*4.1. Mineral Reactions*

Despite the high availability of basalts on the Earth's surface [48–50], only few basaltic types have the appropriate petrophysical and chemical properties [3,6,50–52] to serve as host rocks suitable for CO2 mineral carbonation. The basaltic rocks from the localities of Porphyrio and Microthives possess proper mineralogical, chemical, and textural features to apply CO2 sequestration techniques. These features include the high abundance of Ca-bearing minerals, as well as their silica-undersaturated alkaline composition and high porosity. Mineral chemistry reactions that result from this interaction can be modeled based on data provided from this study.

The physicochemical properties of water strongly affect the formation of carbonate minerals during the interaction of basalts with the CO2 injected fluids. Carbonation with the presence of water can lead to higher amounts of sequestered CO2 compared to the dry carbonation processes [53,54]. The dissolution of CO2 in water further affects the liquid reactivity, due to the high amounts of the released H<sup>+</sup> [3,4,6,50]. The concentration of Mg in water can affect the crystal growth of calcite, whereas, at high temperatures, Mg can precipitate in the form of solid mineral phases [3]. In addition, the water saturation reflected from the water/rock ratio (W/R) determines the dissolution of basaltic rocks, which is higher in CO2 saturated solutions compared to the undersaturated ones (W/R: 10/1 and 2/1 respectively; atmosphere [3]).

The underground water analysed from the region of Microthives is classified as neutral to alkaline (pH = 7.30). Dissolution of CO2 in water produces carbonic acid. The gradual mixing of the alkaline groundwater with the acidic injection fluids starts up with the entrance of the injected fluid into the storage formation and ends up with the entrance of the fluid in the monitoring wells [55]. After the mixing process, the formation fluids become more acidic, presenting lower pH values [55]. This acidic pH is characterised by a high concentration of dissolved inorganic carbon (DIC), making the water reactive with the basaltic rocks, due to the high H<sup>+</sup> contents [50].

Addition of CO2 is expected to lower the pH of water due to the release of H<sup>+</sup> ions, according to the following chemical reactions [50]:

$$\rm{H}\rm{CO}\_{2} + \rm{H}\_{2}\rm{O} = \rm{H}\_{2}\rm{CO}\_{3} \tag{1}$$

$$\rm H\_2CO\_3 = HCO\_3^- + H^+ \tag{2}$$

Basaltic rocks are rich in Ca, Mg, and Fe, providing the potential for CO2 mineralisation in the form of carbonate minerals. The released H<sup>+</sup> ions (chemical reaction-2) increase the reactivity of water, resulting in dissolution of the primary basalt minerals and the precipitation of Ca2+, Mg2+, and Fe2<sup>+</sup> in the form of carbonate minerals [4,50], according to the following chemical reaction:

$$\text{(Ca,Mg,Fe)}^{2+} + \text{H}\_2\text{CO}\_3 \rightarrow \text{(Ca,Mg,Fe)}\text{CO}\_3 + 2\text{H}^+\tag{3}$$

Carbonation of olivine is described by mineral reaction-4. The high MgO contents (MgO: 36.58–48.50%) of the studied olivine phenocrystals will produce high amounts of magnesite. This reaction is developed with slow rates in the natural systems, suggesting that the carbonation of olivine must be enhanced by a large-scale storage method for CO2 mineralisation [56,57]. The formation of hydromagnesite is favoured at low temperatures and can be described by reaction-5 [58]. At low temperatures (*T* < 60 ◦C), indirect precipitation of magnesite can occur via hydromagnesite dehydration [58]. This process is described through the two-way reaction-6 [59].

$$\rm Mg\_2SiO\_{4(s)} + 2CO\_2 \to 2MgCO\_{3(s)} + SiO\_{2(s)}\tag{4}$$

$$2\text{ }\text{5Mg}\_2\text{SiO}\_{4(s)} + 8\text{CO}\_2\text{(gas)} + 2\text{H}\_2\text{O}\_{(liq)} \rightarrow 2\text{ }(\text{4MgCO}\_3\cdot\text{Mg(OH)}\_2\cdot4\text{H}\_2\text{O})\_{(s)} + 5\text{H}\_4\text{SiO}\_{4(aq)}\tag{5}$$

$$4\text{Mg(CO}\_3)\cdot\text{Mg(OH)}\_2\cdot4\text{H}\_2\text{O} \leftrightarrow 4\text{MgCO}\_3 + \text{Mg(OH)}\_2 + 4\text{H}\_2\text{O}\tag{6}$$

The studied olivine crystals of Microthives and Porphyrio basalts are mostly composed by forsterite. In that case, the olivine carbonation can be further described by the following mineral reaction:

$$\text{Mg}\_2\text{SiO}\_{4(s)} + 4\text{H}^+\text{(aq)} \to 2\text{Mg}^{2+} + \text{SiO}\_{2(s)} + 2\text{H}\_2\text{O} \tag{7}$$

Dissolution of clinopyroxene is developed according to the following mineral reaction:

$$\text{MgCaSi}\_2\text{O}\_6 + 4\text{H}^+ \rightarrow \text{Mg}^{2+} + \text{Ca}^{2+} + 2\text{H}\_2\text{O} + 2\text{SiO}\_{2(aq)}\tag{8}$$

The release of Ca2<sup>+</sup> cations is described by the dissolution of anorthite rich plagioclase according to the chemical reaction-9:

$$\text{CaAl}\_2\text{Si}\_2\text{O}\_8 + 8\text{H}^+ \rightarrow \text{Ca}^{2+} + 2\text{Al}^{3+} + 4\text{H}\_2\text{O} + 2\text{SiO}\_{2(aq)}\tag{9}$$

