Silica nanoparticles produced by explosive flash vaporization during earthquakes

Hydrothermal activity in the crust results in the precipitation of large volumes of silica and often involves the formation of ore deposits, the shaping of geothermal systems, and recurring earthquakes. Pore fluid pressures fluctuate between lithostatic and hydrostatic, depending on seismic activity, and some models suggest the possibility of flash vaporization, given that fluid pressures can drop to the level of vapour at fault jogs during seismic slip. The phase changes of water could create extremely high supersaturations of silica, but the mechanisms of quartz vein formation under such extreme conditions remain unclear. Here we describe flash experiments conducted with silica-saturated solutions under conditions ranging from subcritical to supercritical. We found that amorphous silica is produced instantaneously as spherical nano- to micron-scale particles via nucleation and aggregation during the evaporation of water droplets. The nanoparticles are transformed to microcrystalline quartz very rapidly by dissolution and precipitation in hydrothermal solutions, with this process requiring less than one day under supercritical conditions because of the huge surface areas involved. We suggest that such short-lived silica nanoparticles have significant impacts on the dynamic changes in mechanical behaviour and hydrology of hydrothermal systems in volcanic areas.

www.nature.com/scientificreports www.nature.com/scientificreports/ from highly supersaturated solutions [15][16][17] . Metastable silica phases usually contain water, which is easily lost during deformation and transformation into more stable phases. However, the metastable silica phases themselves are not preserved in most quartz veins.
Experimental studies of silica precipitation remain limited. Flow-through experiments [18][19][20] have revealed that various types of silica precipitation occur via a range of mechanisms under supercritical conditions including epitaxial quartz overgrowth, formation of metastable silica minerals, and 3-dimensional homogeneous (or heterogeneous) nucleation of quartz in fluids, depending on temperature and solution chemistry. In the field of engineering, silica nanoparticles have been synthesized by the rapid expansion of silica-dissolved solutions 21 or the evaporation of silica gel dispersed droplets 22 , but their geological significance has not been considered.
The aim of this study was to understand the mechanism of silica precipitation induced by flash vaporization, and to evaluate whether the formation of amorphous silica is a key step in the formation of quartz veins in the crust. We conducted novel experiments on quartz formation comprising two steps: flashing experiments under subcritical or supercritical fluid to air pressures with granite-dissolved aqueous solutions, and batch-type experiments to investigate the transformation of the flashing products under hydrothermal conditions.  (Fig. 1). In runs FL250 and FL350, the P-T paths crossed the liquid-vapour boundary of water. In runs FL400 and FL450, the flashing did not cause a phase transition of water, but the water density decreased gradually from a supercritical fluid to vapour. Within 5 seconds after the isothermal decompression stage, the temperature within the autoclave decreased slightly with decreasing pressure due to the latent heat induced by the evaporation of liquid water in the inlet capillary tube.

Results
Silica precipitates were caught by the alumina filter that was placed at the outlet of the autoclave (Fig. S1). In all runs, the spherical silica particles collided and became stacked on the surface of the alumina filter ( Fig. 2a-e). Raman spectra of the silica precipitates display a broad peak at 300-550 cm −1 , indicating that the particles comprise amorphous silica 23 (Fig. S2). The diameters of the silica particles range from ~100 to ~6000 nm (Fig. 2f). The particle size distributions observed in the SEM images are not normal distributions, but are skewed to smaller sizes. The mode of the particle size was 900 nm in run FL250, 700 nm in FL350 and FL400, and 500 nm in FL450. Most particles had relatively smooth spherical surfaces, but some displayed a cauliflower-like roughness with a wavelength of 100-200 nm (Fig. 2c,d). Highly angular scanning transmission electron microscope (STEM; Hitachi HD-2700A) dark-field (or Z-contrast) images indicate that the spherical silica particles have uniform internal structures without nano-scale pores or evidence of characteristic growth patterns (Fig. 2e).
In classical nucleation theory, the critical radius of the cluster, r c , for homogeneous nucleation is as follows 24 : www.nature.com/scientificreports www.nature.com/scientificreports/ where σ, v, and Ω indicate the interfacial energy between the mineral and water, the molar volume, and the saturation ratio of the mineral, respectively. R is the universal gas constant, and T is temperature (K). In our experiments, amorphous silica nucleated instead of quartz in all runs, because of the lower interfacial energy of amorphous silica with water (65 mJ m −2 ) compared with that of quartz (350 mJ m −2 ; ref. 24 ). Equation 1 also indicates that the nucleus size decreases with increasing temperature and with an increase in the saturation ratio. Equation 1 predicts that an extremely high supersaturation at the phase transition of H 2 O results in the formation of fine nuclei (100-200 nm in run FL250 and 30-200 nm in FL350), whereas during flashing from supercritical fluids (runs FL400 and FL450) the continuous nucleation of amorphous silica is expected to occur over a wide range of P-T conditions during gradual changes in water density, resulting in a wide range of nucleus sizes (30-1000 nm diameter; Fig. 3a). However, the observed particle size distributions of silica particles (Fig. 2f) cannot be explained solely by homogeneous nucleation theory. In particular, the large particles in run FL250 (mode 700-900 nm, maximum ~6000 nm) deviate from the predicted size (100-200 nm). The large range of particle sizes in run FL250 (Fig. 2a) suggests that nucleation occurred in at least two separate events with supersaturation having passed through two maxima during pressure release, although the detailed mechanism is unclear.
The cauliflower-like surface morphology of the silica particles (arrows in Fig. 2b-d) suggests that the aggregation of fine silica particles occurred during flashing. Iskandar et al. 22 created silica particles by spray-drying a nanoparticle sol, and showed that the sizes and shapes of the silica particles were controlled by the sizes of water droplets and the concentrations of silica particles. In our experiments, the silica existed as a monomer in the input solution, and during flashing liquid water was fragmented into numerous droplets. In each droplet, many nuclei of amorphous silica were produced, and they would have aggregated to form a particle with a size that exceeded the size predicted by nucleation theory (Fig. 3b).

