5:15 PM - 7:15 PM
[PPS07-P05] Aqueous alteration of olivine in water with dissolved organic matter simulating the hydrothermal conditions inside carbonaceous meteorite parent bodies
Keywords:olivine, serpentine, pH
Introduction:
Interstellar dust consist of a mineral core composed of silicates, which is surrounded by solid water containing organic matter (Greenberg, 1998). Therefore, during the early stages of solar system formation, solid water containing organic matter accumulated together with minerals in small bodies beyond the snowline. The presence of serpentine produced by the hydrothermal alteration of olivine in carbonaceous chondrites suggests the presence of liquid water in the interior of the small bodies from which they originated. Vinogradoff et al. (2024) conducted hydrothermal experiments with olivine added to formaldehyde solution to simulate conditions in the interior of such small bodies and showed that olivine catalyzes the formose reaction. However, changes in the morphology and chemical composition of olivine have not been investigated. In this study, a hydrothermal experiment simulating the condition in the interior of a small body was carried out using aqueous solutions to investigate the effect of organic matter in the solutions on the initial aqueous alteration stage of olivine.
Method:
A mixture of 37% formaldehyde solution (F), 28% ammonia water (A) and pure water (W) (HCHO:NH3:CH2OH:H2O = 5:1:0.83:100 (molar ratio), denoted FAW) was prepared as an aqueous solution containing organic substances and sealed (10:1 (w/w)) together with San Carlos olivine (45-63 μm, 0.20 g) in PTFE reactors under a nitrogen atomosphere. The reactors were placed in an oven at 130 ℃ for 1, 2, 3, 5, 7, 10, or 30 days. After heating the pH of solutions were mesured. Olivine grains were subjected to XRD analysis to confirm serpentine formation, their surface morphology was observed by SEM and TEM, and chemical composition analysis was performed by EDS.
Results:
pH measurements : The pH of FAW (initial pH8-9) decreased after the initiation of heating and reached approximately pH6 at 3 days. Subsequently, the pH increased to about pH9; however, in some cases, it remained constant at approximately pH6.
XRD: All samples showed an X-ray diffraction pattern of olivine. Peaks of serpentine were not identified.
SEM: The olivine that was heated for longer than 2 days showed etch pits (Pokrovsky et al., 2000) indicating dissolution of olivine surfaces (Fig. 1a). When the pH of the FAW solution after heating changed to acidic, serpentinite precipitation was not observed. Conversely, when the pH after heating was basic, scale-like materials (possibly serpentine) partly covered surface of olivine (Escario et al., 2018)(Fig. 1b).
EDS: No change in chemical composition was observed in any of the samples.
TEM: TEM investigatons revealed flake-like particles (Lafay et al., 2016), which were considered to be the initial stage of serpentinization.
Discussion:
Based on XRD, SEM/EDS, and TEM results, precipitates were present only on the surface of the grains, suggesting that the grains were in the initial stages of serpentinization.
The decrease of pH from 0-3 days suggests that formic acid was formed by the Cannizzaro reaction. The formic acid decreased the pH and accelerated the dissolution of olivine by replacement of Mg2+ on the olivine surface with H+ from the solution. The increase in pH after 3 day is also suggested to be due to the consumption of H+ and formation of OH- by the reaction 2Mg2SiO4 + 4H2O + 4H+ → 4Mg2+ + 2H4SiO4 + 4OH- (Lafay et al., 2014), caused by olivine dissolution. The precipitation of serpentine is caused by the reaction 4Mg2+ + 2H4SiO4 + 4OH- → Mg3Si2O5(OH)4 + Mg(OH)2 + H2O + 4H+ (Lafay et al., 2014).The presence of grains with both etch pits and scale-like material can be explained from these reactions. The maintenance of a pH of approximately 8-9 was probably due to the simultaneous occurrence of both olivine dissolution and serpentine precipitation, but the reactions among organic matter may also have affected the pH. The results above indicate that changes in the pH of the solution attributable to the reaction of formaldehyde affect the dissolution of olivine and the precipitation of serpentine.
