Seventeen simulations for this composition in a microcanonical ensemble of 1300 particles (O + Si + Al + Ca) were conducted at 1700 to 5000K and approximately 1GPa (table 20). A pair-wise potential allowing for Coulombic and Born-Mayer interactions was used. The simulation durations ranged from 50 to 150ps. Particle trajectories were collected and used to build a picture of the structure and dynamics of equilibrium liquid, supercooled liquid and glassy CaAl2Si2O8 as a function of temperature along the 1GPa isobar. A computer glass transition, Tg, was detected at about 2800K by studying thermodynamic properties, speciation equilibria and tracer diffusivity. Marked differences in the mobility of all atoms monitored by tracer diffusion were noted as a function of temperature. The self-diffusivity order at a fixed temperature was DCa > DO > DAl> DSi with DCa about 20% larger than DO and DO equal to about 2DSi. The activation energies for diffusion for all atoms lay in the range of 170 to 190kJ/mol. The small range in tracer diffusivity and activation energy (Ea) found for different atoms suggested that cooperative motion was important. At Tg/T ≥ 1 for the non-equilibrium glass, Ea decreased by about 40% for all atoms as compared with the corresponding high-temperature (equilibrium melt) values. A crossover between continuous (hydrodynamic-like) motion and atomic hopping motion appeared clearly in the time-dependence of the mean square displacement as a function of temperature. The qualitative view was that a given particle and its neighbors remained trapped for a finite waiting time

before undergoing cooperative thermally activated rearrangement. The waiting time distribution was strongly temperature-dependent and was related to the rapid increase in structural relaxation time as the temperature approaches Tg.

A Molecular Dynamics Study of the Glass Transition in CaAl2Si2O8: Thermodynamics and Tracer Diffusion. Morgan, N.A., Spera, F.J.: American Mineralogist, 2001, 86[7-8], 915-26

 

Table 20

Tracer Diffusivity for Ca, Al, Si and O in CaAl2Si2O8

 

T(K)

DCa (m2/s)

DAl(m2/s)

DSi(m2/s)

DO(m2/s)

1707.0

7.338 x 10-12

4.302 x 10-12

3.458 x 10-12

8.053 x 10-12

1871.3

1.783 x 10-11

5.334 x 10-11

1.282 x 10-11

2.450 x 10-11

2013.6

5.569 x 10-11

2.071 x 10-11

1.679 x 10-11

3.449 x 10-11

2239.7

7.203 x 10-11

1.790 x 10-11

1.732 x 10-11

4.942 x 10-11

2320.8

8.531 x 10-11

4.035 x 10-11

2.787 x 10-11

7.943 x 10-11

2542.3

2.188 x 10-10

9.030 x 10-11

7.704 x 10-11

1.451 x 10-10

2670.1

1.478 x 10-10

7.541 x 10-11

5.080 x 10-11

1.555 x 10-10

2868.1

3.148 x 10-10

2.254 x 10-10

1.425 x 10-10

3.095 x 10-10

2947.5

4.830 x 10-10

2.399 x 10-10

1.315 x 10-10

4.063 x 10-10

3086.6

5.949 x 10-10

3.435 x 10-10

1.983 x 10-10

4.543 x 10-10

3150.2

9.300 x 10-10

5.386 x 10-10

3.370 x 10-10

7.236 x 10-10

3475.3

1.703 x 10-9

8.602 x 10-10

5.567 x 10-10

1.174 x 10-9

3481.3

1.167 x 10-9

1.132 x 10-9

7.062 x 10-10

1.351 x 10-9

3983.3

3.157 x 10-9

2.118 x 10-9

1.356 x 10-9

2.612 x 10-9

4151.6

3.319 x 10-9

2.557 x 10-9

1.726 x 10-9

3.113 x 10-9

4500.0

4.605 x 10-9

3.568 x 10-9

2.358 x 10-9

4.283 x 10-9

4976.5

9.264 x 10-9

4.741 x 10-9

3.505 x 10-9

5.848 x 10-9