Transformation of silica particles by dissolution and precipitation processes. We conducted
batch experiments at 350 °C and 16 MPa (vapour-saturated pressure), and 450 °C and 36 MPa (extension of the vapour-saturated curve under supercritical conditions) to investigate the transformation of amorphous silica particles produced by flashing (Fig. 1). The silica mineral changed in different ways during runs BT350 (Fig. 4a) and BT450 (Fig. 4b). For comparative purposes, we also conducted a run under vapour conditions of 350 °C and 5 MPa (run BT350vap; Fig. 4c). Resulting Raman spectra are shown in Fig. 4d.
In run BT350, the amorphous silica occurred as spherical particles on day 1, with adjacent particles being connected to form necking structures (Fig. 4a). Semi-quantitative analyses by energy-dispersive X-ray spectrometry (EDXS) revealed that these particles contained 95-99 wt.% SiO 2 with minor amounts of Al 2 O 3 (<1 wt.%), Na 2 O (1.0-1.5 wt.%) and K 2 O (<1 wt.%) (Table S2). By day 5, surfaces of the silica precipitates had become rough and formed larger stacked aggregates of hemispherical particles (Fig. 4a). By day 15, these aggregates had sizes of up to 5-10 μm, with most displaying a broad amorphous-silica Raman peak, and some a weak peak at ~408 cm −1 . The morphology and Raman spectra (Fig. 4a,d) suggest that amorphous silica was transformed into low-ordered opal-C 18,25 . The chemical composition of the opal-C was similar to that of the spherical amorphous silica (Table S1).
In the experiment under vapour conditions (run BT350vap), the spherical amorphous silica particles remained even after day 28 (Fig. 4c), although some were partly connected. This indicates that dissolution and precipitation processes in the solution played essential roles in the transformation of silica.
The transformation from amorphous silica to cristobalite and quartz has been investigated experimentally using silica gel 26 and natural silica sediments 27 , but transformation rates have not been determined precisely because solid products rather than solutions were analysed. The change in Si concentration in run BT350 is explained by the coupled dissolution of amorphous silica and precipitation of cristobalite (low-ordered opal C). The dissolution of silica occurred more rapidly than cristobalite precipitation for the first three days, with the Si concentration increasing to a level exceeding cristobalite solubility (intermediate between those of quartz and amorphous silica). Then, with the increase in volume and surface area of the cristobalite, its precipitation rate exceeded the dissolution rate of amorphous silica, causing Si solubility to decrease towards that of cristobalite (Fig. 4e). This is consistent with the stepwise decrease in Si concentrations observed in experiments 28 and predicted by models 29,30 involving precipitation from amorphous silica-saturated solutions.
Cristobalite was not observed at 450 °C, but nucleation of quartz occurred rapidly at that temperature. As a result, the coupled dissolution of amorphous silica and precipitation of quartz caused a decrease in Si concentration to near the level of quartz solubility (Fig. 4e). The preferential nucleation of quartz under supercritical www.nature.com/scientificreports www.nature.com/scientificreports/ conditions is consistent with results of flow-through experiments 5 . Impurities such as small amounts of Al, Na and K (Table S2), and water in amorphous silica phases might have facilitated the rapid nucleation of quartz 20 . The rate of precipitation of quartz as growth from substrate seed crystals, R SiO2, Qtz (mol s −1 ), is simply expressed by the first order kinetic equation 31 as follows: where k_ is the precipitation rate constant (mol m −2 s −1 ), and A Qtz is the reactive surface area of quartz (m 2 ). Q and K Qtz,eq respectively represent the activity product and its equilibrium constant for quartz. With using k_ = 2. ). This means that quartz precipitation via amorphous silica, as observed in our two-step experiments, is much more rapid than has been estimated in models of quartz overgrowths on pre-existing crystals in vein walls, probably due to huge surface areas of amorphous silica and microcrystalline quartz (Figs 2, 4). Flashing and subsequent transformation might therefore play primary roles in quartz vein formation.

