The growth and dissolution of carbonate minerals are believed to significantly impact the oceanic CO 2 cycle. However, to date, no effective method has been developed to assess the CO 2 balance at actual mineral surface reactions. This study confirmed the nonstoichiometric dissolution of calcium carbonate (CaCO 3 ) in an aqueous solution using a newly developed method to simultaneously visualize the local pH and Ca 2+ concentration distributions near the solid-liquid interface. This method used two fluorescent probes, 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) and C 39 H 36 N 5 O 13 (Rhod-5N), as the pH and Ca 2+ concentration indicators, respectively. The fluorescence intensity produced by HPTS was pH-dependent, while that produced by Rhod-5N was influenced by both the Ca 2+ concentration and pH. The temporal variations of these distributions during CaCO 3 (calcite) dissolution in aqueous solutions were visualized and quantified by first determining the pH and then quantifying the Ca 2+ concentration at that pH using pre-prepared calibration curves. The observed pH was significantly lower than that predicted from the observed Ca 2+ concentration changes, indicating that CaCO 3 does not dissolve in a 1:1 stoichiometric ratio of Ca 2+ and CO 3 2- but rather dissolves as HCO 3 - . This suggests that the pH does not increase as much as expected during the rapid dissolution of CaCO 3 , thereby enabling a precise estimation of the impact of the carbonate reaction on the global CO 2 budget. This imaging method enables direct observation of the local behavior of multiple ions, thereby revealing interfacial reaction mechanisms that are not evident from bulk measurements.

Hydrocarbons in organic sediments subducted into the interior of the Earth play a crucial role in the deep carbon cycle. In this study, the thermal reaction of pentacosane (n-C25), a long-chain n-alkane, was experimentally investigated under high-pressure and high-temperature conditions relevant to a subducting slab. Gas chromatography-mass spectrometry (GC/MS) analyses of the reaction products revealed the formation of lighter n-alkanes, indicating that thermal cracking of n-C25 occurred between 360 and 400 degrees C at 0.5 GPa in the presence of SiO2. A comparison of the residual n-C25 under dry and wet conditions suggested that the presence of water enhanced thermal cracking, even at 0.5 GPa. Heavier alkanes and amorphous carbon preferentially formed under dry conditions from addition and dehydrogenation reactions, respectively. By contrast, the IR spectra indicated the formation of oxygenated organic compounds containing -OH and -CO groups under wet conditions, implying that polymerization with oxidation preferentially progresses under wet conditions. These results suggest that the presence of water promotes the formation of diverse organic compounds in subducting slabs, including oxygen-bearing compounds, rather than amorphous carbon or graphite.

The phase changes and reactivity of 1-pentadecene (C15H30) were investigated using Raman spectroscopy under high-pressure and high-temperature conditions using diamond anvil cells. At room temperature, the phase changes from liquid phase to solid phase I, and solid phase I to solid phase II were observed at 0.3 GPa and 4.1 GPa, respectively. Another phase change to form phase III progressed at approximately 10 GPa, and a partial irreversible chemical reaction was observed after decompression from 15.7 GPa. Under high-pressure and high-temperature conditions, the irreversible chemical reaction extensively progressed from the solid phase at 180°C, 1.8 GPa, and at 210 °C, 5.4 GPa. The selective reduction of the Raman peak intensities corresponding to sp2 carbon indicated the progression of an addition reaction similar to that observed in the 1-hexene reaction. A new CC stretching mode appeared, which was attributed to the non-terminal CC bond and was more evident at higher pressures. As the chemical reaction progressed from the liquid phase, a gradual reduction of the CC stretching mode peak was observed upon increasing the temperature at approximately 1.0 GPa, indicating the occurrence of a partial chemical reaction.

The phase changes and chemical reactions of 1-hexene under high-pressure and high-temperature (HPHT) conditions were investigated by using diamond anvil cells and Raman spectroscopy. The slow crystallization of 1-hexene was observed after compression to approximately 3 GPa at room temperature. Solid 1-hexene was dominant up to 20.7 GPa without any phase changes. Under HPHT conditions, the intensities of the Raman peaks related to sp(2) carbon were selectively reduced, and a new C & boxH;C vibration mode appeared, indicating an irreversible reaction. The reaction threshold temperature decreased with an increase in pressure to approximately 100 degrees C at 3 GPa, where the reaction progressed from liquid 1-hexene. In contrast, the reaction threshold temperature increased with increasing pressure above 3 GPa, at which point the reaction progressed from a solid 1-hexene. Oligomers of 1-hexene up to at least 8mers were detected from gas chromatography-mass spectrometry analysis of the reaction products recovered from 120 and 240 degrees C at 1.5 GPa by further experiments using a piston-cylinder-type high-pressure and high-temperature apparatus. The major dimers in the reaction products were linear and branched alkenes bearing C12H24.

Amorphous magnesium carbonate (AMC) is an important phase in the early formation stage of magnesium carbonate. In this study, precipitation experiments were conducted to clarify the formation and transformation process of AMC in aqueous solution. Fine AMC particles precipitated, immediately after mixing of Na2CO3 and MgCl2 solutions. Chemical composition of the AMC was determined to be approximately MgCO3 center dot 2H(2)O although two hydration states were expected to exist for AMCs. Subsequently, the AMC transformed in aqueous solutions into needle-like crystals of nesquehonite (MgCO3 center dot 3H(2)O), eventually to tiny polycrystalline particles of dypingite [Mg-5(CO3)(4)(OH)(2)center dot 5H(2)O] via a solvent-mediated processes.

