Original paper

Heat capacity and third-law entropy of kaersutite, pargasite, fluoropargasite, tremolite and fluorotremolite

Dachs, Edgar; Baumgartner, Iris; Arha Bertoldi, Christian; Benisek, Artur; Tippelt, Gerold; Maresch, Walter V.

Abstract

The heat capacities (Cp) of natural kaersutite, tremolite and fluoropargasite, as well as of synthetic pargasite were measured in the temperature range from 5 to 764 K and were used to calculate the standard entropy (S°) of the pure end-members. The data between 5 and 300 K were obtained by relaxation calorimetry, the high-temperature data (282-764 K) by differential scanning calorimetry (DSC). Natural kaersutite was characterized by electron microprobe analysis and Mössbauer spectroscopy, the other samples had been characterized previously. Assuming ideal OH-F mixing, the Cp data of natural fluoropargasite (XF = 0.605) were used to estimate the heat capacity and standard entropy difference between OH and F end-members of pargasite and tremolite.Fits to the DSC data yielded the following polynomials for the corrected pure end-members (valid above 298 K, T [K], Cp [J/mol·K]):Kaersutite - NaCa2(Mg4Ti4+)(Si6Al2)O22(OH)OCp = 1145.3014 − 3356.9700T−0.5 − 4.3995 × 107T−2 + 5.8164 × 109T−3Pargasite - NaCa2(Mg4Al)(Si6Al2)O22(OH)2Cp = 1251.9495 − 5934.2927T−0.5 − 3.6235 × 107T−2 + 4.8999 × 109T−3Fluoropargasite - NaCa2(Mg4Al)(Si6Al2)O22F2Cp = 1218.8568 − 5923.9772T−0.5 − 2.9302 × 107T−2 + 3.4496 × 109T−3Tremolite - Ca2Mg5Si8O22(OH)2Cp = 1278.1996 − 8114.5798T−0.5 − 2.3199 × 107T−2 + 2.8845 × 109T−3Fluorotremolite - Ca2Mg5Si8O22F2Cp = 1145.1069 − 8104.2643T−0.5 − 1.6266 × 107T−2 + 1.4342 × 109T−3.A comparison with the various Cp-T polynomials and additive estimation techniques suggested in the literature for T > 300 K indicates that most of these lie in a corridor within ±1 % of the values derived here. However, none of these individual alternatives works equally well over the whole temperature range 300-1000 K.Fits of the low-temperature Cp's to a combination of Debye, Einstein and Schottky functions yielded a standard entropy of 599.7 ± 0.8 J/mol·K for kaersutite, 591.0 ± 4.7 J/mol·K for pargasite and 550.1 ± 0.8 J/mol·K for tremolite (no site-configurational entropy contributions added). These S° values of pargasite and tremolite are in excellent agreement with previous determinations from phase equilibrium experiments and low-temperature adiabatic calorimetry. The entropy difference between OH and F pargasite was found to be 5.3 J/mol·K and S° of fluoropargasite is 585.7 ± 1.2 J/mol·K. Assuming the same difference for tremolite, S° of fluorotremolite amounts to 544.8 ± 0.8 J/mol·K. Similar to F-phlogopite and F-apatite, fluoropargasite has a larger heat capacity at low temperatures compared to its OH analogue with a cross-over around 50 K and a possible reason for this behaviour is discussed. Based on the S° values of pargasite, tremolite and their F analogues from this study, and on the results of OH-F partitioning experiments from the literature, a standard reaction enthalpy ΔHOR = −5.5 + 1.0 kJ/mol for the pargasite-phlogopite F-OH exchange, and a ΔHOR = − 14.8 ± 2.5 kJ/mol for the tremolite-phlogopite F-OH exchange were calculated. Standard enthalpy of formation values for fluoropargasite and fluorotremolite can then be derived.

Keywords

calorimetryheat capacitythird-law entropyamphiboleskaersutitepargasitetremolitefluoropargasitefluorotremolite