ESR studies of sepiolites

The electron spin resonance (ESR) spectra for each beige and brown sepiolit es from Polatli-Ankara, Turkey, a clay mineral of Mg8Si12O30(OH)(4)(OH2)(4) (H2O)(3), at 77 K have their own single characteristic peak at g approximat e to 1.99, which begins to anneal from 437 and 536 degrees C, respectively. The sextet lines of Mn2+ were also observed in addition to two peaks (g = 4.4 and 5.0) associated with Fe3+. A high-quality sepiolite called "meersch aum" from Sepetci-Eskisehir, Turkey, has two characteristic peaks at g appr oximate to 1.99, which begins to anneal from 419 degrees C, and at g approx imate to 2.03. ESR intensities of the signal at g approximate to 1.99 for b eige, brown sepiolites and meerschaum are enhanced by gamma-ray irradiation to give equivalent doses D-E = 11.8 +/- 3.4. 4.7 +/- 2.3 and 4.6 +/- 1.2 k Gy, respectively. The ESR ages obtained by assessing the annual dose rate f rom the content of U-238, Th-232 and K-40 determined by gamma-ray spectrosc opy are 1.8 +/- 0.8, 0.7 +/- 0.5 and 0.9 +/- 0.2 Ma, respectively, falling into the Pliocene Epoch in geological time scale in agreement with the stra tigraphy.