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Yoshihiro Kuwahara Last modified date:2024.05.08

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Academic Degree
Dr. of Science
Country of degree conferring institution (Overseas)
Field of Specialization
Total Priod of education and research career in the foreign country
Research Interests
  • Dissolution and crystal growth mechanisms of carbonate and sulfate minerals as the index of climate and environmental changes: nanoscale analysis by hot/cool-stage AFM
    keyword : climate change, environmental change, AFM, dissolution and crystal growth mechanisms, carbonate mineral, sulfate mineral
  • In-situ AFM study for smectite dissolution under alkaline conditons
    keyword : smectite, dissolution, kinetics, alkaline solutions, Atomic Force Microscopy, Bentonite, buffer materials
  • Reconstruction of paleoclimatic and paleoenvironmental variations recorded in clay minerals in the Kathmandu Basin sediments
    keyword : clay minerals, crystallinity, paleoclimate, paleoenvironment, the Kathmandu Basin, Hymalaya
  • AFM study on surface structure of micas
    keyword : micas, Atomic Force Microscopy, surface structure, surface relaxation
  • AFM study for the growth mechanism and process of mica clay minerals
    keyword : illite, Atomic Force Microscopy, crystall growth, spiral growth pattern
  • Dissolution process and mechanism of micas under acid conditons
    keyword : micas, acid condition, dissolution, weathering, alteration, kinetics
Current and Past Project
  • Carbonate and sulfate minerals serve as an important indicator of climate and environmental changes because they dissolve and/or precipitate by slight change of temperature even at low temperature. However, the mechanisms are not well understood. In-situ AFM studies on mineral dissolution make it possible to characterize the reactive surfaces and to estimate the essential dissolution rate. Furthermore, AFM with a temperature-controlled fluid-cell system enables in situ observations of mineral dissolution at elevated and lowered temperatures. So far, no in situ AFM study of carbonate and sulfate mineral dissolution at elevated and lowered temperature has been reported. In this study, we attempt an in situ AFM study of carbonate and sulfate mineral dissolution and precipitation at elevated and lowered temperature.
Academic Activities
1. Reconstruction of paleoclimate recorded in clay minerals.
1. Tatsuya Hayashi, Toshiro Yamanaka, Yuki Hikasa, Masahiko Sato, Yoshihiro Kuwahara, Masao Ohno, Latest Pliocene Northern Hemisphere glaciation amplified by intensified Atlantic meridional overturning circulation, Communications Earth & Environment,, 1, 25, 1-10, 2020.09, The global climate has been dominated by glacial–interglacial variations since the intensification of Northern Hemisphere glaciation 2.7 million years ago. Although the Atlantic meridional overturning circulation has exerted strong influence on recent climatic changes, there is controversy over its influence on Northern Hemisphere glaciation because its deep limb, North Atlantic Deep Water, was thought to have weakened. Here we show that Northern Hemisphere glaciation was amplified by the intensified Atlantic meridional overturning circulation, based on multi-proxy records from the subpolar North Atlantic. We found that the Iceland–Scotland Overflow Water, contributing North Atlantic Deep Water, significantly
increased after 2.7 million years ago and was actively maintained even in early stages of
individual glacials, in contrast with late stages when it drastically decreased because of
iceberg melting. Probably, the active Nordic Seas overturning during the early stages of
glacials facilitated the efficient growth of ice sheets and amplified glacial oscillations..
2. Y. Kuwahara, W. Liu, M. Makio, K. Otsuka, In Situ AFM Study of Crystal Growth on a Barite (001) Surface in BaSO4 Solutions at 30°C, Minerals, 10.3390/min6040117, Special Issue Nucleation of Minerals: Precursors, Intermediates and Their Use in Materials Chemistry), 117, 1-18, 2016.11, The growth behavior and kinetics of the barite (001) surface in supersaturated BaSO4 solutions (supersaturation index (SI) = 1.1–4.1) at 30°C were investigated using in situ atomic force microscopy (AFM). At the lowest supersaturation, the growth behavior was mainly the advancement of the initial step edges and filling in of the etch pits formed in the water before the BaSO4 solution was injected. For solutions with higher supersaturation, the growth behavior was characterized by the advance of the and [010] half-layer steps with two different advance rates and the formation of growth spirals with a rhombic to bow-shaped form and sector-shaped two-dimensional (2D) nuclei. The advance rates of the initial steps and the two steps of 2D nuclei were proportional to the SI. In contrast, the advance rates of the parallel steps with extremely short step spacing on growth spirals were proportional to SI2, indicating that the lateral growth rates of growth spirals were directly proportional to the step separations. This dependence of the advance rate of every step on the growth spirals on the step separations predicts that the growth rates along the [001] direction of the growth spirals were proportional to SI2 for lower supersaturations and to SI for higher supersaturations. The nucleation and growth rates of the 2D nuclei increased sharply for higher supersaturations using exponential functions. Using these kinetic equations, we predicted a critical supersaturation (SI =4.3) at which the main growth mechanism of the (001) face would change from a spiral growth to a 2D nucleation growth mechanism: therefore, the morphology of bulk crystals would change..
