to uranium-oxygen groups that form distorted octahedrons. The phosphates and uranium groups lie in sheets that are weakly held together by water molecules. This structure produces the tabular habit, the one perfect direction of cleavage, and the relative softness.

  • Formula mass: 986.26 g/mol
  • Color: Lemon-yellow to sulfur-yellow, greenish yellow to pale green
  • Crystal habit: Tabular crystals, foliated or scaly aggregates, and in crusts
  • Fracture: uneven
  • Mohs scale hardness: 2-2.5
  • Luster: Vitreous – pearly
  • Streak: Pale yellow
  • Diaphaneity: Transparent to translucent
  • Specific gravity: 3.1-3.2
  • Density: 3.15

Autunite crystallizes in the orthorhombic system and often occurs as tabular square crystals. Due to the moderate uranium content of 48.27 %, it is radioactive and also used as uranium ore. If the mineral dries out, it converts to meta-autunite-I, which can turn into meta-autunite-II after heating. These two subsequent minerals are very rare in nature.

Occurrence

It is mainly found in uranium-bearing pegmatites around the world. This mineral was first discovered near Autun, France in 1852. Presently it is found from the Daybreak Mine near Mount Spokane in Washington. Spruce Pine in North Carolina is another rich source of this material. It is also mined in Urgeiriça and Trancoso in Portugal and Mt. Painter in South Australia.

Autunite Uses

This radioactive mineral is a fairly rich source of uranium with approximately 48.27% uranium content. Due to this reason, it is used as a uranium ore in various industries. It is also used as a mineral specimen.

 

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