5:15 PM - 6:45 PM
[MGI28-P06] Characterization of porosity structures produced by Alteration of Feldspar: insights from hydrothermal alteration and persistent homology
Keywords:feldspar, hydrothermal alteration, porosity, 3D, geometry, persistent homology
In the high-temperature crust, the main fluid pathways have been considered to be fractures or the grain
boundaries. Recently, the presence of abundant micropores has been reported in various altered rocks and
minerals such as feldspar, sulfide minerals and carbonate minerals, and thus formation mechanism and roles
of the nano- to microscale pores during alteration reactions with fluids are attracting attention [1]. In particular,
the replacement of feldspars, in which a feldspar (plagioclase (Pl) or K-feldspar (Kfs)) is reacted with brine
(NaCl,aq or KCl,aq) and is replaced by the other feldspars, produces abundant micro porosities are formed[1].
There have been numerous hydrothermal experiments on replacement of feldspars. However, the 3D shape, connectivity, and temporal evolution of the porosity network are still poorly understood, due to the limitation of system of the experiments and observations (2D snapshots of the experimental products). In addition, appropriate ways for the quantitative description of pores with complex geometric shapes are lacking. In this study, we conducted hydrothermal reaction experiments on contrasting reactions: (1) potassium feldspar (Kfs) reacting with NaCl solution (substitution by albite (Ab); ΔV=+8.85%) and (2) albite (Ab) reacting with KCl solution (substitution by Kfs; ΔV=-8.14%). By repeated runs (48h, 96 h) and imaging with high-resolution X-ray CT, we investigated the temporal changes in pore shape. Then, we applied persistent homology, a branch of topology, to reveal the characteristics of the pores and discussed the mechanisms of pore formation from the obtained persistence diagrams [2].
In both reactions, porosity was generated at the reaction front of the replacement that proceeded along fractures or out margin of the parent crystals. In the replacement reaction from Ab to Kfs (solid volume increase), the isolated pores existed showing a platy shape, and aligned almost parallel to the main fractures. Conversely, in the substitution reaction from Kfs to Ab (solid volume decrease), the pores exhibited a 3D dendritic structure with branching, some of which were partially connected to the central main fractures. After 96h runs, the pores of 48h is closed and new pores are generated at the migrated reaction front.
The evolution of 3D pores are evaluated by using 0th order persistence diagrams, in which birth (b) and death (d) of the materials are plotted during filtration operations. In the 48 h run of the replacement from Ab to Kfs, births were distributed over a broad range (-9 to -2, with 1 pixel equivalent to 1.65μm), while deaths were in a narrow range(-6 to -2). In contrast, after the 96 h run, the plot of (b<0, d<0) moved to the plot of (b<0, d>0). The inversion of these characteristic points in PD reveal that the necked large pores (b<0, d<0) are split into multiple isolated pores (b<0, d>0). In the PD for the repalcement from Kfs to Ab, the trend from 48 h to 96 h was opposite to that observed in the replacement from Ab to Kfs, suggesting that multiple isolated pores became connected with the progress of alteration, resulting in the formation of large tree like porosities. Continuous generation of nano-porosity at the reaction front due to the solid volume decrease, and migration by diffusion and aggregation of pores along the chemical potential gradient could be responsible for the formation of such characteristic pore geometry.
[1]Plümper O, Botan A, Los C, Liu Y, et al. 2017, Nature Geoscience, 10, 685-691
[2]Kimura M, Obayashi I, Takeichi Y et al. 2018. Scientific Reports, 8, 3553.
boundaries. Recently, the presence of abundant micropores has been reported in various altered rocks and
minerals such as feldspar, sulfide minerals and carbonate minerals, and thus formation mechanism and roles
of the nano- to microscale pores during alteration reactions with fluids are attracting attention [1]. In particular,
the replacement of feldspars, in which a feldspar (plagioclase (Pl) or K-feldspar (Kfs)) is reacted with brine
(NaCl,aq or KCl,aq) and is replaced by the other feldspars, produces abundant micro porosities are formed[1].
There have been numerous hydrothermal experiments on replacement of feldspars. However, the 3D shape, connectivity, and temporal evolution of the porosity network are still poorly understood, due to the limitation of system of the experiments and observations (2D snapshots of the experimental products). In addition, appropriate ways for the quantitative description of pores with complex geometric shapes are lacking. In this study, we conducted hydrothermal reaction experiments on contrasting reactions: (1) potassium feldspar (Kfs) reacting with NaCl solution (substitution by albite (Ab); ΔV=+8.85%) and (2) albite (Ab) reacting with KCl solution (substitution by Kfs; ΔV=-8.14%). By repeated runs (48h, 96 h) and imaging with high-resolution X-ray CT, we investigated the temporal changes in pore shape. Then, we applied persistent homology, a branch of topology, to reveal the characteristics of the pores and discussed the mechanisms of pore formation from the obtained persistence diagrams [2].
In both reactions, porosity was generated at the reaction front of the replacement that proceeded along fractures or out margin of the parent crystals. In the replacement reaction from Ab to Kfs (solid volume increase), the isolated pores existed showing a platy shape, and aligned almost parallel to the main fractures. Conversely, in the substitution reaction from Kfs to Ab (solid volume decrease), the pores exhibited a 3D dendritic structure with branching, some of which were partially connected to the central main fractures. After 96h runs, the pores of 48h is closed and new pores are generated at the migrated reaction front.
The evolution of 3D pores are evaluated by using 0th order persistence diagrams, in which birth (b) and death (d) of the materials are plotted during filtration operations. In the 48 h run of the replacement from Ab to Kfs, births were distributed over a broad range (-9 to -2, with 1 pixel equivalent to 1.65μm), while deaths were in a narrow range(-6 to -2). In contrast, after the 96 h run, the plot of (b<0, d<0) moved to the plot of (b<0, d>0). The inversion of these characteristic points in PD reveal that the necked large pores (b<0, d<0) are split into multiple isolated pores (b<0, d>0). In the PD for the repalcement from Kfs to Ab, the trend from 48 h to 96 h was opposite to that observed in the replacement from Ab to Kfs, suggesting that multiple isolated pores became connected with the progress of alteration, resulting in the formation of large tree like porosities. Continuous generation of nano-porosity at the reaction front due to the solid volume decrease, and migration by diffusion and aggregation of pores along the chemical potential gradient could be responsible for the formation of such characteristic pore geometry.
[1]Plümper O, Botan A, Los C, Liu Y, et al. 2017, Nature Geoscience, 10, 685-691
[2]Kimura M, Obayashi I, Takeichi Y et al. 2018. Scientific Reports, 8, 3553.