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 K 0.026 Na 0.435 Ca 0.010 ( Si 3.612 Al 0.388 ) ( Al 1.593 Fe 3 + 0.184 Mg 0.228 Fe 2 + 0.038 Ti 0.011 ) O 10 ( OH ) 2 for smectite MX-80, K 0.762 Na 0.044 ( Si 3.387 Al 0.613 ) ( Al 1.427 Fe 3 + 0.292 Mg 0.241 Fe 2 + 0.084 ) O 10 ( OH ) 2 for illite IMt-2 and K 0.530 Na 0.135 ( Si 3.565 Al 0.435 ) ( Al 1.709 Fe 3 + 0.051 Mg 0.218 Fe 2 + 0.017 Ti 0.005 ) 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: C p 0 (J mol −1 K −1) S 0 (J mol −1 K −1) Smectite MX-80 326.13 ± 0.10 280.56 ± 0.16 Illite IMt-2 328.21 ± 0.10 295.05 ± 0.17 Mixed-layer ISCz-1 320.79 ± 0.10 281.62 ± 0.15