Mineral spotlight: Autunite

Autunite is a wonderful mineral for several reasons. First and most obviously is the distinctive bright lime-green color and unusual rectangular tabular crystal habit it can achieve — resembling sheets in a book. Autunite is also radioactive, as it is a minor ore of uranium, but perhaps most interesting is the vivid green color it exhibits when placed under ultraviolet light.

The ability of a mineral to “glow” under ultraviolet light is called fluorescence, and is a relatively uncommon phenomenon for minerals to display. Different wavelengths of ultraviolet light can make some fluorescent minerals glow better than others. For example, rubies will often fluoresce well under long-wave ultraviolet light, whereas the zinc mineral willemite fluoresces only under shortwave ultraviolet light. Autunite glows well under both long and shortwave ultraviolet light. Autunite is a type of fluorescent mineral known as “self-activating,” which means the elements causing the fluorescence are a part of the chemical formula of the mineral itself, not trace elements as with most fluorescing minerals. The uranyl (UO2) ion of Autunite can also causes fluorescence in other uranium minerals.

Autunite is hydrated calcium uranyl phosphate, and is found in the oxidation zone of uranium deposits which is where the ore is in contact with the oxygen from water and air. Over time, water in autunite’s crystal structure can be lost (called dehydration), turning into the mineral meta-autunite. Uranium oxidation zones can yield exceptionally colored minerals with fantastic crystals such as autunite, the bright yellow mineral carnotite, torbernite, which is bright green and has a square platy crystal habit, and andersonite, which is bright yellow and can fluoresce pale blueish-green.

Come see bright green autunite glow in a new display of fluorescent minerals at the Dice Mineralogical Museum, open 12:30 p.m. to 4 p.m. on Wednesdays, Thursdays and Fridays.