J. Gotze et al., Defect structure and luminescence behaviour of agate - results of electronparamagnetic resonance (EPR) and cathodoluminescence (CL) studies, MINERAL MAG, 63(2), 1999, pp. 149
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.