Thermal decomposition of the natural hydrotalcites carrboydite and hydrohonessite

A combination of high-resolution thermogravimetric analysis (HRTGA) coupled to a gas evolution mass spectrometer combined with infrared emission spectroscopy has been used to study the thermal decomposition of two Australian hydrotalcites carrboydite (Ni6Al2(SO4,CO3)(OH)16·4H2O) and hydrohonessite (Ni6Fe2(SO4,CO3)(OH)16·7H2O). High-resolution thermal analysis shows the decomposition takes place in five steps. Each step is related to the loss of water, hydroxyl units, carbonate and sulphate. Infrared emission spectroscopy clearly identifies the presence of these molecular species and the temperature at which they are lost. Minor amounts of carbonate are observed in the minerals. Water is in two states in the structure, namely weakly hydrogen bonded and strongly hydrogen bonded. The symmetry of the sulphate anion is reduced through this hydrogen bonding.