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

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 K0.026Na0.435Ca0.010(Si3.612Al0.388)K0.026Na0.435Ca0.010(Si3.612Al0.388) (Al1.593Al1.593Fe3+0.184Mg0.228Mg0.228Fe2+0.038Ti0.011Ti0.011)O10(OH)2O10(OH)2 for smectite MX-80, K0.762Na0.044(Si3.387Al0.613)K0.762Na0.044(Si3.387Al0.613) (Al1.427Al1.427Fe3+0.292Mg0.241Mg0.241Fe2+0.084)O10(OH)2O10(OH)2 for illite IMt-2 and K0.530Na0.135(Si3.565Al0.435)K0.530Na0.135(Si3.565Al0.435)(Al1.709Al1.709Fe3+0.051Mg0.218Mg0.218Fe2+0.017Ti0.005Ti0.005)O10(OH)2O10(OH)2for mixed-layer ISCz-1. From the heat capacity values, we determined the molar entropies, standard entropies of formation and heat contents of these minerals. The following values were obtained at 298.15 K and 1 bar: Cp0 (J mol−1 K−1)S0 (J mol−1 K−1)Smectite MX-80326.13 ± 0.10280.56 ± 0.16Illite IMt-2328.21 ± 0.10295.05 ± 0.17Mixed-layer ISCz-1320.79 ± 0.10281.62 ± 0.15Full-size tableTable optionsView in workspaceDownload as CSV