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

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 Å. As the decomposition proceeds, the intensity of the 9 Å 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.