• journal_title：Clay Minerals
• Contributor：N. Worasith ; B. A. Goodman
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2012-12-01
• Format：text/html
• Language：en
• Identifier：10.1180/claymin.2012.047.4.10
• journal_abbrev：Clay Minerals
• issn：0009-8558
• volume：47
• issue：4
• firstpage：539
• section：Articles

Electron paramagnetic resonance (EPR) spectra of different particle size fractions of four kaolins from diverse sources in North America, Europe and Asia have been investigated in order to characterize their paramagnetic properties and heterogeneity. There were major differences in the sources of the EPR signals from transition metals; V and Mn were structural, Fe was both structural and as associated oxides, and Cu was in the form of an adsorbed ion. The radiation-induced free radical signals commonly known as the A- and B-centres were observed in three of the deposits; however, in addition to the previously reported ²⁷Al hyperfine structure associated with the B-centre, we also observed much smaller ²⁷Al hyperfine structure on the g_⊥ feature of the A-centre. The other kaolin sample produced four free radical signals that have not previously been reported in kaolins. Each had substantial ¹H hyperfine splitting; three are interpreted as corresponding to defect centres associated with Si-OH groups, and the other to a Si hole surrounded by protonated O atoms. The EPR spectra changed progressively with particle size, and measurements on the Asian specimens after grinding showed major differences in the Fe³⁺ signals from the same particle size fractions separated from the natural samples, thus supporting previous reports that grinding results in major structural changes in the minerals.