Discussion
The vaporization of pore water requires drastic changes in the crustal environment, and such changes during seismic slip have been proposed to occur in two ways: by instantaneous decompression at a fault jog 13 , and by frictional heating on a fault surface 32 . Such vaporization causes a high degree of supersaturation of silica minerals in faults hosted by quartz-bearing crustal rocks. Our experiments have verified that flash vaporization produces a large number of nano-to micron-sized amorphous silica particles on the fault surface (Figs 2, 3). Recent observations of fault materials and the products of frictional experiments have suggested that silica gel is formed on the fault plane as the product of the amorphization of quartz during friction 17,[33][34][35][36] , and that the amorphous silica acts as a lubricant that facilitates seismic slip 33 . We consider that amorphous silica is formed not only mechanically, but also by chemical processes during earthquake rupturing. Co-seismic slip also results in a continuous rise in fluid temperatures to >350 °C on fault surfaces, and this results in significant water-rock interaction 37 . Under such conditions, amorphous silica particles could change to aggregates of microcrystalline quartz within a few days, a process that may contribute to the recovery of fault strength.
Since amorphous silica is not preserved in hydrothermal quartz veins, it remains unclear how common the formation of amorphous silica is within the crust. One simple constant regarding the formation of amorphous silica is that the fluid, which was originally in equilibrium with quartz, should exceed the solubility of amorphous silica at the time of vein formation. The solubility ratio of amorphous silica with respect to quartz ( Fig. 5a; ref. 38 ) increases with increasing pressure and decreasing temperature. Along a cold geothermal gradient (~10 °C km −1 ) such as a subduction zone interface, the solubility of amorphous silica is 3-5 times higher than that of quartz in temperature range of 200 °C-450 °C. Such a condition makes it difficult for amorphous silica to form even by the change from lithostatic to hydrostatic pressure 39 , except in extreme cases such as flash vaporization. In contrast, www.nature.com/scientificreports www.nature.com/scientificreports/ along a high geothermal gradient, such as in volcanic and geothermal areas where the gradient may exceed 100 °C km −1 , the solubility of amorphous silica is only 1-2 times higher than that of quartz, which means that the formation of amorphous silica can be realized by subtle fluctuations in fluid pressure from lithostatic to hydrostatic, as well as boiling processes 12 . The scales of silica in the pipelines of geothermal power plants provide direct evidence for the formation of amorphous silica by flashing under such high temperatures. We suggest, therefore, that amorphous silica forms more commonly than expected, even within hot crust (T > 300 °C). Aggregates of nano-scale amorphous silica are short-lived, and are transformed to microcrystalline quartz with colloform or banded structures ( Fig. 5b; refs 15,16 ), as commonly observed in epithermal or mesothermal quartz veins.
One notable characteristic of silica nanoparticles produced by flashing is their high mobility with fluid flow, which is quite different from the silica that is precipitated as quartz overgrowth from vein walls. Recent seismological observations 2,40,41 and continuous records of groundwater levels 42 indicate that a redistribution of fluid pore pressures and fluid flow is induced by earthquakes, and that a dynamic oscillation of permeability is induced by distant earthquakes 42 . The permeability oscillation is often explained by the clogging and removal of clay mineral particles at pore throats on fault surfaces 43 . In the hydrothermal systems of volcanic areas, the flow of silica nanoparticles is easily self-organized as the result of oscillations in fluid pressure. We speculate that these silica nanoparticles are easily transported and can clog faults effectively, thus affecting the dynamic behaviour of the crust.

