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 anorthite 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 confining pressure. Fourier transform infrared spectrometry indicates a water content of 0.002–0.0035 wt % (300–550 H per 10 6 Si) for specimens prepared at atmospheric pressure. Hot‐isostatically pressed samples contain 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 found. 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 ranging from 1 to 100 MPa. The mechanical data indicate grain boundary diffusion 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 (≅973–923 K), our mechanical data indicate 3–4 orders of magnitude difference in strength between dry samples and specimens containing trace amounts of water.