Natural Mg-Fe clinochlores: Enthalpies of formation and dehydroxylation derived from calorimetric study

Abstract

This paper presents the results of the first experimental thermochemical investigation of two natural trioctahedral chlorites (clinochlores). The study was performed with the help of a high-temperature heat-flux Tian-Calvet microcalorimeter. The samples were characterized by X-ray spectroscopy analysis, X-ray powder diffraction, thermal analysis, and FTIR spectroscopy. The enthalpies of formation of clinochlores were found using the melt solution calorimetry method to be: -8806 ± 16 kJ/mol for composition (Mg 4.9 Fe 2+ 0.3 Al 0.8 )[Si 3.2 Al 0.8 O 10 ](OH) 8 and -8748 ± 24 kJ/mol for composition (Mg 4.2 Fe 2+ 0.6 Al 1.2 ) [Si 2.8 Al 1.2 O 10 ](OH) 8 . The experimental data for natural samples allowed calculating the enthalpies of formation for end-members and intermediate members of the clinochlore (Mg 5 Al)[Si 3 AlO 10 ](OH) 8 and chamosite (Fe 5 Al)[Si 3 AlO 10 ](OH) 8 series. An important feature of the clinochlore structure is the presence of two distinct hydroxyl-containing octahedral layers: the interlayer octahedral sheet and octahedral 2:1 layer; the enthalpies of water removal from these positions in clinochlore structure were determined as: 53 ± 20 kJ/(mol·H 2 O) and 131 ± 10 kJ/(mol·H 2 O), respectively. These obtained first thermodynamic characteristics of Mg-Fe clinochlores can be used for quantitative thermodynamic modeling of geological and industrial processes including clinochlores of different composition.

Structural features and stability of Spanish sepiolite as a potential catalyst

“…Although the thermal treatment process is essential for the decomposition of the metal precursor, there is a significant change in the catalyst properties caused by heating above 250 o C, which is associated with the loss of water and the collapse of the layered structure. In agreement with previous studies (Hayashi et al, 1969;Kok, 2013;Ogorodova et al, 2016), our TGA-DSC data confirmed the stepwise removal of water from Sep, which accounts for ~9% of the mass loss at 100 o C and ~4% at 250-300 o C (both steps are endothermic processes as expected) with the total mass loss of ~18% by 900 o C ( Figure 5). The exothermic peak observed at o C corresponds to a high temperature phase transition resulting in a complete loss of the Sep structure.…”