autunite

autunite. Autunite is a hydrated calcium uranyl phosphate mineral, renowned for its vibrant yellow to greenish-yellow color and tabular, often platy, crystals. It forms in the secondary oxidation zones of uranium deposits and is a significant ore of uranium, prized by both mineral collectors and the nuclear industry. The mineral is named for its type locality near Autun in France and is known to dehydrate to a related phase called meta-autunite upon prolonged exposure to air.

Properties

Autunite crystallizes in the tetragonal crystal system, typically forming thin, square or octagonal plates with a perfect basal cleavage. Its color ranges from lemon-yellow to greenish-yellow, and it exhibits a vitreous to pearly luster, often fluorescing a bright greenish-yellow under ultraviolet light. With a Mohs hardness of only 2 to 2.5, it is a relatively soft mineral, and its specific gravity varies between 3.1 and 3.2. Chemically, it is a hydrated phosphate, with its structure containing the uranyl ion (UO₂²⁺), and it is notably radioactive due to its uranium content. A key characteristic is its alteration to meta-autunite, a dehydration product, when it loses water molecules upon exposure to dry air, a process studied extensively at institutions like the United States Geological Survey.

Occurrence and formation

Autunite occurs globally as a secondary mineral in the oxidized zones of hydrothermal uranium deposits, often associated with granitic pegmatites and sedimentary rocks. It forms through the alteration of primary uranium minerals like uraninite in the presence of phosphate-bearing groundwater. Notable localities include the type area near Autun in the Saône-et-Loire department of France, as well as significant deposits in Portugal, Germany, and Australia. In North America, fine specimens have been found in the Lakeview district of Oregon and at the Daybreak Mine in Washington. It is commonly associated with other secondary uranium minerals such as torbernite and uranophane, and is frequently found in proximity to granite and pegmatite bodies, as seen in the historic mines of Cornwall.

History and etymology

The mineral was first described in 1852 by the French mineralogist Henry J. Brooke, who named it after its discovery locality near the town of Autun in eastern France. This region, part of the Massif Central, has a long history of mining activity. The study of autunite contributed to the early understanding of radioactive elements, with researchers like Antoine Henri Becquerel and Marie Curie investigating uranium-bearing minerals. Its identification and classification were advanced through the work of institutions like the Muséum national d'Histoire naturelle in Paris. The mineral's propensity to dehydrate was a subject of study in the early 20th century, linking it to the broader field of mineralogy explored by figures such as Charles Palache.

Uses and applications

Primarily, autunite is mined as a minor ore of uranium, a critical element for the nuclear fuel cycle used in power reactors like those operated by Électricité de France and for historical weapons programs such as the Manhattan Project. Its bright fluorescence makes it a popular specimen for mineral collectors and for educational displays in museums like the Smithsonian Institution. While its direct industrial applications are limited due to its relative scarcity and radioactivity, the study of its alteration products informs geologists about weathering processes in uranium deposits. Research into its crystalline structure has also contributed to materials science, particularly in understanding hydrated phases relevant to nuclear waste containment at facilities like the Waste Isolation Pilot Plant.

Related minerals

Autunite is the calcium-dominant member of the autunite group of hydrated uranyl phosphates and arsenates. Its immediate dimorph, meta-autunite, has an identical chemical formula but a different crystal structure due to dehydration. The closely related mineral torbernite is the copper analogue, forming a solid-solution series in some deposits. Other group members include uranocircite (barium-dominant) and zeunerite (copper-dominant arsenate). It is also paragenetically associated with secondary minerals like carnotite and saleeite, and often alters from primary minerals such as uraninite and coffinite. These relationships are frequently documented in studies from the International Mineralogical Association and geological surveys worldwide.

Category:Phosphate minerals Category:Uranium minerals Category:Minerals in space group 89