Polycrystalline quartz was synthesized from amorphous silica with sea-and distilled-water at temperatures from 50degreesC to 450degreesC, and pressures from 50 to 3000 MPa. The results show that pressure enhances significantly the transformation rates of amorphous silica to quartz, thus shortening the time for the grain to grow before completion of the transformation. This leads to the finding of a new condition to synthesize polycrystalline nanoquartz crystals in near-neutral solutions at pressures above 500 MPa. The minimum average grain size of single-phase nanoquartz synthesized so far is 80 nm (with sigma = 30 nm) at 300degreesC, 2500 MPa (2.5 GPa) and 20 h. The study concludes that well crystallized nanoquartz powder with desired grain sizes may be synthesized by selecting, properly the experimental conditions, in particular, the pressure. (C) 2002 Elsevier Science B.V. All rights reserved.