Thermal decomposition of NH4+-vermiculite from Santa Olalla (Huelva, Spain) and its relation to the metal ion distribution in the octahedral sheet

Abstract

The investigation of the NH3 loss in the NH4+-vermiculite (Santa Olalla) by thermogravimetry, evolved gas analysis, chemical analysis, X-ray diffraction and IR spectroscopy is reported here. The mass loss during heating takes place in two steps at about 650 and 825 °C. Additionally, the releases of H2O and NH3 occurs simultaneously. The experimental results indicate that the protons remaining in the interlayer space after NH3 removal trigger the H2O release. X-ray diffraction shows that during the decomposition of NH4+-vermiculite there are two domains with different interlayer spaces at ~9 and ~10 A. As the decomposition proceeds, the intensity of the 9 A peak increases at the expense of the second one. The change in the IR-stretching modes of the structural OH groups during heating indicates that the OH groups surrounded by 3Mg2+ or 2Mg2+Fe2+ are released at lower temperatures than those with environments like 2Mg2+Fe3+, 2Mg2+Al3+ or more complex ones.