Electron spin resonance and distribution of some elements in shales

Abstract

Five types of ESR signals were observed in shales. The resonance composed of six lines near g_eff = 2 (type A) is attributed to Mn(II). Type A resonance is divided into two groups on the basis of its line shape; one exhibits six sharp lines (type A-1), and another shows relatively broad lines (type A-2). The g_eff = 2 broad resonance (type B) may be attributed to Fe(III). The g_eff sharp resonance line (type C) probably results from stable organic free radicals. The g_eff = 4.1 resonance (type D) can be attributed to high spin Fe(III) occupying sites of orthorhombic symmetry in clay minerals. Some of the shales show an inclined signal covering a wide magnetic field (type E). Combination of ESR measurements and leaching experiments made it possible to survey the distribution and states of some elements in shales. In shales which exhibit type A-1 resonance overlapped with type A-2 resonance, Mn(II) is randomly distributed both in calcite and clay minerals. In shales which exhibit only type A-2 resonance, Mn(II) is associated mainly with clay minerals. Most of the iron in shales is bound to clay minerals. A part of the iron in clay minerals is present as Fe(III) which shows type D resonance.