SUPPLEMENTARY DATA
Llana-Fúnez, Wheeler and Faulkner
Determination of equilibrium line
The equilibrium line of reaction gypsum to bassanite plus water at 1 bar is
based on original data in Kelley et al (1941) and subsequent calculations (p. 36-40
of that work). Bassanite- is the form used as it is the one generated in the
presence of liquid water, rather than in vacuum when a  form results. To resolve
a discrepancy between calculated (123 ºC) and measured (97 ºC) dehydration
temperature at 1 bar, these authors note that bassanite- has a slightly higher
amount of molar water, contained in solid solution, than the ideal formula, for
example 0.6 rather than 0.5 (page 40, Kelley et al. 1941). The reaction considered
is then:
1/(1-x)CaSO4.2H2O = 1/(1-x)CaSO4.(1/2 +3x/2)H2O + 3/2 H2O
They argue that the extra H2O gives rise to an entropy of mixing: hence the
entropy increase during reaction is greater than that calculated using
stoichiometric bassanite, and the predicted equilibrium temperature lower. They
assume that the calculated values for all other thermodynamic data (such as rCpº)
will remain the same in this reaction as in the end-member reaction (when x = 0).
This is equivalent to assuming that the properties of bassanite-* (the * indicating
excess water) are equal to those of a mix of bassanite- and gypsum, except for
the entropy. We follow their assumption and, in the absence of other evidence, use
it for volume change too. The leading constant in the expression for entropy
change during reaction (eqn. 4 below) is modified from -440.45 J to -436.56
J/mol. The calculated temperature of dehydration at 1 bar is then 97 ºC, consistent
1
with what is established independently by other methods (Posjnak 1938; Prasad et
al. 2001). The current equilibrium line is also in accord with experimental data
above 40 MPa (McConnell et al. 1987).
Supplementary Tables 1 and 2 show the basic data on which calculations are
based.
For reaction enthalpy, using
rHTº = rH298º + 298T rCpº dT
(1)
and the expressions from Supplementary Table 1 we obtain
rHTº = -3591.06 + 92.705 T - 0.0775 T 2 .
(2)
For reaction entropy, using
rSTº = rS298º + 298 T rCpº/T dT,
(3)
the heat capacity and the adjusted leading constant (discussed above), we obtain
rSTº = -436.56 + 92.705 ln (T) - 0.155 T .
(4)
For reaction free energy change at 1 bar we use
rGº 1 bar = rHTº - T rSTº,
(5)
to find
rGº 1 bar = -3591.06 + 529.265 T + 0.0775 T 2 - 92.705 T ln (T) .
(6)
Here,
rH298º is molar enthalpy of reaction at a reference temperature of 298 K.
rHTº is molar enthalpy of reaction at a temperature
rCpº is heat capacity of reaction
rS298º is the molar entropy of reaction at a reference temperature of 298 K
rSTº is the molar entropy of reaction at a temperature
rGº is the free energy of reaction
2
To calculate the pressure dependence of the free energies, we need to
integrate the volume change with respect to pressure and then add it to equation 6
after recalculating the leading constant.
rGº P= -3591.06 + 529.265 T + 0.0775 T 2 - 92.705 T ln (T)
+ 5·10-6 P3 - 5.54·10-3 P2 + 6.8665 P
.
(7)
References
Brewer L (1982) Thermodynamic Values for Desulfurization Processes. In:
Hudson J (ed) In Flue Gas Desulfurization, vol. American Chemical Society,
Washington, p 39
Kelley KK, Southard JC, Anderson CT (1941) Thermodynamic properties of
gypsum and its dehydration products. Bureau of Mines Technical Paper, US
Department of Interior 625:1-73
McConnell JDC, Astill DM, Hall PL (1987) The Pressure-Dependence Of The
Dehydration Of Gypsum To Bassanite. Mineral Mag 51(361):453-457
Mirwald PW (2008) Experimental study of the dehydration reactions gypsumbassanite and bassanite-anhydrite at high pressure: Indication of anomalous
behavior of H2O at high pressure in the temperature range of 50-300 degrees
C. Journal of Chemical Physics 128(7) doi:10.1063/1.2826321
Posjnak E (1938) The system CaSO4-H2O. American Journal Of Science
35A:247-272
Powell R, Holland TJB (1988) An Internally Consistent Dataset with
Uncertainties and Correlations .3. Applications to Geobarometry, Worked
Examples and a Computer-Program. Journal of Metamorphic Geology
6(2):173-204
Prasad PSR, Pradhan A, Gowd TN (2001) In situ micro-Raman investigation of
dehydration mechanism in natural gypsum. Current Science 80(9):1203-1207.
3