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Publication Type
Journal Article
Author, Analytic
Antao, Sytle M.; Mulder, Willem H.; Hassan, Ishmael; Crichton, Wilson A.; Parise, John B.
Author Affiliation, Ana.
Department of Chemistry
Article Title
Cation disorder in dolomite, CaMg(CO3)2, and its influence on the aragonite + magnesite dolomite reaction boundary
Medium Designator
n/a
Connective Phrase
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Journal Title
American mineralogist
Translated Title
n/a
Reprint Status
Refereed
Date of Publication
2004
Volume ID
89
Issue ID
7
Page(s)
1142-1147
Language
n/a
Connective Phrase
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Location/URL
n/a
ISSN
0003-004x
Notes
n/a
Abstract
The structure of dolomite, CaMg(CO3)2, was determined from 298 to 1466 K at a constant pressure of about 3 GPa using in situ synchrotron X-ray diffraction data to investigate the state of disorder. An order parameter s, defined as 2 xca -1, varies from s = 1 (where xca = 1) for a completely ordered dolomite to s = 0 (where xca = 0.5) for a completely disordered dolomite. On heating, there is no measured change in s until the temperature is high enough to cause exchange of Ca2+ and Mg2+ cations. Significant disorder began to occur at about 1234 K [s = 0.83(1)] and increases along a smooth pathway to T = 1466 K [s = 0.12(5)]. The R3 R3c transition in dolomite is described by a modified Bragg-Williams thermodynamic model with the following molar free energy of disorder, Gd(T, s)=Rtc[1-s2 + a(s4-1) - (T/Tc) {2 In 2 - (1+s) In(1+s) - (1-s) In(1-s)}]. Using Tc = 1466 K and a = -0.29, this model provides an excellent agreement with experimental data. Moreover, the maximum enthalpy of disorder, Hd(s=0) = RTc(1- a) ~ 14 kJ/mol, agrees with published calorimetric data. A thermodynamic description of the aragonite + magnesite dolomite reaction boundary is also presented and it reproduces the main qualitative features correctly.....
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