Optical Absorption and Epr Behaviours of Two Pyralspite Garnets in Solid
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OPTICAL ABSORPTION AND EPR BEHAVIOURS OF TWO PYRALSPITE GARNETS IN SOLID 1 1 1 2 Lucio P. Neves , Ana P. Perini , Antonio A.R. Marques , T.K. Gundu Rao , Natasha N. Lucas2, Karina M. Sato2, S. Watanabe2 1Institute for Energy and Nuclear Researches, São Paulo, Brazil 2Institute of Physics, University of São Paulo, Brazil Abstract Two piralspite garnet, one called G5, a solid solution of 79.51% almandine, 11.73% pyrope, 4.50% spessartine and 4.27% grossular and the other one called G6, a solid solution of 86.092% almandine, 4.007% pyrope, 1.192% spessartine and 8.709% grossular were investigated as to optical absorption and EPR properties. In G5 a small percentage of iron is found in (2+)-state, but in G6 a large percentage of iron is in (2+)- state. This fact explains why in G6 the EPR spectrum contains only few very weak signals. In natural G5 sample iron-ions form magnetite particles, in large number, so that a large EPR signal around g + 9.0 is observed experimentally. Annealing at 1000 to 1100 ºC both G5 and G6 samples present typical Fe3+ magnetic dipole interaction signal around g = 2.0. Keywords: EPR, Optical Absorption, Piralspite Garnets. Introduction Six silicate minerals are component of the so called Garnet Group: pyrope, almandine, spessartine, grossular, andradite and uvarovite. The three first minerals constitute a subgroup called pyralspite and the three last ones the subgroup called ugrandite. The three pyralspites garnets have a general chemical formula X3Al2Si3 O12, X = Mg for pyrope, X = Fe for almandine and X = Mn for spessartine. In nature it is not common to find pure pyrope or pure almandine, etc. Manning [1] reported 12 optical absorption bands in both G5 and G6 most of them attributed to Fe3+ in octahedral environment, but 1299, 1695, and 2380 nm bands have been attributed to Fe2+ in dodecahedral site. We did not, however, find any work on EPR properties in the literature. In this work, we investigated the optical absorption (OA) and electron paramagnetic resonance (EPR) properties of two solid solutions found in nature, both them being members of pyralspite subgroup. Material and methods Two pyralspite garnets, one called G5, a solid solution of 79.51% almandine, 11.73% pyrope, 4.50% spessartine and 4.27% grossular and the other one called G6, a solid solution of 86.092% almandine, 4.007% pyrope, 1.192% spessartine and 8.709% grossular have been investigated as to optical absorption and EPR properties. For OA measurements the samples were cut into lamina with 5 mm x 5 mm x 1.5 mm size and polished. For EPR measurements portions of samples were crushed and sieved, retaining grains of sizes between 0.080 and 0.180 mm, they were called PS5 and PS6. Both measurements have been carried out after annealing at high temperatures of 800 to 1100 °C. Results In this work we added 370 nm UV band and 2532 nm IR band. For reason that becomes clear later we concentrated on 1299 nm Fe2+ band, which in G5 sample has an absorbance of about 3.5 (a.u.) and in G6, 1.5 (a.u.). Since the X ray fluorescence analysis indicated 34.1 w% of iron in G5 and 12,9 w% in G6, we infer that large concentration of iron is in the form of Fe3+ state in G5 while in G6 small concentration of Fe is in (3+)-state. Fig. 1(a) shows the absorption spectra of natural and annealed at 200 to 10000C samples of G5 in the interval from 800 nm to 2000 nm, Fig. 1(b) is of G6. (a) (b) Fig. 1 Optical absorption spectra after the heat treatment between 200 and 1000 °C/1hr. (a) G5 sample and (b) G6 sample. The EPR spectrum of natural G6 sample consisted of very weak signals around g = 2.0 and 4.3; that of the sample annealed at 1100 °C presented an intense signal at g = 2.0. G5 sample, on the other hand, presented spectra shown in Fig. 2. The natural sample has shown a spectrum with a signal with g = 9.0, samples annealed at around 800-900 0C spectrum with g = 6.0 and annealed at higher temperature, signal at g = 2.0. Fig. 2 EPR spectra of G5 sample annealed at 800, 900, 1000 and 1100 for 1 hr in the field interval of 500 to 6500 G. Conclusion Although G5 and G6 samples are solid solutions, with large percentage of almandine, they presented a distinct behavior concerning their optical absorption around Fe2+ - 1300 nm band under pre-irradiation annealing below 800 °C. In G5 this band indicate that a large fraction of Fe is in (3+)-state, whereas in G6 is (2+)-state. This fact gives rise to a EPR spectrum with very weak signals in G6 sample; in G5 natural sample the EPR signal seem to indicate that iron in (3+)-state form a large number of magnetite particles. The signal is a large one with g=9.0, typically of a magnetite sample. With high temperature annealing above 900 ºC, magnetite particles transform into hematite particles and an EPR signal has g=6.0. At very high temperature annealing individual Fe3+ ions result their magnetic dipole interaction with each other gives rise to a large signal around g=2.0. Acknowledgments This work was supported in part by the Brazilian agencies São Paulo Research Foundation (FAPESP, Grants No. 2013/15669-3 and 2013/21741-9) and CAPES (Projeto CAPES Pró-Estratégia No. 1999/2012). The work of Dr. T. K. Gundu Rao was supported by a Visiting Professor grant from CAPES, Brazil. References [1] P. G. Manning, The optical absorption spectra of some andradites and the identification of the 6A1→4A14E(G) transition in octahedrally bonded Fe3+. Canadian Journal of Earth Sciences, 1967, 4(6): 1039-1047. .