Optical absorption and EPR studies on some natural carbonate minerals

Abstract

Limestone and dolomite minerals have been investigated by EPR and optical absorption studies. The optical absorption results indicate the presence of ferrous and ferric ion in both the minerals. The bands observed at 24,750, 22,780, 19,415 and 14,450 cm −1 are assigned to 6A 1 → 4T 2 ( 4D), 6A 1 → 4E, 4A 1 ( 4G), 6A 1 → 4T 2 ( 4G) and 6A 1 → 4T 1 ( 4G) d–d transitions of Fe 3+ ions, respectively. A low energy band at 10,638 cm −1 is identified as being due to Fe 2+ ion and can be attributed to 5T 2g → 5E g transition. The weak band in the region 30,000–40,000 cm −1 corresponds to Fe–O charge transfer. Crystal field and Racah parameters evaluated for the Fe 2+ ion are Dq = 990 cm −1, B = 885 cm −1 and C = 3860 cm −1 and that for Fe 3+ ions are Dq = 1040 cm −1, B = 703 cm −1 and C = 3150 cm −1. The room temperature 9 and 35 GHz EPR spectra of the minerals exhibit a sextet hyperfine pattern characteristic of Mn 2+. The EPR parameters obtained for Mn 2+ in limestone are g = 2.00399, A = −9.411 mT, D = −8.19 mT and these values confirm that the Mn 2+ ion are located in the calcite impurity. For Mn 2+ in dolomite are g = 2.0004, A = −9.45 mT for Mn 2+ substituted in the Ca lattice site and g = 2.00984, A = −9.37 mT, D = −9.94 mT for substitution at the Mg site. The EPR spectra of heat-treated limestone and dolomite samples at 950 °C show a signal corresponding to CO 2 − ion.