Spectroscopic investigations on Fe3+, Fe2+ and Mn2+ bearing antigorite mineral

A dark green antigorite, a variety of serpentine mineral from Salem, Tamilnadu State of India and supplied by M/s Alminrock, Bangalore is investigated. The AAS analysis show that the mineral contains Fe(0.463), Cr(0.049), Mn(0.014) and Ni(0.012 %). Of the eleven bands observed in the optical absorption spectrum, 15428, 17852, 19602, 22773, 24869 and 26448 cm(-1) are assigned to trivalent iron and the other at 9122, 10635, 11679, 15380, 22929 cm(-1) to divalent iron. The crystal field parameters evaluated for Fe3+ and Fe2+ are Dq = 875, B = 700 and C = 2800 cm(-1) and Dq = 990, B = 885 and C = 3860 cm(-1) respectively. The EPR spectrum clearly indicates two parts, one part having a g-value in the range 5.4-3.1 indicating the presence of Fe3+ in the lattice. The other has a g-value of 2.0 the sextet with a spacing of 92.5 G. This sextet can be assigned to Mn2+ ion in substitution for Mg2+ in the mineral. The weak lines in between the sextet are due to the forbidden transitions.