ESR SIGNALS FROM NATURAL GYPSUM SINGLE-CRYSTALS - IMPLICATIONS FOR ESR DATING

Electron spin resonance (ESR) spectra of ancient gypsum single crystals were examined at room temperature to investigate the paramagnetic centers, which may be useful for ESR dating. The angular dependence of the ESR signals revealed four paramagnetic centers: G1, nearly isotropic with g = 2.0028-2.0042; G2, nearly axial with g(x) = 2.0084 +/- 0.0002, g(y) = 2.0088 +/- 0.0002, and g(z) = 2.0192 +/- 0.0002; G3, nearly axial with g(x) = 2.0029 +/- 0.0002, g(y) = 2.0027 +/- 0.0002, and g(z) = 1.9973 +/- 0.0002; G4, two magnetically non-equivalent centers (g = 1.999-2.008), each having two lines of hyperfine structure. G1 is attributed to a SO3- radical (an electron trapped at an oxygen vacancy), based on its g-factor and the observed hyperfine structure of S-33. Two signals (g = 2.004 and 2.009 in powder spectra) that have been previously used for ESR dating correspond to G1 and G3 for the former and G2 for the latter. Isochronal (20 min) annealing at 60-160-degrees-C implied that all four signals were thermally stable as long as gypsum (CaSO4.2H2O) did not convert into bassanite (2CaSO4.H2O). Transfer of spin from more unstable centers to G1 was found during the annealing. To avoid the superposition of different centers and the possible effect of spin transfer, the signal at g = 2.009 seems to be more suitable than the signal at g = 2.004 for dating gypsum powders.