Heat capacities and entropies of rhodochrosite (MnCO/sub 3/) and siderite (FeCO/sub 3/) between 5 and 600 K
Journal Article
·
· Am. Mineral.; (United States)
OSTI ID:7010889
The heat capacities of rhodochrosite from Alma, Colorado, having the composition (Mn/sub 0.994/Fe/sub 0.005/Mg/sub 0.001/)CO/sub 3/ and siderite of composition (Fe/sub 0.956/Mn/sub 0.042/Mg/sub 0.002/)CO/sub 3/ were measured between 5 and 550 K by combined cryogenic-adiabatic and differential scanning calorimetry. Rhodochrosite has a ..gamma..-type transition in C/sub p//sup 0/ at 34.27 +/- 0.05 K, and siderite has a similar heat-capacity anomaly at 39.71 +/- 0.03 K, arising from the antiferromagnetic ordering of the magnetic moments of the Mn/sup 2 +/ and Fe/sup 2 +/ ions, respectively. At 298.15 K, the heat capacity and entropy of rhodochrosite are 80.62 +/- 0.10 and 98.03 +/- 0.10 J/(mol x K), respectively. For siderite, the equivalent values are 82.44 +/- 0.10 and 95.47 +/- 0.15 J/9mol x K). Between 298.15 and 600 K, the heat capacity of rhodochrosite measured by differential scanning calorimetry may be represented by the equation C/sub p//sup 0/ = 149.70 + 0.01876T - 1314.2T/sup -/ /sup 5/ + 1.417 x 10/sup 5/T/sup -2/ to +/- 0.3%, for the same temperature range, the heat capacity of siderite is C/sub p//sup 0/ = 257.38 - 0.04620T - 3081.9T/sup -/ /sup 5/ + 1.523 x 10/sup 6/T/sup -2/ also with an average deviation of 0.3%. When the authors combine this entropy data with extant equilibrium decomposition and aqueous solubility studies, the authors otain ..delta..H/sub f,298//sup 0/ = -891.91=/-0.52 kJ/mol and ..delta..G/sub f,298//sup 0/ = -818.13+/-0.55 kJ/mol for rhodochrosite.
- Research Organization:
- Geological Survey, Reston, VA
- OSTI ID:
- 7010889
- Journal Information:
- Am. Mineral.; (United States), Journal Name: Am. Mineral.; (United States) Vol. 69:3-4; ISSN AMMIA
- Country of Publication:
- United States
- Language:
- English
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