Uranium and uranyl luminescence in agate/chalcedony


Uranium and uranyl luminescence in agate/chalcedony

Götze, J.; Gaft, M.; Möckel, R.

A systematic investigation of agates from occurrences around the world revealed that macrocrystalline quartz and chalcedony of these samples have unusual high U content. The content of U in agates may reach more than 70 ppm and is in contrast to crystalline quartz from magmatic and metamorphic rocks as well as pegmatite quartz which have U concentrations in the sub-ppm level. Spatially resolved trace-element analyses by LA-ICP-MS illustrate that the distribution of U within the agate samples is heterogeneous and is related to the structural banding. The results indicate that U is incorporated as uranyl ion which is bound to the silica surface and originates from the parallel accumulation of Si and U by alteration processes of surrounding host rocks during agate formation.
The uranyl ion is the cause for the greenish photoluminescence (PL) in agate, which can only be excited by short wavelengths (< 300 nm). The green PL is due to the electron transition from an excited level 1φ1u(1δ1u) back to the 1π3g level and is shown by a typical emission line at ca. 500 nm accompanied by several equidistant lines. These are due to the harmonic vibration of oxygen atoms along the uranyl axis. Luminescence can be detected in samples with U content down to the 1 ppm level.

Keywords: Luminescence; green PL; uranium; uranyl; agate; chalcedony; quartz

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Publ.-Id: 21681