Characterization of Indian serpentine by X-ray diffraction, photoacoustic spectroscopy and electron paramagnetic resonance spectroscopy

A variety of serpentines collected from Cuddapah region of India was investigated through X-ray diffraction (XRD), photoacoustic spectroscopy (PAS) and electron paramagnetic resonance (EPR) studies. XRD studies indicated the presence of only one phase in one of the serpentines (sample 1) whereas three other serpentines (samples 2, 3 and 4) were either biphasic or triphasic. The deconvoluted PAS spectra showed prominent peaks around 350, 374, 440, 510, 630 and 850 nm. According to the ligand field theory, these peaks were attributed to the presence of isolated Fe3+ and Fe2+ ions. EPR studies revealed the presence of Mn2+ in octahedral site (g = 2.004, D = 80 G, AMn = 95 G) and spin counting suggested that it was present at trace level. Apart from this, two more signals at g = 4.2 and g = 2.00 were observed and were assigned to Fe3+ ions at tetrahedral and octahedral sites. An unusually broad EPR signal was observed at room temperature in samples 2, 3 and 4, which was attributed to the presence of iron clusters or MgFe2O4.