Electron spin resonance of sulfite radicals in irradiated calcite and aragonite

Radiation-induced radicals in sulfite-doped calcite and aragonite were characterized by ESR. Among the many ESR signals produced, only four signals are dependent on the SO^2-₃ concentration; an axial signal (g_⊥=2.0038 and g|=2.0024) in calcite, and an orthorhombic signal (g₁=2.0042, g_2=2.0038 and g₃=2.0025), an isotropic signal (g=2.0034) and an isotropic signal (g=2.0060) in aragonite. The former two signals exhibit hyperfine structures arising from ^33S$\text{(I=}\text{3}\text{2}\text{, 0.75\%}$ natural abundance), and are attributed to SO^-₃ radicals. The isotropic signal at g=2.0034 in aragonite is considered to be due to rotating SO^-₃ radicals. Rotating or tumbling molecules (probably SO^-₂) produced by thermal decomposition of SO^2-₃ seem to be responsible for the isotropic signal at f=2.0060.