Heat-pulse calorimetry measurements on natural chlorite-group minerals

Bertoldi, C., Dachs, E. and Appel, P. (2007) Heat-pulse calorimetry measurements on natural chlorite-group minerals American Mineralogist, 92 (4). pp. 553-559. DOI 10.2138/am.2007.2247.

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Abstract

Low- and high-temperature heat capacities of five natural chlorite-group samples were measured using the heat-capacity option of the Physical Properties Measurement System (Quantum Design), which is based on the principles of heat-pulse calorimetry, and by differential scanning calorimetry. Comprehensive chemical analyses were performed on these samples by electron microprobe analysis, by inductively coupled plasma mass spectrometry, and by Karl-Fischer titration (for H2O). The natural chlorites span a range in X-Fe from 0.052 to 0.885 with increasing Al-content due to the Tschermak substitution with increasing X-Fe. The measured heat capacities were extrapolated to the end-member compositions of chamosite and clinochlore. Integration of heat-capacity data yields the calorimetric standard entropies of chamosite (Fe5Al)[Si3AlO10](OH)(8) and clinochlore (Mg5Al)[Si3AlO10](OH)(8), with values of 572.0 +/- 0.2 and 425.6 +/- 0.4 J/(mol-K), respectively. The C-p-polynomial for end-member chamosite is C-p = 1151.7 - 8.4564 x 10(3)-T-0.5 - 13.206 x 10(6)-T-2 + 15.233 x 10(8).T-3 [J/(mol-K)], valid in the temperature range of 298.15-900 K, and that for end-member clinochlore is C-p = 1160.5 - 9.9819 x 10(3)-T-0.5 - 5.9534 x 10(6).T-2 + 3.8677 x 10(8)-T-3 [J/(mol-K)], valid in the temperature range of 298.15-1000 K. The Fe-rich chlorites exhibit an asymmetric distribution of the excess heat capacity in a plot of C-p(ex) vs. T, with a maximum at about 52 K. By analogy to annite, we interpret this peak to represent the magnetic ordering temperature. Based on our standard entropy value for chamosite, the enthalpy of formation of berthierine (Fe2.5Al0.5)[Si1.5Al0.5O5Y](OH)(4) was estimated as -3570.30 kJ/mol using a reported onset temperature of 70 degrees C at 16 MPa for the berthierine-chamosite polymorphic transition.

Document Type: Article
Keywords: clinochlore chamosite heat pulse calorimetry heat capacity excess heat capacities standard entropy chlorite group minerals consistent thermodynamic data system na2o-k2o-cao-mgo-feo-fe2o3-al2o3-sio2-tio2-h2o-co2 tuscaloosa formation low-temperature gulf-coast x-ray rocks capacities origin berthierine
Research affiliation: Kiel University > Faculty of Mathematics and Natural Sciences > Institute of Geosciences
DOI etc.: 10.2138/am.2007.2247
ISSN: 0003-004X
Date Deposited: 12 Jan 2012 06:06
Last Modified: 08 Oct 2012 10:20
URI: http://eprints.uni-kiel.de/id/eprint/15895

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