SILICON DIFFUSION IN SILICATE MINERALS

Recently obtained silicon self-diffusion data, D-Si = D(0)exp(-Delta H /RT), in silicates (quartz, vitreous silica, forsterite, San Carlos ol ivine and diopside) show a compensation law that is a linear, log D-0 = log D + Delta H/2.303 RT*. We find Delta H = 532.6 + 30.4 log D-0 ( Delta H in KJ/mol and D-0 in cm(2)/s), which corresponds to log D sim ilar or equal to -17.5 and T similar or equal to 1588 K. D* represent s a common value of D-Si for all these silicates at T, but also the v a of D-Si at Delta H = 0. It has, therefore, a pure entropic significa tion: D = fa(2) (v) over tilde exp(Delta S/R); f is a geometrical fac tor, a the jump distance and (v) over tilde the Si-O vibrational stret ching optic frequency almost common to all these silicates. Despite th e wide range of enthalpies for Si diffusion in silicates, we propose t hat the compensation law outlines a unique mechanism for Si migration in minerals with structures based on the SiO4 tetrahedron. (C) 1997 El sevier Science B.V.