Synthesis of transparent polycrystalline jadeite under high pressure and temperature

Attempts to synthesize transparent polycrystalline jadeite have been made by direct conversion from bulk glass at pressures 10 – 20 GPa and temperatures 900 – 1300 °C using Kawai – type multianvil apparatus. The grain size of jadeite tends to decrease with increasing pressure, but we failed to synthesize polycrystalline jadeite with grain sizes in nano – regime (<100 nm) and obtained the sample with the smallest average grain size of ~ 240 nm at 20 GPa and 1300 °C for 20 min. Polycrystalline jadeite of the minimum grain size exhibits high optical transparency with a transmittance of ~ 70% for a typical wavelength in the visible region. The highest Vickers hardness (H v ) of 14.2 GPa was observed for the polycrystalline jadeite sample with the minimum grain size of ~240 nm, which is about 7% higher than the hardness (H v = 13.3 GPa) of the sample with the largest grain size of ~ 390 nm. Further increases in optical transparency and hardness of polycrystalline jadeite would be realized if we get nano – polycrystalline samples by optimizing pressure, temperature, heating duration, etc. of the ultrahigh – pressure synthesis experiment.