Thermodynamic properties of anhydrous smectite MX-80, illite IMt-2 and mixed-layer illite–smectite ISCz-1 as determined by calorimetric methods. Part I: Heat capacities, heat contents and entropies

Abstract : The heat capacities of the anhydrous international reference clay minerals, smectite MX-80, illite IMt-2 and mixed-layer illite–smectite ISCz-1, were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 6 to 520 K (at 1 bar). The samples were chemically purified and Na-saturated. Dehydrated clay fractions <2 μm were studied. The structural formulae of the corresponding clay minerals, obtained after subtracting the remaining impurities, are ( ) for smectite MX-80, ( for illite IMt-2 and ( )for mixed-layer ISCz-1. From the heat capacity values, we determined the molar entropies, standard entropies of formation and heat contents of these minerals.