Interstellar dust consist of a mineral core composed of silicates, which is surrounded by solid water containing organic matter (Greenberg, 1998). Therefore, during the early stages of solar system formation, solid water containing organic matter accumulated together with minerals in small bodies beyond the snowline. The presence of serpentine produced by the hydrothermal alteration of olivine in carbonaceous chondrites suggests the presence of liquid water in the interior of the small bodies from which they originated. Vinogradoff et al. (2024) conducted hydrothermal experiments with olivine added to formaldehyde solution to simulate conditions in the interior of such small bodies and showed that olivine catalyzes the formose reaction. However, changes in the morphology and chemical composition of olivine have not been investigated. In this study, a hydrothermal experiment simulating the condition in the interior of a small body was carried out using aqueous solutions to investigate the effect of organic matter in the solutions on the initial aqueous alteration stage of olivine.
Method:
A mixture of 37% formaldehyde solution (F), 28% ammonia water (A) and pure water (W) (HCHO:NH3:CH2OH:H2O = 5:1:0.83:100 (molar ratio), denoted FAW) was prepared as an aqueous solution containing organic substances and sealed (10:1 (w/w)) together with San Carlos olivine (45-63 μm, 0.20 g) in PTFE reactors under a nitrogen atomosphere. The reactors were placed in an oven at 130 ℃ for 1, 2, 3, 5, 7, 10, or 30 days. After heating the pH of solutions were mesured. Olivine grains were subjected to XRD analysis to confirm serpentine formation, their surface morphology was observed by SEM and TEM, and chemical composition analysis was performed by EDS.
Results:
pH measurements : The pH of FAW (initial pH8-9) decreased after the initiation of heating and reached approximately pH6 at 3 days. Subsequently, the pH increased to about pH9; however, in some cases, it remained constant at approximately pH6.
XRD: All samples showed an X-ray diffraction pattern of olivine. Peaks of serpentine were not identified.
SEM: The olivine that was heated for longer than 2 days showed etch pits (Pokrovsky et al., 2000) indicating dissolution of olivine surfaces (Fig. 1a). When the pH of the FAW solution after heating changed to acidic, serpentinite precipitation was not observed. Conversely, when the pH after heating was basic, scale-like materials (possibly serpentine) partly covered surface of olivine (Escario et al., 2018)(Fig. 1b).
EDS: No change in chemical composition was observed in any of the samples.
TEM: TEM investigatons revealed flake-like particles (Lafay et al., 2016), which were considered to be the initial stage of serpentinization.
Discussion:
Based on XRD, SEM/EDS, and TEM results, precipitates were present only on the surface of the grains, suggesting that the grains were in the initial stages of serpentinization.
The decrease of pH from 0-3 days suggests that formic acid was formed by the Cannizzaro reaction. The formic acid decreased the pH and accelerated the dissolution of olivine by replacement of Mg2+ on the olivine surface with H+ from the solution. The increase in pH after 3 day is also suggested to be due to the consumption of H+ and formation of OH- by the reaction 2Mg2SiO4 + 4H2O + 4H+ → 4Mg2+ + 2H4SiO4 + 4OH- (Lafay et al., 2014), caused by olivine dissolution. The precipitation of serpentine is caused by the reaction 4Mg2+ + 2H4SiO4 + 4OH- → Mg3Si2O5(OH)4 + Mg(OH)2 + H2O + 4H+ (Lafay et al., 2014).The presence of grains with both etch pits and scale-like material can be explained from these reactions. The maintenance of a pH of approximately 8-9 was probably due to the simultaneous occurrence of both olivine dissolution and serpentine precipitation, but the reactions among organic matter may also have affected the pH. The results above indicate that changes in the pH of the solution attributable to the reaction of formaldehyde affect the dissolution of olivine and the precipitation of serpentine.