Temperature dependence on the electron paramagnetic resonance spectra of natural jasper from Taroko Gorge (Taiwan)

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.