Abstract
Structural properties of natural jasper from Taroko Gorge (Taiwan) have been investigated by means of powder X-ray diffraction, electron paramagnetic resonance (EPR) and Fourier transform infrared spectroscopic techniques. The EPR spectrum at room temperature exhibits a sharp resonance signal at g = 2.007 and two more resonance signals centered at g ≈ 4.3 and 14.0. The resonance signal at g = 2.007 has been attributed to the E′ center and is related to a natural radiation-induced paramagnetic defect. Two more resonance signals centered at g ≈ 4.3 and 14.0 are characteristic of Fe3+ ions. The EPR spectra recorded at room temperature of jasper samples, heat-treated at temperatures ranging from 473 to 1,473 K exhibit marked temperature dependence. The resonance signal corresponding to E′ center disappears at elevated temperatures. A broad, intense resonance signal centered at g ≈ 2.0 appears at elevated temperatures. This resonance signal is a characteristic of Fe3+ ions, which are present as hematite in the jasper sample. The intensity of the resonance signal becomes dominant at elevated temperatures at ≥873 K, masking g ≈ 4.3 and g ≈ 14.0 resonance signals. The EPR spectra of jasper heat-treated at 673 K have been recorded at temperatures between 123 and 296 K. The population of spin levels (N) has been calculated for the broad g ≈ 2.0 resonance signal. It is found that N decreases with decreasing temperature. The linewidth (ΔH) of g ≈ 2.0 resonance signal of the heat-treated jasper is found to increase with decreasing temperature. This has been attributed to spin–spin interaction of the Fe3+ ions present in the form of hematite in the studied jasper sample.
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