The Raman spectra indicate that the silicate concentration and speciation in the fluid at given temperature and pressure is dependent on the nature of the crystalline materials in the equilibrium with the fluid.
Solubility
Silicate solubility in the aqueous fluids might be estimated by combining silica solubility in the fluid in the system SiO2–H2O (Manning 2004) with the intensity of relevant Raman bands in order to generate a calibration curve from the intensity of Raman bands in spectra of fluid. This calibration curve then may be used to compute an approximate solubility of SiO2 in the fluids from the other two series of experiments (SiO2–NaOH–H2O and SiO2–CaO–H2O).
The silicate contents of fluid in equilibrium with quartz in the SiO2–H2O system were calculated with the algorithm of Manning (1994) in which the temperature and the density of pure H2O are variables. For the density of the fluid, the calculations by Withers, using the formulism of Pitzer and Sterner (1994), were employed [URL: https://www.esci.umn.edu/people/researchers/withe012/fugacity.htm]. Those SiO2 solubilities (Table 2) were then used to create a calibration curve based on the 770 + 850 cm−1 integrated Raman intensities from the fluid spectra (see Fig. 4). The data points defining the solubility relations were fitted to the function (Fig. 5):
$$ \log\ {m}_{\mathrm{SiO}2}\left(\mathrm{mol}/\mathrm{kg}\right)=1.6-2.0\bullet {\mathrm{Int}}^{-0.32}\left(\mathrm{cts}/\mathrm{s}\right), $$
(1)
where “Int” is the integrated intensity of the 770 + 850 cm−1 Raman bands in counts per second, and mSiO2 is silica molality.
The SiO2 solubility in the SiO2–NaOH–H2O and SiO2–CaO–H2O fluids was then derived from the calibration curve in Fig. 6 and the integrated Raman intensities at 770 + 850 cm−1 from the SiO2–NaOH–H2O and CaO–SiO2–H2O fluid spectra. In doing this, it was assumed that the different cations, H+, Na+, and Ca2+, in the aqueous solutions had no effect on the Raman intensities and that possible temperature and pressure effects in the pressure range under study on Raman intensity can be ignored.
With the above caveats in mind, the SiO2 solubility in SiO2–CaO–H2O fluids from the calibration curve in Fig. 6 is an order of magnitude, or more, lower than in the SiO2–NaOH–H2O and SiO2–H2O fluids in the fH2O (H2O fugacity) range defined by the temperature and pressure conditions of these experiments (Fig. 6; Table 2). The solubility difference does, however, decrease with increasing fugacity of H2O.
The significant solubility depression resulting from 10 mol% CaO to SiO2 appears similar to that from MgO in the system MgO–SiO2–H2O (Zhang and Frantz 2000). This effect contrasts with adding NaOH where, despite the dilution of SiO2 concentration, the SiO2 solubility in aqueous fluids is similar to that in the simpler SiO2–H2O system. The solubility is, however, more sensitive to fH2O in the SiO2–NaOH–H2O system so that at high H2O fugacity, the SiO2 solubility in the SiO2–NaOH–H2O system exceeds that in SiO2–H2O fluids (Fig. 6). The temperature/pressure coordinates of the solubility cross-over (750–800 °C at ~ 900 MPa), from linear extrapolation of the curves in Fig. 6, occur at the conditions just below the second critical point in the SiO2–H2O system at 1080 °C and 970 MPa (Kennedy et al. 1962). In contrast, linear extrapolation of the SiO2–CaO–H2O line leads to a cross-over at an fH2O corresponding to ~ 1300 MPa pressure at 800 °C. These pressure and temperature conditions are inside the supercritical region of the SiO2–H2O system, which means that conditions of greater silica solubility in SiO2–CaO–H2O fluids than in SiO2–H2O and SiO2–NaOH–H2O cannot be realized.
