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Nature 390, 596 (1997) Vanishing Atomic Migration Barrier in SiO2
 

Summary: Nature 390, 596 (1997)
Vanishing Atomic Migration Barrier in SiO2
Michael J. Aziz, Susan Circone* and Carl B. Agee*
Division of Engineering and Applied Sciences, Harvard University, Cambridge MA 02138;
maziz@harvard.edu
*Department of Earth and Planetary Sciences, Harvard University, Cambridge MA 02138
Understanding the high-pressure behaviour of SiO2, a prototypical network-forming material, is
important for resolving many problems in the Earth sciences. For pressures up to 1-3 GPa (1-3 104
atm), it has been shown that increases in pressure result in higher rate constants for atomic transport
processes such as diffusion, viscous flow and crystal growth in SiO2 as well as in some silicate melts 1-5.
Structural transitions and coordination changes observed beyond 10 GPa (refs. 5-9 ) may also be related to
this pressure-induced increase in atomic mobility. There must be limits, however, on the extent to which
pressure can enhance mobility, as a migration barrier decreasing linearly with pressure should vanish at a
critical pressure, beyond which a sudden change in behaviour should be observed 10, 11. Here we report
measurements of the pressure-dependence of the growth rate of quartz from amorphous SiO2 for
pressures up to 6 GPa. We observe a sharp peak in growth rate -- implying a minimum in viscosity -- at
3 GPa, which we interpret as evidence that the critical pressure is being traversed. The corresponding
depth below the Earth's surface at which this peak occurs (~ 100 km) suggests that this critical pressure
may be related to the ubiquitous cut-off in subduction-related volcanism observed when oceanic plates
reach approximately this depth.

  

Source: Aziz, Michael J.- School of Engineering and Applied Sciences, Harvard University

 

Collections: Physics; Materials Science