Phase formation of Mn-doped zinc silicate (Zn2SiO4:Mn2+, ZSM) in high-temperature and high-pressure water was studied by in situ observations with a hydrothermal diamond anvil cell (HDAC). Precursor was prepared with zinc oxalate dihydrate, manganese oxalate, and silica, where the Zn/Mn/Si molar ratio was 192/8/120 to 199/1/120. Conditions of particle formation were at temperatures up to 650 C and at pressures up to 1250 MPa. Precursors dissolved at temperatures of 145-203 degrees C and needle-like particles formed through homogeneous nucleation at temperatures from 357 to 374 degrees C. close to the critical point of water. The needle-like particles grew at growth rates of 0.5-3.8 mu m/s and were identified to be ZSM as evident from their green luminescence. ZSM synthesized in supercritical water (400 degrees C for 180 min) by batch reactions had comparable luminescence with that of ZSM produced by solid-state reaction (1200 degrees C for 240 min) using the same precursor. The key finding in this work is that the precursors can be made to dissolve in near-critical water and that this allows ZSM to form via a homogeneous nucleation process. (C) 2007 Elsevier B.V. All rights reserved.