5:15 PM - 6:45 PM
[SCG45-P13] Experimental study of sulfide mineral replacement reactions to elucidate the power generation process at submarine hydrothermal vents
Keywords:chimney, sulfide mineral replacement reaction, thermoelectromotive force
At submarine hydrothermal vents, the hydrothermal water mixes with cold seawater resulting in particles of sulfide minerals to form chimney. Sulfide minerals are known as semiconductors and may have thermoelectric performance, but it is not well understood how much thermoelectromotive force is generated by chimneys with complex structures. Recently, our group conducted systematic electrical measurements along the zonal structure of chimneys collected from the Izu-Ogasawara region, and revealed that (1) the outer part of the chimney with high porosity composed of sphalerite and barite has high resistance and does not conduct electricity, whereas (2) thermoelectromotive force (EMF) was found to be generated in the dense regions around the hydrothermal vents, where highly conductive minerals such as pyrite and galena are replacing the pre-existing sphalerite (Takahashi et al., 2023 JpGU). In this study, to understand the effect of mineral replacement on the electrical properties of chimney, we conduct hydrothermal experiments on the replacement of sphalerite, pyrite, and galena by analogue hydrothermal water. Based on the microstructural observations and measurement of resistivity and EMF, the influence of sulfide mineral replacement reactions in chimneys on power generation phenomena on the seafloor will be discussed.
In the experiments, the cube of sphalerite, galena and pyrite was enclosed with aqueous solutions containing Fe2+ and Cu2+ ions (0.03 M) with or without NaCl (1M). Experiments were conducted at 200, 250, and 300degreeC at vapor-saturated pressure for 12 days. In the cases of experiments with sphalerite and galena, Cu-S sulfide film was formed on the surface of the starting minerals, in which the Cu valence changed between Cu2S and CuS. In these cases, the resistivity values decreased significantly by 10 and 4 orders of magnitude. On the other hand, when pyrite was used as the starting material with an aqueous solution containing Cu2+ ions, distinct cracks were formed in the interiors of pyrite crystals and the product minerals systematically changed to Cu2S on the surface and chalcopyrite inside the cracks. The measured resistivity values decreased by four orders of magnitude. Whether the replacement occurs from the surface of the starting material or with precipitation within the cracks were related to the solid volume change of the replacement reaction; the latter was probably formed by the reaction-induced stresses associated with the increase in volume. The change of products into the interior of the pyrite may reflect the chemical potential gradient between Fe and Cu.
Thermoelectromotive force generated when the experimental product was bridged on Peltier devices. The Seebeck coefficient S, which is defined as the electromotive force per temperature difference of sphalerite is very high (96 mV/K) compared to pyrite (0.2 mV/K). The Cu-Fe-S sulfides after the replacement reaction are basically n-type and show small absolute values of -2.5-95 (mV).
The thermoelectric performance of a material is evaluated by its index ZT (= S2 x electrical conductivityσx absolute temperature / thermal conductivityκ). Focusing on the power factor (S2 x σelectrical conductivity), it is found that the performance of the product Fe-Cu sulfide minerals 10~107 times better than that of the sphalerite. Our results suggests that the thermoelectric performance can be greatly improved from the high-porosity sphalerite aggregates in the early stage of chimney to the formation of highly conductive Cu-Fe sulfides around the hydrothermal vents. We plan to proceed with further replacement reaction experiments using young chimneys as starting materials.
In the experiments, the cube of sphalerite, galena and pyrite was enclosed with aqueous solutions containing Fe2+ and Cu2+ ions (0.03 M) with or without NaCl (1M). Experiments were conducted at 200, 250, and 300degreeC at vapor-saturated pressure for 12 days. In the cases of experiments with sphalerite and galena, Cu-S sulfide film was formed on the surface of the starting minerals, in which the Cu valence changed between Cu2S and CuS. In these cases, the resistivity values decreased significantly by 10 and 4 orders of magnitude. On the other hand, when pyrite was used as the starting material with an aqueous solution containing Cu2+ ions, distinct cracks were formed in the interiors of pyrite crystals and the product minerals systematically changed to Cu2S on the surface and chalcopyrite inside the cracks. The measured resistivity values decreased by four orders of magnitude. Whether the replacement occurs from the surface of the starting material or with precipitation within the cracks were related to the solid volume change of the replacement reaction; the latter was probably formed by the reaction-induced stresses associated with the increase in volume. The change of products into the interior of the pyrite may reflect the chemical potential gradient between Fe and Cu.
Thermoelectromotive force generated when the experimental product was bridged on Peltier devices. The Seebeck coefficient S, which is defined as the electromotive force per temperature difference of sphalerite is very high (96 mV/K) compared to pyrite (0.2 mV/K). The Cu-Fe-S sulfides after the replacement reaction are basically n-type and show small absolute values of -2.5-95 (mV).
The thermoelectric performance of a material is evaluated by its index ZT (= S2 x electrical conductivityσx absolute temperature / thermal conductivityκ). Focusing on the power factor (S2 x σelectrical conductivity), it is found that the performance of the product Fe-Cu sulfide minerals 10~107 times better than that of the sphalerite. Our results suggests that the thermoelectric performance can be greatly improved from the high-porosity sphalerite aggregates in the early stage of chimney to the formation of highly conductive Cu-Fe sulfides around the hydrothermal vents. We plan to proceed with further replacement reaction experiments using young chimneys as starting materials.