Hydrothermal flow-through experiments were conducted to investigate the mechanism of silica precipitation at 430°C, 31 MPa using the input solutions of C Si = -300 ppm (the degree of supersaturation, Ω = C Si/C Si,qtz,eq was ∼3). The mineralogy and occurrence of silica minerals changed depending on the Si concentration, substrate type, and additive cations (Na, K and Al). At near equilibrium conditions (Ω < 1.5), silica precipitation was restricted to overgrowth of substrate quartz. On the other hand, the mechanism was more complex in the higher Si concentration. In pure Si solution, the dominant silica mineral changed as opal-A → opal-C → quartz with decreasing Si concentration along the flow path, and stable minerals formed on next less stable phases. In multi-component solutions, quartz predominantly occurred in the entire parts of the vessel, indicating that the additive cations enhanced the direct nucleation of quartz.