A systematic investigation of agates from different occurrences in Europe, Northern and Southern America reveals that macrocrystalline quartz and chalcedony within them have an unusually high uranium content. Whereas agates may contain >70 ppm of U, quartz from magmatic and metamorphic rocks as well as pegmatite quartz commonly exhibit U concentrations at sub-ppm levels. Spatially resolved trace-element analyses by laser ablation inductively coupled plasma mass spectrometry show that the distribution of U within the agate samples is heterogeneous and coincides with the structural banding. The results indicate that U is incorporated into agate as uranyl ions. These ions, which are bound to the silica surface, are interpreted to originate from the parallel accumulation of Si and U by alteration processes of surrounding host rocks during agate formation.
The uranyl ion is the cause of 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 to a ground state of the uranyl ion and is shown by a typical emission line at ~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 a U content down to the 1 ppm level.