Meta-autunite from the Daybreak Mine, Washington
Contributed by: Michael Crawford
Date: Jun 22nd, 2025
Locality: Daybreak Mine, Mount Kit Carson, Spokane County, Washington, USA (See on Mindat)
Size: 4 x 6 cm
Description:
A thumbnail specimen of meta-autunite from the Daybreak Mine, Spokane County, Washington. Meta-autunite is a secondary mineral formed from the oxidation of primary uranium minerals in hydrothermal veins. Meta-autunite is a uranyl phosphate mineral, (Ca(UO2)2(PO4)2·2-6H2O). The uranyl ion in the meta-autunite crystalline structure activates the green fluorescence. The uranyl ion is an integral component of the chemical makeup of autunite, the fluorescence is considered to be intrinsic. Trace amounts of the uranyl ion can replace some of the ions in minerals such as chalcedony, opal-AN, aragonite, adamite, etc. and activate fluorescence. This impurity activation of fluorescence is called extrinsic. Minerals with intrinsic fluorescence are usually very radioactive (>700 CPM to tens of 1000’s CPM). This meta-autunite specimen registered 29,000 CPM. Extrinsic fluorescence can be activated with uranium content in the sub-ppm level. Minerals with extrinsic uranyl activated fluorescence have background levels to slightly elevated levels of radioactivity (0 to 500 CPM).
The uranyl ion has a linear structure with a hexavalent uranium atom between two oxygen atoms. Short uranium-oxygen distances between atoms indicate multiple bonds between uranium and oxygen. The uranium in the uranyl ion has an oxidation state of 6+, so there are two bonds with each oxygen atom leaving the uranyl ion with an oxidation state of 2+. There are 2 bonds between each oxygen atom and the uranium atom. Linear vibrations and bending of the atoms occur when exposed to ultraviolet illumination causing multiple peaks in the emission spectrum. The multiple peaks are called vibronic. The position and strength of the vibronic peaks depend on the coordination with the other atoms in the mineral.
The meta-autunite shortwave emission spectrum has 5 peaks at 501 nm, 523 nm, 546 nm, 571 nm and 600 nm. The brightest peak is at 523 nm. The longwave emission spectrum is identical to the shortwave spectrum. Manual Robbins stated in his book that meta-autunite and autunite have identical fluorescence.
Summary of luminescence responses:
Meta-autunite
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Green