Methods
Input solutions. We used a single high-Si solution as the input solution for the flash and batch experiments.
The solution was prepared by the dissolution of quartz (100 g) and granite sand (Iidate granite, Japan, 100 g) with a grain size of 1-2 mm in the flow-through apparatus. Distilled water entered the cylindrical autoclave at a constant flow rate of 0.7 mL min −1 . The temperature was 370 °C and the fluid pressure was regulated by the back-pressure valve to be 41 MPa. The solution was cooled and stored in the plastic tank until each experiment. The concentrations of Si, Al, Na, K, Ca, Mg, and Fe in the input solutions and the solutions after the batch experiments were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Hitachi P-4000) at Tohoku University, Sendai, Japan. The solubility of quartz used in this study was calculated as a function of water density following Manning 7 . The solubility ratio of amorphous silica with respect to quartz is from Karásek et al. 38 . The pH of the input solution was 6.8 at room temperature (Table S1).

Flash experiments.
A cylindrical autoclave (volume 110 mL) was used for the flash experiments, and its inner wall and inner caps were made of a titanium alloy (Ti-6Al-4V) (Fig. S1a). The autoclave was placed vertically in the furnace, connected with a syringe pump on the inlet side, and connected to 6 mm internal diameter stainless steel tubes (SUS316) on the flashing side. The thermocouple was set at the centre of the autoclave and fluid pressure was measured at the inlet. To catch the silica particles, we installed a cylindrical alumina filter (Fuji Chemical, FA220, diameter 6.3 mm, height 5.0 mm, average pore diameter 60 μm) within the cap of the autoclave on the flashing side (Fig. S1a). Before flashing, the autoclave was filled with the input solution by the syringe pump at room temperature. The solution was then pressurized to ~36 MPa by controlling the back-pressure valve of the capillary line, and then heated to the target temperature. When the temperature reached the target temperature, the stop valve was opened. The fluid pressure instantaneously dropped to the air pressure, and the silica particles were caught by the alumina filter. We conducted a series of flashing experiments at temperatures of 261 °C at 35.2 MPa (run FL250), 353 °C at 36.7 MPa (FL350), 400 °C at 37.8 MPa (FL400), and 450 °C at 35.8 MPa (FL450). Changes in the P-T conditions were recorded in the data logger at time intervals of 0.1 s. After flashing, the autoclave was cooled to room temperature within 30 min, and the silica samples the on the alumina filter were collected from the autoclave. Observations of the surface morphology of the silica particles, and measurements of particle diameters, were carried out using a field emission-scanning electron microscope (FE-SEM, Hitachi SU-8000) at Tohoku University. The internal structures of silica particles in the flash experiment at 400 °C (run FL400) were assessed by scanning transmission electron microscope (STEM; Hitachi HD-2700A) at Tohoku University. The thin sample for STEM observation (~100 nm thickness) was prepared using a focused ion-beam instrument (FIB, Hitachi FB2000A) at Tohoku University.
Batch experiments for silica transformation. Two series of batch experiments were conducted at 350 °C and vapour-saturation pressure (run BT350), and 450 °C (run BT450; Fig. 1; Table S1), using the products of flash experiments conducted at ~350 °C and ~450 °C, respectively. Starting silica materials for these experiments were prepared in 8-12 flash experiments at each temperature. The layers of silica particles were removed from the surface of the alumina filters to reduce the influence of alumina on silica transformation during the batch experiments. About 5 mg of silica was enclosed in the gold inner tube (2 mm diameter and 10 mm height) with holes. The inner tube and the input solution were then enclosed in a cylindrical stainless-steel autoclave with an inner volume of 8 mL (inner diameter 10.8 mm, height 100 mm; Fig. S1). The inner tube facilitated collection of the silica sample after the batch experiments. Pressure was controlled by the water-filling ratio, with pressures of run BT350 being set at the saturated vapour pressure with a water-filling ratio of 40%. Pressures in runs at 450 °C were set at the extension of the vapour-saturation curve under supercritical conditions, with a water-filling ratio of 20% (36 MPa; Fig. 1). The amount of input solutions was ~3.3 g for run BT350 and ~1.7 g for BT450, corresponding to water-rock mass ratios of ~670 and ~340, respectively. In the runs under vapour-saturated conditions, the silica sample was completely immersed in liquid. Different autoclaves were used for runs with different reaction times in individual series of batch experiments. The durations of the experiments were 1, 3, 5, 10, 15 days (Table S1). For comparison, runs were also conducted under vapour conditions at 350 °C and 5 MPa for 21 and 28 days (run BT350vap). In these experiments, the silica particles on alumina filters produced by the flash experiments were