Silicate speciation in fluid
The approximate concentration of the silicate species in fluid (Qn species) can be obtained from the integrated intensities of the 770 and 850 cm−1 bands of the Raman spectra of fluid. The concentrations thus obtained depend, however, on the assumption that the relative intensities of these two Raman bands are equal to the mol fraction of the species. Given the similar Si–O− force constants for Si–O− stretch vibrations in Q0 and Q1 in hydrous melts (Cody et al. 2005), the equivalence of relative intensities of the 770 and 850 cm−1 bands and the mol fraction of Q0 and Q1 species seems a reasonable assumption.
In the SiO2–H2O and SiO2–NaOH–H2O systems, silica-saturated fluids contain both Q1 and Q0 species, whereas in the SiO2–CaO–H2O system, only Q0 species were detected in the temperature and pressure range. For the SiO2–H2O and SiO2–NaOH–H2O fluids, the Q0 abundance decreases with increasing temperature and pressure although this decrease is more pronounced in the SiO2–NaOH–H2O system. This decrease is coupled with increasing Q1 abundance. The rate of change of the abundance ratio, XQ1/XQ0, with temperature and pressure is considerably faster in the SiO2–NaOH–H2O system and, in fact, can be expressed as linear functions of SiO2 molality, mSiO2 (Fig. 7). Qualitatively, this behavior resembles that for SiO2–H2O and SiO2–MgO–H2O fluids at pressures above 2 GPa (Mysen et al. 2013). This behavior also is qualitatively similar to that of increased silicate polymerization with increasing total SiO2 concentration in silicate melts (Buckermann et al. 1992). The difference in silicate species abundance between SiO2–H2O and SiO2–NaOH–H2O fluids probably occurs because steric hindrance associated with nonbridging oxygen (NBO) bonding to H+ is greater than Na+–NBO bonding given the much smaller ionic radius of H+. Protons will, therefore, favor nonbridging oxygen in Q0 species, whereas Na+ will form bonding with nonbridging oxygen in Q1 species. Hence, the more rapid increase in XQ1/XQ0 in the SiO2–NaOH–H2O systems than in SiO2–H2O.
Thermodynamic considerations
When Q0 and Q1 species are present in the fluid, the equilibrium is:
$$ {2\mathrm{Q}}^0\iff {\mathrm{Q}}^1, $$
(2)
with the equilibrium constant:
$$ {K}_{(2)}={X}_{\mathrm{Q}1}/{\left({X}_{\mathrm{Q}0}\right)}^2, $$
(3)
where XQ1 and XQ0 are mol fractions of Q1 and Q0, respectively. This equilibrium applies to the SiO2–H2O and SiO2–NaOH–H2O fluids in the temperature and pressure range discussed here. We note that it differs from that of SiO2–H2O at higher pressures (1.8–5.2 GPa) where the silica concentration is sufficiently high to stabilize even more polymerized Q2 species as well (Mysen et al. 2013).
The silicate speciation in the SiO2–CaO–H2O fluids is simpler than in the other two systems as the only species throughout the temperature and pressure is Q0 only (Fig. 4). Under such circumstances, the equilibrium constant equals concentration in the fluid, mSiO2:
$$ K={m}_{\mathrm{SiO}2}. $$
(4)
From the temperature dependence of the equilibrium constants, the enthalpy change of the reactions follows from:
$$ \ln\ K=-\Delta H/ RT+\Delta S/R, $$
(5)
where ∆H and ∆S are the enthalpy and entropy changes, T is absolute temperature, and R is the gas constant. It is assumed in using this equation that there is no pressure effect on the equilibria in the pressure range of these experimental results (Table 1). This conclusion is substantiated by the SiO2–H2O and SiO2–MgO–H2O fluid data of Mysen et al. (2013) who did not detect a pressure dependence in the same pressure range as the present experiments.
A linear fit to ln K versus 1/T yields significantly different ∆H values for the equilibria among the Qn species in the fluid (Fig. 8). For the SiO2–CaO–H2O system (Fig. 8a), equilibrium (4) describes the situation. It is more sensitive to temperature (∆H = 322 ± 4 kJ/mol) than equilibrium (2), which describes the speciation variations in the SiO2–H2O (22 ± 12 kJ/mol) and SiO2–NaOH–H2O systems (51 ± 17 kJ/mol; see Fig. 8).