5:15 PM - 6:30 PM
[MIS14-P02] Experimental study on the growth process of iron rind in Fe-oxide concretions
Keywords:Fe-oxide concretion, CaCO3 concretion
Introduction
The Navajo Sandstone in Utah, U.S.A. and the Gobi Desert in Mongolia have geological formations containing Fe-oxide concretions. They are several mm to cm in size and have spherical shape. The surface is covered with a rind of iron and the interior is filled with sandgrain. An analog of Fe-oxide concretion was discovered on Mars by NASA’s Opportunity rover. Fe-oxide concretions form only under limited conditions. Therefore, identification of the formation process of Fe-oxide concretions on Earth will lead to clarification of the ancient environment of Mars. In 2018, a research group at Nagoya University proposed a new model for the formation of Fe-oxide concretions on the Earth. The hypothesis is that Fe-oxide concretions were originally CaCO3 concretions. They were replaced by iron through processes including (1) dissolution and (2) precipitation due to neutralization reactions with iron rich acidic groundwater (Yoshida et al., 2018). However, the growth rate and width of the iron rind have not been clarified in this model.
Objectives
The purpose of this study is to generate artificial Fe-oxide concretions and to measure the (1) growth rate (infiltration, precipitation, and dissolution rate) and (2) width of the iron rind.
Based on geological evidence, the formation process of Fe-oxide concretions on Earth can be applied to one on Mars. Thus, this study will provide clues to the ancient environment of Mars.
Methods
We used artificial CaCO3 concretions that made by kneading CaCO3 powder, glass beads with the same particle size as desert sand (150-180 μm), and distilled water. Also, groundwater was simulated with FeCl2 solution (Molarity=1.07 [mol/l]). We fixed an artificial CaCO3 concretion in a Petri dish and simulated the environment where groundwater with iron ion reacts with the CaCO3 concretion. We set a sample on a fixed-point imaging system that we made ourselves.
Results and Discussion
We investigated the relationship between width of rind and time. We analyzed width of a rind on 300 images. The results showed that the solution rapidly penetrated the CaCO3 concretion (about 5.2x10-4[mm/s]) in five minutes, and the width slowly grew (about 3.4x10-7[mm/s])(Fig.1). This indicates that the iron rind grows through the processes of (1) penetration, (2) dissolution, and (3) precipitation. We describe the relationship between natural CaCO3 concretions and artificial CaCO3 concretion. We performed the same experiment with the moisture content of the concretion set to 0[%] in another preliminary experiment. A thicker iron rind was formed than in this experiment. From the above, It is expected that the porosity (in artificial CaCO3 concretions, moisture content) determines the penetration rate and width of rind. Porosity of natural CaCO3 concretions is about 10.2[%]. It is technically difficult to reproduce this porosity in artificial CaCO3 concretions. We will create an artificial CaCO3 concretion with a feasible porosity about 15-25 [%] and calibrate the porosity, penetration velocity and width of rind. We use this as a basis for correcting the results of this experiment and can derive the penetration rate and width of rind for a porosity of 10.2 [%]. This mean that it is possible to estimate the reaction of natural CaCO3 concretions. Also, The existence of a penetration process means that we need to consider a "dissolution from the part where the solution has finished penetration " model, rather than the traditional "dissolution from the surface" model.
Conclusion
We have succeeded in experimentally photographing the growth of the iron rind over time. As a result, we discovered a new process, " penetration " that was not previously hypothesized. We propose to incorporate the effects of penetration into numerical calculations and formation processes. In the future, we will conduct similar experiments by varying the soil concentration and moisture content. In addition, elemental mapping will be created using SXAM.
References
Yoshida et al., (2018) Fe-oxide concretions formed by interacting with carbonate-acidic waters on Earth and Mars. Sci. Adv. 4:eaau0872. doi:10.1126/sciadv.aau0872
The Navajo Sandstone in Utah, U.S.A. and the Gobi Desert in Mongolia have geological formations containing Fe-oxide concretions. They are several mm to cm in size and have spherical shape. The surface is covered with a rind of iron and the interior is filled with sandgrain. An analog of Fe-oxide concretion was discovered on Mars by NASA’s Opportunity rover. Fe-oxide concretions form only under limited conditions. Therefore, identification of the formation process of Fe-oxide concretions on Earth will lead to clarification of the ancient environment of Mars. In 2018, a research group at Nagoya University proposed a new model for the formation of Fe-oxide concretions on the Earth. The hypothesis is that Fe-oxide concretions were originally CaCO3 concretions. They were replaced by iron through processes including (1) dissolution and (2) precipitation due to neutralization reactions with iron rich acidic groundwater (Yoshida et al., 2018). However, the growth rate and width of the iron rind have not been clarified in this model.
Objectives
The purpose of this study is to generate artificial Fe-oxide concretions and to measure the (1) growth rate (infiltration, precipitation, and dissolution rate) and (2) width of the iron rind.
Based on geological evidence, the formation process of Fe-oxide concretions on Earth can be applied to one on Mars. Thus, this study will provide clues to the ancient environment of Mars.
Methods
We used artificial CaCO3 concretions that made by kneading CaCO3 powder, glass beads with the same particle size as desert sand (150-180 μm), and distilled water. Also, groundwater was simulated with FeCl2 solution (Molarity=1.07 [mol/l]). We fixed an artificial CaCO3 concretion in a Petri dish and simulated the environment where groundwater with iron ion reacts with the CaCO3 concretion. We set a sample on a fixed-point imaging system that we made ourselves.
Results and Discussion
We investigated the relationship between width of rind and time. We analyzed width of a rind on 300 images. The results showed that the solution rapidly penetrated the CaCO3 concretion (about 5.2x10-4[mm/s]) in five minutes, and the width slowly grew (about 3.4x10-7[mm/s])(Fig.1). This indicates that the iron rind grows through the processes of (1) penetration, (2) dissolution, and (3) precipitation. We describe the relationship between natural CaCO3 concretions and artificial CaCO3 concretion. We performed the same experiment with the moisture content of the concretion set to 0[%] in another preliminary experiment. A thicker iron rind was formed than in this experiment. From the above, It is expected that the porosity (in artificial CaCO3 concretions, moisture content) determines the penetration rate and width of rind. Porosity of natural CaCO3 concretions is about 10.2[%]. It is technically difficult to reproduce this porosity in artificial CaCO3 concretions. We will create an artificial CaCO3 concretion with a feasible porosity about 15-25 [%] and calibrate the porosity, penetration velocity and width of rind. We use this as a basis for correcting the results of this experiment and can derive the penetration rate and width of rind for a porosity of 10.2 [%]. This mean that it is possible to estimate the reaction of natural CaCO3 concretions. Also, The existence of a penetration process means that we need to consider a "dissolution from the part where the solution has finished penetration " model, rather than the traditional "dissolution from the surface" model.
Conclusion
We have succeeded in experimentally photographing the growth of the iron rind over time. As a result, we discovered a new process, " penetration " that was not previously hypothesized. We propose to incorporate the effects of penetration into numerical calculations and formation processes. In the future, we will conduct similar experiments by varying the soil concentration and moisture content. In addition, elemental mapping will be created using SXAM.
References
Yoshida et al., (2018) Fe-oxide concretions formed by interacting with carbonate-acidic waters on Earth and Mars. Sci. Adv. 4:eaau0872. doi:10.1126/sciadv.aau0872