Precipitation of silica minerals by hydrothermal flow-through experiments at 200-430 °C and 30 MPa

Hydrothermal flow-through experiments were conducted to understand the kinetics and mechanism of precipitation of silica minerals from Si-saturated aqueous solutions. The Si-saturated solutions were created by dissolution of silica glass at 350 °C. The first series of runs consisted of precipitation experiments using 31 grams of quartz sand (grain size of 1-2 mm) at 200-430 °C and 30 MPa with flow rate of 0.5 g/min. The decrease in Si concentration during the pass through the quartz sand was less than 50 ppm at temperatures below 350 °C, whereas the Si concentration after pass-through at 430 °C reaches close to equilibrium solubility. This indicates that temperatures of > 400 °C are suitable for observing the precipitation of a substantial amount of silica minerals at 30 MPa within several days, because of the low equilibrium solubility and the high reaction rate. The second series of runs consisted of the precipitation experiments (430 °C, 30 MPa) from a supersaturated solution (C _si = 638 ppm) with using granite substrates with a flow rate of 0.7-1.0 g/min for 80 hours. The total amount of precipitated silica minerals was 3.4 g, and the amount decreased with increasing distance from inlet, indicating that Si concentration of the solution decreased during the pass through the reaction tube. The precipitated silica minerals and textures varied along the flow path. Near the inlet, Opal C was precipitated on the surface of the stainless tube. At 14-20 cm from the inlet, quartz overgrew on quartz grains within the granite substrate, but fine-grained quartz crystals (10-100 μm) were deposited on other minerals and on the stainless tube. At 20-30 cm from the inlet, quartz precipitation was restricted to the pre-existing quartz surfaces of the granite. Our results suggest that nucleation and formation of SiO ₂ polymorphs other than quartz were important for the formation of the quartz veins under high degree of supersaturation (C/C _eq > 2), whereas precipitation sites within the crack was substantially controlled by mineral species within the crack wall under low supersaturated conditions. This suggests that the internal crystal growth textures of vein minerals provide constraints on the fluid compositions within flow paths.