## 1 Introduction
### Single Mineral Interactions
#### 1.9.3 Feldspars  
Orthoclase is a \"framework\" silicate mineral (tectosilicate) of the alkali (potassium) feldspar group. As is typical, this mineral is found in conjunction with the plagioclase feldspar albite. This intergrowth of two feldspars is called perhtite. Due to the high content of alkali and alkaline earth metals found in these minerals, their potential for carbonation is high [21]. Plagioclase feldspars such as albite have been seen to be the most reactive crystalline phase in basalt and granite host formations [43; 44]. Reversible reactions of K-feldspar and plagioclase with dissociated ions are [32]:  
\[\text{Orthoclase }+4\text{H}^{+}\leftrightarrow\text{K}^{+}+\text{Al}^{3+}+3 \text{SiO}_{2}+2\text{H}_{2}\text{O} \tag{5}\]  
\[\text{Albite}+4\text{H}_{2}\text{O}+4\text{H}^{+}\leftrightarrow\text{Na}^{+ }+\text{Al}^{3+}+3\text{H}_{4}\text{SiO}_{4} \tag{6}\]  
Wet CO\({}_{2}\) treatment (14 days at 50 \({}^{\circ}\)C and 138 bar) experiments on albite conducted by [PERSON] et al. [41] showed dissolution features and the suspected precipitation of silica and phyllosilicates. A similar result was seen by [PERSON] et al. [45] when investigating another plagioclase feldspar, anorthite. Again, the authors found feldspar dissolution and phyllosilicate precipitation following exposure to wet CO\({}_{2}\) for 60 days at 200 \({}^{\circ}\)C and 10.5 MPa.  
[PERSON] et al. [21] studied the carbonation of a plagioclase sample (labradorite) using in situ IR. These authors carried out four experiments, which were conducted with varying amounts of water present in the CO\({}_{2}\) (values relative to saturation). The sample was held at 50 \({}^{\circ}\)C and 90 bar and IR measurements were taken every 3 h up to 24 h. Between 1466 and 1520 cm\({}^{-1}\), asymmetric C-O stretching bands appeared on all but the 0% H\({}_{2}\)O sample. This is an indication of mineral carbonation [46; 47], with calcite and minor amounts of sodium carbonate being the most probable product [21]. A dependence with time was also seen. When H\({}_{2}\)O was present, H-O-H bending was observed at circa 1640 cm\({}^{-1}\), which also coincided with O-H stretching between 3000-3500 cm\({}^{-1}\). This suggested that a thin H\({}_{2}\)O film was forming on the mineral surface. This film increased reactivity as it acted as a solvate, transporting ions to the mineral surface. Plagioclase dissolution and the precipitation of silica were shown by the negative net absorbance and subtle peaks between 750-1400 cm\({}^{-1}\). Here, \(v_{2}\) bending and Si-O stretching bands indicated precipitation and dissolution, respectively [21]. The extent of reaction was estimated at 0.3% for the excess water sample, determined using the net change in absorbance at 958 cm\({}^{-1}\). Similar features were found in experiments using anorthite and albite. The extent of reaction of albite was estimated at 0.5%. For this evaluation, a peak at c. 760 cm\({}^{-1}\) was used, corresponding to Al-O-Si symmetric stretching modes. These bands are negative due to the dissolution of feldspar.  
[PERSON] et al. [21] also investigated the potassium feldspar microcline. The sample was obtained under the same conditions with varying water saturation levels. Only at excess water saturation was evidence for mineral carbonation and a large adsorbed H\({}_{2}\)O film seen, signalling that this film layer is necessary to initialise reactions. Some carbonation was seen at 75% water saturation, but [PERSON] et al. [21] suggested that a thicker water film may be needed to sustain the reactions. Feldspars seem largely unreactive in dry scCO\({}_{2}\), but with a thick enough film carbonic acid can form. This then dissociates to a hydrated proton and bicarbonate. Proton-promotion occurs with the metal silicate mineral, dissolving it [21].