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The structure of dolomite, CaMg[(C[O.sub.3]).sub.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 [x.sub.Ca] - 1, varies from s = 1 (where [x.sub.Ca] = 1) for a completely ordered dolomite to s = 0 (where [x.sub.Ca] = 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 [Ca.sup.2+] and [Mg.sup.2+] 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 R[bar.3] [left and right arrow] R[bar.3]c transition in dolomite is described by a modified Bragg-Williams thermodynamic model with the following molar free energy of disorder, [[bar.G].sub.d] (T; s) = R[T.sub.c][1 - [s.sup.2] + 1/2 a([s.sup.4] - 1) - (T/[T.sub.c]) {2 ln2 - (1+s)ln(1 + s) - (1 - s) ln(1 - s)}]. Using [T.sub.c] = 1466 K and a = -0.29, this model provides an excellent agreement with experimental data. Moreover, the maximum enthalpy of disorder, [[bar.H].sub.d](s = 0) = R[T.sub.c](1 - 1/2 a) ~ 14 kJ/mol, agrees with published calorimetric data. A thermodynamic description of the aragonite + magnesite [left and right arrow] dolomite reaction boundary is also presented and it reproduces the main qualitative features correctly.

The polarizable valence state potential energy curve (pVS-PEC) is derived through the systematic improvement of the valence state potential energy curve (VS-PEC) and is given as U(R) = -[(K1/R) + (K2/R (super 4)) + (K3/R (super 7)) + T/R exp (-Lambda R). The potential is tested on a broad variety of homonuclear diatoms and heteronuclear halides and hydrides.

The theory of solvatochromism based on a dielectric continuum description of the solvent and the classical Onsager cavity model was investigated and was extended to include the effect of an added 1:1 salt. The equations differed markedly for salt-free case, from several others reported in the literature dealing with the pure solvent effect and the discrepancy was clarified.

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 + ½ a(s4- 1) - (T/Tc) {2 ln2 - (l+s) ln(1 + s) - (1 - s) ln(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 - ½ 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. [ABSTRACT FROM AUTHOR]