The aim of this study was to perform experiments at conditions, similar to those of the actual injection (270˚C, total pressure of 62 bar and a starting pressure of 8 bar CO2), to probe to what extend mineral carbonation of crystalline basaltic rock can be expected. The reacted material was analysed by X-Ray Diffraction (XRD) to identify the phases present after 5, 47, 155 and 244 hours
of reaction. These patterns were then fitted to that expected for known minerals by Rietveld refinement to determine the relative abundances of the present phases. Furthermore, Scanning Elecetron Microscopy combined with Energy Dispersive X-ray Spectroscopy (SEM-EDXS) analysis were performed to identify any morphological changes in the mineral assemblage. The
geochemical software, PHREEQC was deployed to model the kinetic dissolution of the basaltic minerals and to determine the secondary phase assemblage and their temperature dependence.

The model suggested calcite precipitation occurring within the first hour while the first signs of calcite precipitation in the experimental data appeared after 5 hours of reaction. After 47 hours of reaction, peaks from calcite were clear in the XRD pattern and SEM-EDXS showed rhombohedral crystals with an elemental compositions consistent with calcite. After 244 hours of reaction, XRD and SEM-EDXS indicated that calcite had started to redissolve. Moreover, at that time, the analyses indicated a high degree of contamination originating from corrosion of the reactor vessel. This process undermined the robustness of the results, at least at longer reaction times. In SEMEDXS, forsterite showed a rim of reaction products, including hematite, which slowed down the process of forsterite dissolution. XRD analysis of two basalt samples from the injection depth at Hellisheiði showed peaks from calcite, showing that calcite is stable in the reservoir. The results of the experiments and the presence of calcite in the samples from Hellisheiði indicates that calcite most likely forms when CO2 is injected into the deeper subsurface of Hellisheiði.