1. Formation processes of paper-thin raft and coated bubble: Calcium carbonate deposition at gas–water interface.
- Author
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Shiraishi, Fumito, Akimoto, Takayuki, Tomioka, Naotaka, Motai, Satoko, and Takahashi, Yoshio
- Subjects
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CALCIUM carbonate , *HOT springs , *WATER springs , *RAFTS , *HOT water , *CRYSTAL growth , *DISSOLVED air flotation (Water purification) - Abstract
Travertines are hot spring carbonate deposits that exhibit characteristic fabrics, including a paper-thin raft and a coated bubble formed by the calcification of water and bubble surfaces, respectively. A previous study interpreted that compared with the water surface, the bubble surface displays more active CO 2 degassing and resultant active CaCO 3 precipitation. However, considering the CO 2 partial pressure and the volumes of atmosphere and bubbles, it is possible that more active CO 2 degassing occurs on the water surface. In addition, the surfaces of water and bubbles at the travertine-depositing sites are negatively charged, but it is still unclear whether the CaCO 3 nucleation on these surfaces occurs via amorphous precursors, like the CaCO 3 nucleation on negatively charged organic matter. This study provides a solution to these uncertainties by examining the aragonitic travertines formed in the Nagayu area of Japan. Through field observations, two types of paper-thin rafts were recognized: one with a smooth surface and formed in approximately 1 h, and the other with a rough surface and formed in approximately 3 h. In addition, the bubbles generated on the microbial mat during the daytime were covered with white minerals within an hour, and with ivory-colored minerals over 8 h after sunset, forming firm coated bubbles. Microelectrode measurements revealed that the active CO 2 degassing on the water surface significantly increased the CaCO 3 saturation state to cause active CaCO 3 precipitation. In contrast, less active CO 2 degassing on the bubble surface did not increase the saturation state, and moderate CaCO 3 precipitation occurred due to the moderately high saturation state of the hot spring water. Various microscopic observations revealed that the smooth-surfaced paper-thin raft comprises a micritic layer of approximately 10–20 μm, which represents tightly arranged submicron-sized granular aragonite. At the lower surface of this layer, hemispherical aragonite partially grew toward the hot spring water. In addition, the rough-surfaced paper-thin raft and coated bubble comprise bundled and acicular aragonites arranged along the surfaces of water and bubble, from which hemispherical aragonite grew toward the hot spring water. Neither the paper-thin raft nor the coated bubble contains primary amorphous CaCO 3. These results suggest that the smooth-surfaced paper-thin raft is formed by active CaCO 3 nucleation on the water surface and subsequent crystal growth, and that the water surface is unfavorable for preserving the amorphous precursors potentially involved in the nucleation. The rough-surfaced paper-thin raft and the coated bubble could have been formed by the attachment of externally formed acicular aragonite to the water/bubble surface and subsequent crystal growth. The revealed formation processes of the paper-thin raft and coated bubble provide valuable information for interpreting their formation mechanism in other travertine deposits, including those in the geological past. [ABSTRACT FROM AUTHOR]
- Published
- 2023
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