Orthopyroxene appears in the form of accessory enstatite crystals. Dissolution of enstatite is described by mineral reaction-10 [60]:

$$\text{MgSiO}\_3 + 2\text{H}^+ \rightarrow \text{Mg}^{2+} + \text{SiO}\_2 + \text{H}\_2\text{O} \tag{10}$$

Precipitation of calcite (reaction-11 [50]) during hydrothermal alteration of basaltic rocks is strongly associated with temperature and depth. The Ca2<sup>+</sup> required for calcite precipitation is mostly derived from the primary calc–silicate minerals and the glass matrix of the basaltic protolith. These minerals mostly include clinopyroxene (CaO: 21.58–23.57%), plagioclase (CaO: 13.84–14.34%), and amphiboles (CaO: 9.81–11.66%).

$$\text{Ca}^{2+} + \text{CO}\_2 + \text{H}\_2\text{O} \rightarrow \text{CaCO}\_3 + 2\text{H}^+ \tag{11}$$

Calcite formation is not favoured at temperatures higher than 290 ◦C [61] and depths between 200 and 400 m [62]. The time required for carbonate minerals precipitation strongly depends on the abundance of divalent cations, the fluid *P*–*T*, the liquid chemistry, the CO2 saturation, and the pore surface area [4]. Diagrams of basalt dissolution rates vs. pH (Figure 5a,b) were designed using data from the literature [50,63–65]. The aforementioned diagrams indicate that during the mixing of the background water with the CO2 injected fluids, the pH decrease enhances the dissolution rate of forsterite (Mg-olivine) and augite (clinopyroxene). The crystalline basalts in Microthives and Porphyrio localities are mostly composed by clinopyroxene and olivine phenocrystals within a glass-rich matrix. Clinopyroxene is mostly classified as augite, whereas olivine is characterised by relatively high MgO contents (Table 1). The glass-rich basalts are characterised by relatively constant dissolution rates for pH values between 4 and 7.3, whereas their dissolution rates increase with further pH decrease. For pH values lower than 4, the dissolution rate will be rapidly increased and become similar with that of forsterite. This indicates that during the initial stages of the CO2 injection, more glass-rich basalts will be dissolved with lower rates compared to the crystalline ones. During the interaction of Microthives and Porphyrio basalts with CO2 injected fluids, clinopyroxene-olivine porphyroblasts [3,6,50,66] and the anorthite-glass rich matrix will be dissolved with similar rates against their pH [3,6,50,67]. The aforementioned results indicate that clinopyroxene and olivine porphyroblasts will be the first mineral phases to be dissolved during the CO2 injection.

**Figure 5.** (**a**) Modified diagram of the dissolution rate of the forsterite (*T*: 25 ◦C; [64]) and basaltic glass (*T*: 30 ◦C) [65] vs. pH. The dissolution rate is normalised to the BET surface area of the dissolving mineral and glass grains. (**b**) Modified diagram [68] of the dissolution fluxes (mol m−<sup>2</sup> s<sup>−</sup>1) at *T*: 25 ◦C of crystalline and glassy basalts. Forsterite and augite dissolution rates taken from [63].

Based on the experimental results from Gislason et al. [50] (Figure 5a), the dissolution rate of olivine increases from 10−<sup>10</sup> to 10−8.5 (mol/m2/s) for pH values ranging from 7.3 (Microthives water pH) to 1.5. These results are in agreement with the experiments of Palandri and Kharaka [63] that indicate a comparable increase of forsterite dissolution rate from 10−10.5 to 10−8.5 (mol/m2/s) for pH values ranging from 7.3 to 2. Dissolution of augite vs. pH follows similar trends, ranging from 10−<sup>12</sup> to 10−8.5 (mol/m2/s) for the same pH range with augite (Figure 5b). Experimental results suggest that dissolution rate of diopside will be three orders of magnitude slower compared to other silicate minerals, such as olivine at 25 ◦C [69,70]. Data provided by Palandri and Kharaka [63] point to an increase of the plagioclase dissolution rate from 10−11.5 to 10−<sup>10</sup> (mol/m2/s) for pH values ranging from 7 to 2.

Dissolution rate of CO2 in water strongly depends on the water temperature, the partial pressure of CO2, and the salinity of the medium [50]. Carbonation rate of secondary minerals is strongly associated with the acidic or alkaline nature of the water. Experimental results at a temperature of 25 ◦C under acidic and neutral conditions show that the carbonation rate of calcite, magnesite, and siderite ranges are 10<sup>−</sup>0.3–10−5.81 mol/(m2/s) [63], 10<sup>−</sup>6.38–10−9.34 mol/(m2/s) [63], and 10<sup>−</sup>3.74–10−8.90 mol/(m2/s) [71], respectively. This further suggests that precipitation of carbonate minerals is mostly favoured during the late stages of the CO2 injection, characterised by lower pH values compared to the formation groundwater (pH: 7.3 for Microthives groundwater). Availability of divalent cations is the main limiting step during CO2 mineralisation in basalts [5]. Basalts of 8% average MgO correspond to 0.087 CO2 g/g basalt converted to magnesite [3]. Abundance of Mg-olivine in the studied basalts from the regions of Microthives and Porphyrio support their high potential for magnesite precipitation. The relatively low alteration grade of the studied basalts provides additional advantages regarding their potential for mineral carbonation, due to their higher carbonation grades compared to the altered ones [72].
