Organic acids are abundant biogenic organic molecules in the earth's surface environments. To evaluate the effect of organic acids on the precipitation rate and polymorphism of calcium carbonate (CaCO_3) minerals, we conducted precipitation experiments of CaCO_3 minerals by the batch method using 100 ml solutions containing citric acid, malic acid, and acetic acid with Mg^<2+> ions at 25℃ for 10 days. Each solution contained 5.0 mM Ca^<2+> and Mg^<2+> in addition to 20.0 mM HCO_3^- and 0.01, 0.05, 0.1, 0.5, and 1.0 mM organic acid. In addition, adsorption experiments with these organic acids were conducted on the surfaces of calcite and aragonite to reveal the adsorption affinity of organic acids for CaCO_3 minerals. Results confirmed that citric acid significantly decreased the precipitation rate of CaCO_3 minerals and strongly favored the formation of calcite as the predominant polymorph by increasing its concentrations. Moreover, malic acid decreased the precipitation rate and favored the formation of calcite to a lesser extent than with citric acid. Acetic acid showed no significant effect on the precipitation rate and polymorphism of CaCO_3 minerals. Adsorption experiments revealed that adsorption affinity of organic acids was much higher for the surface of aragonite than that for the calcite surface, and showed adsorption amounts in the following order: citric acid > malic acid > acetic acid. Thus, the effect of organic acids on the precipitation rate and polymorphism of CaCO_3 minerals were fundamentally caused by the adsorption of organic acids on the CaCO_3 surface. In this process, higher adsorption affinity of the aragonite surface with organic acids led to a greater adsorption, resulting in greater inhibition of aragonite precipitation and favorable formation of calcite as the dominant polymorph.

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Smectites and related phases were synthesized systematically from quenched glasses with stoichiometric dehydrated Na-smectite compositions in a montmorillonite-stevensite pseudo-binary join under hydrothermal conditions. Experiments were conducted at temperatures from 250 to 500℃ and at 100 MPa, and durations ranged from 1 to 151 days. The existence of a chemical composition gap in the pseudo-binary join between dioctahedral and trioctahedral smectites at 350℃ or lower was confirmed from the time-temperature-transformation relationships obtained. The smectite with intermediate composition was recognized to be metastable; it changed to rectorite through coexisting dioctahedral and trioctahedral smectites. Regularly interstratified chlorite-smectite was then obtained above 500℃. In the montmorillonite composition system, a reaction sequence comparable to that of the natural system was obtained from montmorillonite to rectorite through beidellite plus saponite. Above 500℃, a mica and mica-like phase expanded by ethylene glycol appeared. In the region of trioctahedral composition, unusual high-crystallinity smectite appeared after the low-crystallinity smectite (which is comparable to natural smectite), and then mica was obtained. In the stevensite composition system, stevensite as an initial phase was transformed to a novel regularly interstratified talc-talc-stevensite (r.i.TTS) and then to mica. The structure of r.i.TTS was confirmed by a computer simulation of the XRD line profile.

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Influence on acetaldehyde adsorption ability of talc by mechanochemical treatment was investigated. Talc was subjected to mechanochemical treatment for 5, 10, 20, 30, and 60 min using a planetary ball mill. The treated samples were characterized by X-ray diffraction, N_2-adsorption, and Fourier transform-infrared spectroscopy. The mechanochemical treatment led to a decrease in crystallinity and a significant increase in specific surface area. The specific surface area reached a maximum at 10 min. The number of surface hydroxyl groups increased with increase in treatment time. The mechanochemical treatment of talc resulted in an improvement in acetaldehyde adsorption ability. The highest adsorption ability occurred for the 30 min treated sample.

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