Grain boundary diffusion creep of synthetic anorthite aggregates: The effect of water

To investigate the effect of trace amounts of water on plastic deformation of feldspar, we fabricated synthetic polycrystalline aggregates of pure ano rthite from a glass. The glass powder was either densified and crystallized at 1473 K and 0.1 MPa or hot isostatically pressed at 1443 K and 300 MPa c onfining pressure. Fourier transform infrared spectrometry indicates a wate r content of 0.002 - 0.0035 wt % (300 - 550 H per 10(6) Si) for specimens p repared at atmospheric pressure. Hot - isostatically pressed samples contai n 0.05 wt % to 0.1 wt % (8000 - 15000 H per 10(6) Si), depending on whether they were crystallized from glass powder predried at 1073 K for 2 - 3 days or from glass powder as received. In the wet samples, < 1 vol% glass was f ound. Transmission electron microscopy showed that two-grain boundaries are glass-free to a resolution of 5 nm. We performed creep experiments at 0.1 MPa pressure with temperatures ranging from 1373 to 1573 K and stresses ran ging from 1 to 100 MPa. The mechanical data indicate grain boundary diffusi on controlled creep with a stress exponent n = 1.0 +/- 0.1 and a grain size exponent m = - 2.9 +/- 0.2. The activation energy for creep is 585 +/- 45 kJ mol(-1) or 377 +/- 38 kJ mol(-1) for dry or wet specimens, respectively. If extrapolated to natural temperatures (congruent to 973 - 923 K), our me chanical data indicate 3 - 4 orders of magnitude difference in strength bet ween dry samples and specimens containing trace amounts of water.