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 I bar). The samples were chemically purified and Na-saturated. Dehydrated clay fractions < 2 mu m were studied. The structural formulae of the corresponding clay minerals, obtained after subtracting the remaining impurities, are K0.026Na0.435Ca0.010(Si3.612Al0.388) (Al1.593Fe0.1843+ Mg-0.228 Fe0.0382+Ti0.011)O-10(OH)(2) for smectite MX-80, K0.762Na0.044(Si3.387Al0.613) (Al-1.427 Fe-0.292(3+) Mg-0.241 Fe2+ (0.084))O-10(OH)(2) for illite IMt-2 and K0.530Na0.135(Si3.565Al0.435)(Al-1.709 Fe0.0513+Mg0.218Fe0.0172+Ti0.005)O-10(OH)(2) 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. The following values were obtained at 298.15 K and 1 bar [GRAPHICS] (c) 2007 Elsevier Ltd. All rights reserved.