5:15 PM - 7:15 PM
[SMP28-P14] Theoretical analysis of the brucite/olivine interface associated with deserpentinisation:
Investigation based on molecular dynamics calculations
Keywords:MD simulation, olivine, brucite, topotaxy, mineral interface, deserpentinization
In the shallow part of the wedge mantle, antigorite (Mg,Fe)6Si4O10(OH)8 and brucite Mg(OH)2 undergo a dehydration reaction and form olivine (Mg,Fe)2SiO4. Recently, a topotactic relationship between brucite and olivine associated with this reaction was reported by Nagaya et al. (2022). Assuming this is correct, it may force a reconsideration of the previously considered reaction conditions and mineral distribution in the wedge mantle. It is therefore extremely important to clarify whether brucite shows topotactic relations to olivine. In this study, we focused on the interface structure between brucite and olivine. Using Molecular Dynamics (MD) calculations, we analyzed whether the crystal orientation of brucite constrains that of the newly formed olivine.
In the natural and experimental sample analyzed in Nagaya et al. (2022), the (001) plane of the brucite (brc), (001)brc, can be identified as the reaction surface. At this brucite/olivine (ol) interface, the olivine c-axis is slightly (~20°) inclined with respect to the brucite c-axis. Investigation of the orientation relationships using pole figures shows that the (011)ol and the (001)brc are in contact with each other in both natural and experimental cases. Both (001)brc and (001)ol are stable planes that appear in the equilibrium morphology. In contrast, (011)ol is less stable than (001)ol (de Leeuw et al., 2000) and does not appear in the equilibrium morphology. To investigate the factors that cause the less stable (011)ol contact to (001)brc, surface energies of brucite and olivine, we compared the results of MD calculations of the surface energies of (001)brc, (001)ol, and (011)ol, as well as the interfacial energies between brucite and olivine. As deserpentinization is a dehydration reaction, water is considered to be present at the reaction interface. Therefore, models with water added to the olivine surface was also analyzed. The software MXDTRCL (Kawamura, 1998) was used for the MD calculations and potential parameters were taken from Sakuma et al. (2003) and Miyake (1998), which simulate the crystal structure of each mineral well. We note that the calculations in this study are performed for the Mg-endmember of olivine, forsterite Mg2SiO4.
MD calculations show that (001)ol is a more stable olivine surface than (011)ol. This is consistent with previous results. These calculations also show that the surface energy of (011)ol+H, i.e. the (011) surface of the olivine with hydrogen, was smaller than that of (001) ol+H. Furthermore, the (001)brc/(011)ol+H interfacial energy was smaller than that of the (001)brc/(001)ol+H. The replacement of Si4+ ⇔ 4H+ in brucite-rich serpentinite has been reported by Kempf et al. (2018) and accordingly we also analyzed interface models in which Si4+ ions in the topmost layer of (001)ol+H and (011)ol+H were replaced with 4H+. As a result, we found that the replaced (001)brc/(011)ol+H interface was more stable than the replaced (001)brc/(001)ol+H interface. This result suggests that the (001)brc/(011)ol+H interface is more likely to form even in the Si4+-poor environment of the early stage of olivine formation on the brucite surface. The greater stability of (001)brc/(011)ol+H interface revealed in this study is consistent with both natural observations and experimental results and supports the proposal that a topotactic relationship exists between brucite and olivine formed by a deserpentinization reaction.
References
1. Nagaya, T. et al., 2022. Contrib. Mineral. Petrol. 177, 87.
2. de Leeuw, N. et al., 2000. Phys Chem Min 27, 332–341.
3. Kawamura, K., 1997. Japan Chemical Program Exchange #77
4. Sakuma, H. et al., 2003. Surface Science 536, 1–3, L396-L402.
5. Miyake, 1998. Mineral. Jour. Lett. 20, 4, 189-194.
6. Kempf, E.D., Hermann, J., 2018. Geology 46 (6): 571–574.
In the natural and experimental sample analyzed in Nagaya et al. (2022), the (001) plane of the brucite (brc), (001)brc, can be identified as the reaction surface. At this brucite/olivine (ol) interface, the olivine c-axis is slightly (~20°) inclined with respect to the brucite c-axis. Investigation of the orientation relationships using pole figures shows that the (011)ol and the (001)brc are in contact with each other in both natural and experimental cases. Both (001)brc and (001)ol are stable planes that appear in the equilibrium morphology. In contrast, (011)ol is less stable than (001)ol (de Leeuw et al., 2000) and does not appear in the equilibrium morphology. To investigate the factors that cause the less stable (011)ol contact to (001)brc, surface energies of brucite and olivine, we compared the results of MD calculations of the surface energies of (001)brc, (001)ol, and (011)ol, as well as the interfacial energies between brucite and olivine. As deserpentinization is a dehydration reaction, water is considered to be present at the reaction interface. Therefore, models with water added to the olivine surface was also analyzed. The software MXDTRCL (Kawamura, 1998) was used for the MD calculations and potential parameters were taken from Sakuma et al. (2003) and Miyake (1998), which simulate the crystal structure of each mineral well. We note that the calculations in this study are performed for the Mg-endmember of olivine, forsterite Mg2SiO4.
MD calculations show that (001)ol is a more stable olivine surface than (011)ol. This is consistent with previous results. These calculations also show that the surface energy of (011)ol+H, i.e. the (011) surface of the olivine with hydrogen, was smaller than that of (001) ol+H. Furthermore, the (001)brc/(011)ol+H interfacial energy was smaller than that of the (001)brc/(001)ol+H. The replacement of Si4+ ⇔ 4H+ in brucite-rich serpentinite has been reported by Kempf et al. (2018) and accordingly we also analyzed interface models in which Si4+ ions in the topmost layer of (001)ol+H and (011)ol+H were replaced with 4H+. As a result, we found that the replaced (001)brc/(011)ol+H interface was more stable than the replaced (001)brc/(001)ol+H interface. This result suggests that the (001)brc/(011)ol+H interface is more likely to form even in the Si4+-poor environment of the early stage of olivine formation on the brucite surface. The greater stability of (001)brc/(011)ol+H interface revealed in this study is consistent with both natural observations and experimental results and supports the proposal that a topotactic relationship exists between brucite and olivine formed by a deserpentinization reaction.
References
1. Nagaya, T. et al., 2022. Contrib. Mineral. Petrol. 177, 87.
2. de Leeuw, N. et al., 2000. Phys Chem Min 27, 332–341.
3. Kawamura, K., 1997. Japan Chemical Program Exchange #77
4. Sakuma, H. et al., 2003. Surface Science 536, 1–3, L396-L402.
5. Miyake, 1998. Mineral. Jour. Lett. 20, 4, 189-194.
6. Kempf, E.D., Hermann, J., 2018. Geology 46 (6): 571–574.