14:15 〜 14:30
[MIS18-02] ガラス質物質の水和反応の地球化学解析に向けて
キーワード：ガラス質物質、溶解、熱力学、速度論
1. Context
Glassy materials are widespread on the Earth's surface and serve as important precursors to the transition into stable crystalline forms during weathering. These materials are primarily formed through volcanism. However, due to the relatively short lifespan of glass, geologically old specimens are rare and often only preserved as magmatic melt inclusions within crystals. The scarcity of old glass is largely due to the presence of water, which contributes to its degradation over time. As a result, simulations of glass weathering have been limited [1,2,3], mainly due to less understanding their kinetics and thermodynamic properties.
2. Experimental
In this study, two types of glassy materials were selected: (1) anorthite glass and (2) tuff. The dissolution of anorthite glass was directly measured using optical interferometry, while the dissolution rate of tuff was analyzed through batch reaction methods. The obtained dissolution rate data were analyzed according to established procedures [4], with equilibrium LogK values estimated by polyhedoral model [5]. Based on this information, it is possible to model the alteration process of these glassy materials, encompassing dissolution and precipitation. Geochemical simulations with PHREEQC may provide more realistic insights into the process of glass weathering.
4. Results
The measured logarithmic rates for anorthite crystal [6] and glass at pH 7 to 12.8 were within the ranges of 1.04E-13 to 1.34E-12 (with a pH reaction order, n, of 0.191) and 1.32E-10 to 1.43E-8 (n = 0.351) mol/m2/s, respectively. These rates may be thermodynamically driven not only by pH but also by supersaturation, indicated by ΔGr (with a LogK value of 26.70). Comparing these rates with ΔGr suggests that the glassy material may be slightly more stable than its crystalline state. Additionally, the dissolution rate of tuff, yielding a rate of 4.10E-10 mol/m2/s under normal pH conditions, can also be influenced by pH and ΔGr. Although these ΔGr values have not been experimentally determined, they can be estimated from thermodynamically obtained LogK values as 16.87 (indeed lower than the crystal) and -2.66 for anorthite and tuff glasses, respectively. Using the obtained data, geochemical simulations were conducted on the system involving a concrete facility built on the tuffaceous host-rock.
3. Discussion
The results of geochemical simulations on the tuff interface with concrete suggest that the weathering of glassy material within the tuff likely played a crucial role in the formation of secondary clay phases such as kaolinite and zeolite, contributing to natural soil formation processes. Therefore, detailed investigations into the thermodynamics and kinetics of glassy materials could help verify and validate weathering and alteration processes, which are of interest for understanding early Earth environments and addressing concerns regarding the safety of waste disposal facilities.
References:
[1] Aertsens, M., van Iseghem, P. (1995) MRS Online Proc. Lib. 412, 271. [2] Shikazono, N. et al. (2005) Geochem. Jour., 39, 185. [3] Kerisit, S. and Du, J. (2019) Jour. Non-crystal. Sol. 522, 15. [4] Lasaga, A. (1998) Kinetic Theory Earth Sci. 797. [5] Chermak, J.A., Rimstidt, J.D. (1989) Ame. Min. 74, 1023. [6] Satoh, H. et al. (2007) Ame. Min. 92, 503.
Glassy materials are widespread on the Earth's surface and serve as important precursors to the transition into stable crystalline forms during weathering. These materials are primarily formed through volcanism. However, due to the relatively short lifespan of glass, geologically old specimens are rare and often only preserved as magmatic melt inclusions within crystals. The scarcity of old glass is largely due to the presence of water, which contributes to its degradation over time. As a result, simulations of glass weathering have been limited [1,2,3], mainly due to less understanding their kinetics and thermodynamic properties.
2. Experimental
In this study, two types of glassy materials were selected: (1) anorthite glass and (2) tuff. The dissolution of anorthite glass was directly measured using optical interferometry, while the dissolution rate of tuff was analyzed through batch reaction methods. The obtained dissolution rate data were analyzed according to established procedures [4], with equilibrium LogK values estimated by polyhedoral model [5]. Based on this information, it is possible to model the alteration process of these glassy materials, encompassing dissolution and precipitation. Geochemical simulations with PHREEQC may provide more realistic insights into the process of glass weathering.
4. Results
The measured logarithmic rates for anorthite crystal [6] and glass at pH 7 to 12.8 were within the ranges of 1.04E-13 to 1.34E-12 (with a pH reaction order, n, of 0.191) and 1.32E-10 to 1.43E-8 (n = 0.351) mol/m2/s, respectively. These rates may be thermodynamically driven not only by pH but also by supersaturation, indicated by ΔGr (with a LogK value of 26.70). Comparing these rates with ΔGr suggests that the glassy material may be slightly more stable than its crystalline state. Additionally, the dissolution rate of tuff, yielding a rate of 4.10E-10 mol/m2/s under normal pH conditions, can also be influenced by pH and ΔGr. Although these ΔGr values have not been experimentally determined, they can be estimated from thermodynamically obtained LogK values as 16.87 (indeed lower than the crystal) and -2.66 for anorthite and tuff glasses, respectively. Using the obtained data, geochemical simulations were conducted on the system involving a concrete facility built on the tuffaceous host-rock.
3. Discussion
The results of geochemical simulations on the tuff interface with concrete suggest that the weathering of glassy material within the tuff likely played a crucial role in the formation of secondary clay phases such as kaolinite and zeolite, contributing to natural soil formation processes. Therefore, detailed investigations into the thermodynamics and kinetics of glassy materials could help verify and validate weathering and alteration processes, which are of interest for understanding early Earth environments and addressing concerns regarding the safety of waste disposal facilities.
References:
[1] Aertsens, M., van Iseghem, P. (1995) MRS Online Proc. Lib. 412, 271. [2] Shikazono, N. et al. (2005) Geochem. Jour., 39, 185. [3] Kerisit, S. and Du, J. (2019) Jour. Non-crystal. Sol. 522, 15. [4] Lasaga, A. (1998) Kinetic Theory Earth Sci. 797. [5] Chermak, J.A., Rimstidt, J.D. (1989) Ame. Min. 74, 1023. [6] Satoh, H. et al. (2007) Ame. Min. 92, 503.