Monitoring in situ local pH changes to dissolving/growing crystal surfaces is critical to decipher the related mechanisms. To date, however, reliable techniques to acquire such measurements remain under-developed. Here we report the first successful attempt to observe time-resolved, two-dimensional distribution of pH around crystals dissolving in aqueous solutions. The local pH change around dissolving calcium carbonate polymorphs (calcite and aragonite) was visualized by using the fluorescent probe 8-hydroxypyrene-1,3,6-trisul-fonic acid HPTS under an optical microscope. The observations unambiguously showed that pH increased and saturated rapidly immediately adjacent to the reactive surfaces on the cleavage rhombs of calcite. Approximately 1 mm away, however, the increase slowed down significantly, generating a steep pH gradient at the crystal–water interface. Aragonite, which has rough fractured surfaces in contrast to the well cleaved calcite, dissolved much faster and showed a stronger tendency for t...

Pressure-induced irreversible chemical reactions of naphthalene, 2,3-naphthyridine (2,3-Nap), and 1,5-naphthyridine (1,5-Nap) were observed after compression (> 16-18 GPa) at room temperature. Regardless of the initial materials, amorphous products in which sp(3) carbon was formed were obtained after the samples were recovered at ambient pressure. The X-ray photoelectron spectroscopy (XPS) of the product from naphthalene indicated that the carbon sp(3)/sp(2) ratio was much lower than that of benzene nanothreads. Nitrogen remained in the amorphous products from 1,5-Nap and 2,3-Nap and bonded to both sp(3) and sp(2) carbons.

Dehydroxylation of clay minerals within fault gouges is significant for assessing transient thermogenesis due to high‐velocity, frictional slip along fault zones. The clay minerals kaolinite and chlorite are common in fault zones hosted in sedimentary rocks at subduction margins. To better understand the dehydroxylation processes of these clay minerals, high‐temperature X‐ray diffraction analyses were carried out by using a 1:1 mixture of kaolinite and chlorite standard samples. We evaluated the kinetic parameters of each dehydroxylation reaction by thermogravimetric analysis using the Friedman method. For kaolinite, the thermogravimetric data are fitted with a one and a half order equation ( F 3/2 ) with an activation energy of 171 kJ/mol and a frequency factor of 5.6 × 10 8 s −1 . The data for chlorite are analyzed by the geometrical contracting model equation ( R 2 ) with an activation energy of 197 kJ/mol and a frequency factor of 4.5 × 10 9 s −1 . Thermal models of frictional heating employing this calibration show that the frictional heating can explain the reported clay mineralogy in a fossil imbricate thrust from a shallow part in an ancient accretionary prism (Shirako Fault, Japan). This result supports the previous assertion, and the observed temperature anomaly appears to demonstrate the frictional heating caused by coseismic slip on this fault.

At ambient pressure, the hydrogen bond in materials such as ice, hydrates, and hydrous minerals that compose the Earth and icy planets generally takes an asymmetric O-H···O configuration. Pressure significantly affects this configuration, and it is predicted to become symmetric, such that the hydrogen is centered between the two oxygen atoms at high pressure. Changes of physical properties of minerals relevant to this symmetrization have been found; however, the atomic configuration around this symmetrization has remained elusive so far. Here we observed the pressure response of the hydrogen bonds in the aluminous hydrous minerals δ-AlOOH and δ-AlOOD by means of a neutron diffraction experiment. We find that the transition from P21nm to Pnnm at 9.0 GPa, accompanied by a change in the axial ratios of δ-AlOOH, corresponds to the disorder of hydrogen bond between two equivalent sites across the center of the O···O line. Symmetrization of the hydrogen bond is observed at 18.1 GPa, which is considerably higher than the disorder pressure. Moreover, there is a significant isotope effect on hydrogen bond geometry and transition pressure. This study indicates that disorder of the hydrogen bond as a precursor of symmetrization may also play an important role in determining the physical properties of minerals such as bulk modulus and seismic wave velocities in the Earth’s mantle.

In-situ single-crystal and powder X-ray diffraction (XRD) experiments were performed on diaspore at high temperatures. The powder XRD experiments showed that the dehydration reaction from diaspore to corundum occurs between 703 and 733 K. The in-situ single-crystal XRD measurements of diaspore could successfully determine the cell parameters, fractional atomic coordinates and anisotropic displacement parameters at high temperatures, i.e., from 295 to 698 K. Temperature variations in the cell parameters indicate that thermal expansion of the a-axis is a little higher than those of the b-axis and the c-axis. However, the axial thermal expansivity is not as anisotropic as was previously suggested. The results of structure refinements indicate that such lattice expansion behavior is the result of thermal expansion of the tunnels through O2–H···O1 hydrogen-bond separation in the diaspore structure. To the best of our knowledge, this is the first time that the thermal expansion of diaspore has been investigated at an atomic level by in-situ single-crystal XRD experiments at high temperatures.