Defect structure and luminescence behaviour of agate - results of electronparamagnetic resonance (EPR) and cathodoluminescence (CL) studies

Samples of agate and quartz incrustations from different parent volcanic ro cks of certain world-wide localities were investigated by EPR, CL and trace element analysis. In all agate samples the following paramagnetic centres were detected: O-2(3-), E-1', [AlO4](0), [FeO4/M+](0), and [GeO4/M+](0). Ce ntres of the type [TiO4/Li+](0) and [TiO4/H+](0), which were detected in qu artz of the parent volcanics, are absent in agate. Generally, the abundance of O-2(3-) centres (silicon vacancy) and E-1' centres (oxygen vacancy) in agate is remarkably higher than in quartz. The high defect density in agate s points to rapid growth of silica from a strongly supersaturated solution probably with a noncrystalline precursor. CL microscopy reveals internal structures and zoning in agates and quartz i ncrustations which clearly differ from those discernible by conventional po larizing microscopy. The CL spectra of agates differ from those of quartz f rom crystalline rocks. At least three broad emission bands were detected in the CL spectra: a blue band of low intensity, a yellow band at about 580 n m, and an intense red band at 650 nm. The CL emission at 650 nm shows some relations to the hydroxyl or alkali content and the abundance of O-2(3-) ce ntres and E-1' centres. The emission intensity increases during electron bo mbardement due to the conversion of different precursors (e.g. = Si-O-H, = Si-O-Na groups) into hole centres. Another conspicuous feature in the CL sp ectra of agates is the existence of a yellow emission band centred at aroun d 580 nm. The predominance of the yellow CL emission band and the high conc entration of E-1' centres are typical for agates of acidic volcanics and ar e indicative of a close relationship between the two.