10:45 〜 12:15
[SMP26-P11] Experimental investigation of reaction-induced fracturing in brucite-rich serpentinite
キーワード：蛇紋岩、炭酸塩化、反応誘起破壊
In situ carbonation of serpentinite has been expected as a potential method to store a large amount of carbon dioxide permanently. However, its poor reactivity on rock specimen has been considered as a challenge. While 78% of pre-heated powder samples of serpentinite has carbonated in 30 minutes (e.g., Conner et al. 2002), rock samples reacted almost only on the surface, and the reaction rate was less than 3% per half year (e.g., Lacinska et al. 2017). Field observation suggest that reaction-induced fracturing due to volume increasing reactions would enhance the reaction inside the rocks (Plümper et al. 2012). Experiments on hydration of periclase have achieved reaction-induced fracturing (e.g., Uno et al. 2022), however, no experimental studies have ever clearly proven that reaction-induced fracturing can occur for natural serpentinite rocks.
To explore the condition of reaction-induced fracturing for serpentinite, this study presents the results 6 different conditions of batch experiments of serpentinite carbonation. Core samples (diameter : 6mm, height : 5mm) of brucite-rich serpentinite collected in northeastern China were heated to 90, 150, and 200℃ in CO2-saturated water (CO2 pressure: 10 MPa) and 1M NaHCO3 solution (steam pressure) for one week. The starting materials are largely composed of serpentinite phase and brucite phase. The serpentinite phase consists of a fine-grained serpentinite-brucite mixture, cut by brucite-serpentinite veins.
The phases of run products varied with each condition. The greatest amount of magnesite precipitated in the serpentine phase at 200℃ in 1M NaHCO3 solution. In CO2-saturated water, the dissolution of brucite veins decreased with increasing temperature, while the amount of reacted serpentine and magnesite precipitation increased. In NaHCO3 solution, the amount of dissolution of brucite veins, reacted serpentine, and magnesite precipitation increased with increasing temperature. In addition, unknown Mg-Si carbonates were formed in all conditions, mostly in NaHCO3 solution 150℃. Reaction-induced fractures were clearly observed only in the serpentinite phase of the samples under CO2 -saturated water 150℃, NaHCO3 solution 150℃ and 200℃. Further observation of the run product of NaHCO3 solution 200 confirmed 2 types of cracks - diagonal cracks inside the sample and vertical cracks on the surface(Fig.1a). In the reacted area in the sample, porous serpentine was formed around the original serpentine-brucite mixture, and magnesite-serpentinite mixture was formed in a mesh texture outside of it.
Based on the above, the reaction mechanism of the sample in NaHCO3 solution at 200℃ was discussed as follows. (1) Selective dissolution of brucite from the serpentinite-brucite mixture on the sample surface supplies Mg ions, locally raises the pH, and leaves behind porous serpentine. (2) Magnesite precipitates in the veins and gaps in the serpentine formed by the dissolution of brucite, causing the expansion in the reacted area. (3) Diagonal cracks are generated inside the sample due to the expansion of the reacted surface. (4) Fluid permeates through the cracks, and the same reaction and expansion as in (1)-(2) occur on the crack surfaces. (5) Vertical cracks are formed on the surface due to the expansion of the inside. (6) Fluid permeates through the crack, and the same reaction and expansion as in (1)-(2) occur on the crack surface. (Repeat from (3)) Therefore, the local reaction and expansion by selective reaction of brucite may play an important role in the continuous reaction-induced fracturing process in serpentinite.
[References]
Conner, Dahlin, Rush, Dahlin, Collins, Minerals & metallurgical processing. 19, 95-101 (2002).
Lacinska, Styles, Bateman, Hall, Brown, Frontiers 5, 1-20 (2018).
Plümper, Royne, Magraso, Jamtveit, Geology 40 1103-1106 (2012)
Uno, Koyanagawa, Kasahara, Okamoto, Tsutiya, The Proceedings of the National Academy of Sciences (PNAS), 119, 1-8(2022).
To explore the condition of reaction-induced fracturing for serpentinite, this study presents the results 6 different conditions of batch experiments of serpentinite carbonation. Core samples (diameter : 6mm, height : 5mm) of brucite-rich serpentinite collected in northeastern China were heated to 90, 150, and 200℃ in CO2-saturated water (CO2 pressure: 10 MPa) and 1M NaHCO3 solution (steam pressure) for one week. The starting materials are largely composed of serpentinite phase and brucite phase. The serpentinite phase consists of a fine-grained serpentinite-brucite mixture, cut by brucite-serpentinite veins.
The phases of run products varied with each condition. The greatest amount of magnesite precipitated in the serpentine phase at 200℃ in 1M NaHCO3 solution. In CO2-saturated water, the dissolution of brucite veins decreased with increasing temperature, while the amount of reacted serpentine and magnesite precipitation increased. In NaHCO3 solution, the amount of dissolution of brucite veins, reacted serpentine, and magnesite precipitation increased with increasing temperature. In addition, unknown Mg-Si carbonates were formed in all conditions, mostly in NaHCO3 solution 150℃. Reaction-induced fractures were clearly observed only in the serpentinite phase of the samples under CO2 -saturated water 150℃, NaHCO3 solution 150℃ and 200℃. Further observation of the run product of NaHCO3 solution 200 confirmed 2 types of cracks - diagonal cracks inside the sample and vertical cracks on the surface(Fig.1a). In the reacted area in the sample, porous serpentine was formed around the original serpentine-brucite mixture, and magnesite-serpentinite mixture was formed in a mesh texture outside of it.
Based on the above, the reaction mechanism of the sample in NaHCO3 solution at 200℃ was discussed as follows. (1) Selective dissolution of brucite from the serpentinite-brucite mixture on the sample surface supplies Mg ions, locally raises the pH, and leaves behind porous serpentine. (2) Magnesite precipitates in the veins and gaps in the serpentine formed by the dissolution of brucite, causing the expansion in the reacted area. (3) Diagonal cracks are generated inside the sample due to the expansion of the reacted surface. (4) Fluid permeates through the cracks, and the same reaction and expansion as in (1)-(2) occur on the crack surfaces. (5) Vertical cracks are formed on the surface due to the expansion of the inside. (6) Fluid permeates through the crack, and the same reaction and expansion as in (1)-(2) occur on the crack surface. (Repeat from (3)) Therefore, the local reaction and expansion by selective reaction of brucite may play an important role in the continuous reaction-induced fracturing process in serpentinite.
[References]
Conner, Dahlin, Rush, Dahlin, Collins, Minerals & metallurgical processing. 19, 95-101 (2002).
Lacinska, Styles, Bateman, Hall, Brown, Frontiers 5, 1-20 (2018).
Plümper, Royne, Magraso, Jamtveit, Geology 40 1103-1106 (2012)
Uno, Koyanagawa, Kasahara, Okamoto, Tsutiya, The Proceedings of the National Academy of Sciences (PNAS), 119, 1-8(2022).