Ingvi Gunnarsson

Amorphous silica solubility and the thermodynamic properties of H4SiO°4 in the range of 0° to 350°C at Psat

Abstract The solubility of amorphous silica was determined in the temperature range 8° to 310°C at 1 bar below 100°C and at Psat at higher temperatures. Our results are consistent with previous experiments between 100° and 200°C, but at higher temperatures they indicate lower solubility. Below 100°C our result are lower than the results of some researchers, but in good agreement with others. Our solubility data have been combined with previously reported data to retrieve a temperature equation describing amorphous silica solubility. Quartz solubility data have also been assessed. The solubility equations for the reaction SiO2,s + 2H2O = H4SiO°4 are: logK am.silica = −8.476 − 485.24 × T −1 − 2.268 × 10 −6 × T 2 + 3.068 × logT logK quartz = −34.188 + 197.47 × T −1 − 5.851 × 10 −6 × T 2 + 12.245 × logT where T is in K. They are valid in the temperature range 0° to 350°C at 1 bar below 100°C and at Psat at higher temperatures. From the quartz solubility equation and the thermodynamic properties of quartz and liquid water, the standard partial molal Gibbs energy of formation and the third law entropy of H4SiO°4 were calculated as −1,309,181 J/mole and 178.85 J/mole/K at 25°C. The difference in the standard apparent Gibbs energy of H4SiO°4 as calculated from quartz solubility, on one hand, and amorphous silica solubility, on the other, is about the same over the temperature range 0° to 350°C indicating that the solubility temperature equations obtained for the two solids in this study are internally consistent. This indicates that the quartz solubility data of Rimstidt (1997) , which were used in this study to retrieve the quartz solubility equation, are valid and also our data on ΔḠ°f and S° for H4SiO°4 at 25°C and 1 bar as well as the ΔḠ° temperature equation presented for this species. These new Gibbs energy values for H4SiO°4 indicate that all silicate minerals are considerably more soluble under Earth’s surface conditions than generally accepted to date, or by about 0.6 log K units at 0°C per silicon atom in the unit formula.

Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions

Inject, baby, inject! Atmospheric CO2 can be sequestered by injecting it into basaltic rocks, providing a potentially valuable way to undo some of the damage done by fossil fuel burning. Matter et al. injected CO2 into wells in Iceland that pass through basaltic lavas and hyaloclastites at depths between 400 and 800 m. Most of the injected CO2 was mineralized in less than 2 years. Carbonate minerals are stable, so this approach should avoid the risk of carbon leakage. Science, this issue p. 1312 Basaltic rocks may be effective sinks for storing carbon dioxide removed from the atmosphere. Carbon capture and storage (CCS) provides a solution toward decarbonization of the global economy. The success of this solution depends on the ability to safely and permanently store CO2. This study demonstrates for the first time the permanent disposal of CO2 as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO2 injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. This result contrasts with the common view that the immobilization of CO2 as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years. Our results, therefore, demonstrate that the safe long-term storage of anthropogenic CO2 emissions through mineralization can be far faster than previously postulated.

Major element chemistry of surface- and ground waters in basaltic terrain, N-Iceland.: I. primary mineral saturation

This contribution describes primary basalt mineral saturation in surface- and up to 90°C ground waters in a tholeiite flood basalt region in northern Iceland. It is based on data on 253 water samples and the mineralogical composition of the associated basalts. Surface waters are significantly under-saturated with plagioclase and olivine of the compositions occurring in the study area, saturation index (SI) values ranging from −1 to −10 and −5 to −20, respectively. With few exceptions these waters are also significantly under-saturated with pigeonite and augite of all compositions (SI = −1 to −7) and with ilmenite (SI = −0.5 to −6). The surface waters are generally over-saturated with respect to the titano-magnetite of the compositions occurring in the basalts of the study area, the range in SI being from −2 to +10. For crystalline OH-apatite, SI values in surface waters range from strong under-saturation (−10) to strong over-saturation (+5) but for crystalline F-apatite they lie in the range 0 to 15. Systematic under-saturation is, on the other hand, observed for “amorphous apatite,” i.e. an apatite of the kind Clark (1955) prepared by mixing Ca(OH)2 and H3PO4 solutions. Like surface waters, ground waters are under-saturated with plagioclase and olivine, its degree increasing with increasing Ca content of the plagioclase and increasing Fe content of the olivine, the SI values being −2 to −7 and 0 to −4 for the Ca-richest and Ca-poorest plagioclase, respectively, and about −3 to −18 and 0 to −15 for forsterite and fayalite, respectively. Ground waters are generally close to saturation with pigeonite and augite of all compositions. However, some non-thermal ground waters in highland areas are strongly under-saturated. Above 25°C the ground waters are ilmenite under-saturated but generally over-saturated at lower temperatures. These waters are titano-magnetite over-saturated at temperatures below 70°C, the SI values decreasing with increasing temperature from about 6 to 8 at 10°C to 0 at 70°C. The ground waters are highly over-saturated with both crystalline OH- and F-apatite, or by approximately 10 to 15 SI units but close to saturation with “amorphous apatite” containing about equal amounts of F and OH. The results presented here for the pyroxenes carry an unknown error because available thermodynamic data do not permit but a simple solid solution model for the calculation of their solubility. Published values on the dissociation constants for ferrous iron hydroxide complexes are very variable and those for ferric iron are limited. This casts an error of an unknown magnitude on the calculated SI values for all iron bearing minerals. This error may not be large for waters with a pH of less than 9 but it is apparently high for waters with a higher pH. Improved experimental data on the stability of ferrous and ferric hydrolysis constants are needed to improve the accuracy by which Fe-mineral saturation can be calculated in natural waters.

High reactivity of deep biota under anthropogenic CO2 injection into basalt

Basalts are recognized as one of the major habitats on Earth, harboring diverse and active microbial populations. Inconsistently, this living component is rarely considered in engineering operations carried out in these environments. This includes carbon capture and storage (CCS) technologies that seek to offset anthropogenic CO2 emissions into the atmosphere by burying this greenhouse gas in the subsurface. Here, we show that deep ecosystems respond quickly to field operations associated with CO2 injections based on a microbiological survey of a basaltic CCS site. Acidic CO2-charged groundwater results in a marked decrease (by ~ 2.5–4) in microbial richness despite observable blooms of lithoautotrophic iron-oxidizing Betaproteobacteria and degraders of aromatic compounds, which hence impact the aquifer redox state and the carbon fate. Host-basalt dissolution releases nutrients and energy sources, which sustain the growth of autotrophic and heterotrophic species whose activities may have consequences on mineral storage.The impacts of carbon capture and storage (CCS) on subsurface microorganisms are poorly understood. Here, the authors show that deep ecosystems respond quickly to CO2 injections and that the environmental consequences of their metabolic activities need to be properly assessed for sustainable CCS in basalt.