THE HEAT CAPACITY OF TWO NATURAL CHLORITE GROUP MINERALS DERIVED FROM DIFFERENTIAL SCANNING CALORIMETRY

Abstract

The heat capacity of natural chamosite (XFe=0.889) and clinochlore (XFe=0.116) were measured by differential scanning calorimetry (DSC). The samples were characterised by X-ray diffraction, microprobe analysis and Mössbauer spectroscopy. DSC measurements between 143 and 623 K were made following the procedure of Bosenick et al. (1996). The fitted data for natural chamosite (CA) in J mol−1 K−1 give: Cp,CA = 1224.3–10.685 × 103 × T −0.5 − 6.4389 × 106 ×T −2 + 8.0279 × 108 × T −3 and for the natural clinochlore (CE): Cp,CE = 1200.5–10.908 × 103 ×T −0.5 − 5.6941 × 106 × T −2 + 7.1166 × 108 × T −3. The corrected Cp-polynomial for pure end-member chamosite (Fe5Al)[Si3AlO10](OH)8 is Cp,CAcor = 1248.3–11.116 × 103 × T −0.5 − 5.1623 × 106 × T −2 + 7.1867 × 108×T −3 and the corrected Cp-polynomial for pure end-member clinochlore (Mg5Al)[Si3AlO10](OH)8 is Cp,CEcor = 1191.3–10.665 × 103 × T −0.5 − 6.5136 × 106 × T −2 + 7.7206 × 108 × T −3. The corrected Cp-polynomial for clinochlore is in excellent agreement with that in the internally consistent data sets of Berman (1988) and Holland and Powell (1998). The derived Cp-polynomial for chamosite (Cp,CAcor) leads to a 4.4% higher heat capacity, at 300 K, compared to that estimated by Holland and Powell (1998) based on a summation method. The corrected Cp-polynomial (Cp,CAcor) is, however, in excellent agreement with the computed Cp-polynomial given by Saccocia and Seyfried (1993), thus supporting the reliability of Berman and Brown's (1985) estimation method of heat capacities.