The thermal decomposition of hydrotalcite (HT), with chemical composition Mg_1-xAl_x(OH)₂(CO₃)_x/2.(1-3x/2)H₂O, (0.20 < x ≤ 0.33), is a complex sequence of dehydration, dehydroxylation and decarbonization and leads to the formation of a series of metaphases: HT-D (dehydrated HT), HT-B (partially dehydroxylated HT) and MO (mixed oxides with periclase-like structure). The evolution of water and CO₂ in natural and synthetic hydrotalcites (a Mg/Al ratio between 2:1and 3.7:1), heated to 800°C, was investigated by differential thermal analysis, thermogravimetry and evolved gas analysis. At least six endothermic and two exothermic effects were established by computer-aided resolving of the curves. The formation of each HT metaphase was related to the release of a discrete number of water molecules depending on the Al content in the samples and each appeared as a corresponding endothermic peak in the DTA curves. The exothermic processes associated with the crystallization of HT-B and MO metaphases were specified by decomposition of DTA curves. The evolution of CO₂ during the thermal decomposition of the carbonate groups was found to be different for the samples studied. The preservation of CO₃ even at high temperatures was established for synthetic samples with a high Al content. The release of volatile H₂O and CO₂ (which comprise ~40% of the sample mass) provokes fine cracking both along and across the layers.

The genesis of the products of the alteration of acid pyroclastic material is discussed and interpreted on the basis of the distribution, sedimentation conditions, post-sedimentation activity, mineral and chemical compositions of pyroclastic deposits in Bulgaria and Slovakia.It is found that the disordered nonequilibrium nature of the volcanic glass induces a diagenetic devitrification of the tuffs and formation of clinoptilolitic, adularia-cristobalite and bentonite rocks. With increasing temperature, the volcanic glass tends forward a stable state through a series of zeolite mineral associations: clinoptilolite-mordenite-analcime-feldspar. The change in mineral composition of this series of rocks occurs without a change in the chemical composition of the rocks, which could be explained by the closed nature of zeolite systems. The formation of bentonites is associated with the removal of alkaline ions under diagenetic conditions, while the formation of halloysite rocks is caused by hydrothermal activity.