The enthalpy of fusion of anorthite |
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| Authors: | D F Weill J F Stebbins R Hon I S E Carmichael |
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| Institution: | (1) Department of Geology, University of Oregon, 97403 Eugene, Oregon, USA;(2) Department of Geology and Geophysics, University of California, 94720 Berkeley, California, USA;(3) Department of Geology and Geophysics, Boston College, 02167 Chestnut Hill, Massachusetts, USA |
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| Abstract: | The high-temperature enthalpies of liquid and glassy CaAl2Si2O8 were measured by drop calorimetry using a diphenyl ether drop calorimeter. These data are combined with published values of the high-temperature enthalpy of crystalline anorthite and the enthalpy of vitrification of anorthite to obtain the enthalpy of fusion of anorthite. Analysis of the data yields the following preferred values (enthalpy in kcal/mol, uncertainty limits correspond to two standard deviations):enthalpy of vitrification at 985 K, 
v
H
v
985=18.6±0.6; enthalpy of the liquid at 1,830 K, H
1830
l
300
g
=130.4±1.2; enthalpy of the glass at 985 K, H
985
g
-H
300
g
=46.7±0.4; enthalpy of crystalline anorthite between 985 and 1,830 K, H
1830
c
-H
985
c
=69.9±1.4; calculated enthalpy of fusion of anorthite at 1,830 K,
f
H
1830= 32.4±2.1.The average heat capacity of supercooled liquid CaAl2Si2O8 between the glass transition (T
g 1,086 K) and the melting point (T
f7=1,830 K) is 102 ± 2 cal/mol/K. The large difference between the enthalpy of fusion and the enthalpy of vitrification for the minerals anorthite and diopside is emphasized. The practice of assuming
fH  vH should be discontinued for silicate compounds for which T
f
 T
g. |
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| Keywords: | |
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