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 106 Si) for specimens prepared at atmospheric pressure. Hot-isostatically pressed samples contain 0.05 wt % to 0.1 wt % (8000--15000 H per 106 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. ¿ 1999 American Geophysical Union