3. Y. Kuwahara, M. Makio, In situ AFM study on barite (001) surface dissolution in NaCl solutions at 30 C, Applied Geochemistry,, 51, 12, 246-254, 2014.12, This paper reports in situ observations on barite (001) surface dissolution behavior in 0.1–0.001 M NaCl solutions at 30°C using atomic force microscopy (AFM). The step retreating on barite (001) surfaces changed with increasing NaCl solution concentrations. In solutions with a higher NaCl concentration (>0.01 M), many steps showed curved or irregular fronts during the later experimental stage, while almost all steps in solutions with a lower NaCl concentration exhibited straight or angular fronts, even during the late stage. The splitting phenomenon of the initial hhk0i one-layer steps (7.2 Å) into two half-layer steps (3.6 Å) occurred in all NaCl solutions, while that of the initial [010] one-layer steps observed only in the 0.1 M NaCl solution. The step retreat rates increased with an increasing NaCl solution concentration. We observed triangular etch pit and deep etch pit formation in all NaCl solutions, which tended to form late in solutions with lower NaCl concentrations. The deep etch pit morphology changed with increasing NaCl solution concentrations. A hexagonal form elongated in the [010] direction was bounded by the {100}, {310}, and (001) faces in a 0.001 M NaCl solution, and a rhombic form was bounded by the {510} and (001) faces in 0.01 M and 0.1 M NaCl solutions. An intermediate form was observed in a 0.005 M NaCl solution, which was defined by {100}, a curved face tangent to the [010] direction, {310}, and (001) faces: the intermediate form appeared between the hexagonal and rhombic forms in solutions with lower and higher NaCl concentrations, respectively. The triangular etch pit and deep etch pit growth rates also increased with the NaCl solution concentration. Combining the step and face retreat rates in NaCl solutions estimated in this AFM study as well as the data on the effect of water temperature on the retreat rates reported in our earlier study, we produced two new findings. One finding is that the retreat rates increase by approximately two-fold when the NaCl solution concentration increases by one order of magnitude, and the other finding is that the retreat rate increase due to a one order of magnitude increase in the NaCl concentration corresponds to an increase of approximately 8°C in water temperature. This correlation may help to understand and evaluate increasing dissolution kinetics induced by the different mechanisms where barite dissolution is promoted by the catalytic effect of Na+ and Cl ions (through an increase in the NaCl solution concentration) or by an increase in the hydration of Ba2+ and SO4 2- (through an increase in water temperature)..
4. Y. Kuwahara, K. Ishida, S. Uehara, I. Kita, Y. Nakamuta, T. Hayashi, and R. Fujii, Cool-stage AFM, a new AFM method for in situ observations of mineral growth and dissolution at reduced temperature: Investigation of the responsiveness and accuracy of the cooling system and a preliminary experiment on barite growth, Clay Science, 16, 4, 111-119, 2012.12, Using in situ cool-stage atomic force microscopy (AFM), we can observe sample surfaces in air or fluid directly at the site or step level at low or reduced temperatures (~ -35°C in air or ~ 4°C in fluid). The main components of the new AFM system include a heater/cooler element (cool-stage), specialized heater/cooler piezoelectric scanner with a heat exchanger, Thermal Applications Controller, AFM fluid cell, and fluid cooling system to cool the piezo scanner. We investigate the responsiveness and accuracy of the temperature control in the cooling AFM system by varying the setpoint temperature of the cool-stage and flow rate of the water flowing through the AFM fluid cell and thereby obtain suitable experimental conditions for in situ cool-stage AFM observations of mineral growth and dissolution in aqueous solutions at low or reduced temperatures. We also report the results of a preliminary experiment on in situ observations of the growth behavior on the barite (001) surface in a supersaturated BaSO4 solution at reduced temperature (25°C to 15°C) using the new AFM method..
5. Yoshihiro Kuwahara, In situ hot-stage AFM study of the dissolution of the barite (001) surface in water at 30–55 °C, American Mineralogist, doi:10.2138/am.2012.4130, 97, 10, 1564-1573, 2012.10, This paper reports in situ observations of the dissolution behavior of the barite (001) surface in pure water at 30–55 °C using hot-stage atomic force microscopy (AFM). The dissolution at 30 and 40 °C occurred in three stages; however, at 55 °C, the dissolution behavior observed at the former temperatures started immediately after injecting water into the AFM fluid cell. The first stage of the dissolution was characterized by the retreat of the initial steps and continued for about 60 min at 30 °C and about 10 min at 40 °C. The second stage of the dissolution was characterized by the splitting of the initial one-layer step into two half-layer steps [fast (“f”) and slow (“s”) retreat steps] with different retreat rates and by the formation of etch pits. The large difference in the retreat rate of the “f” and “s” steps led to the formation of a new one-layer step, which showed slightly faster retreat rates than the “s” half-layer step at all temperatures. The splitting of the [010] one-layer step into two half-layer steps was observed only at 55 °C. During the third stage, the development of angular deep etch pits from an initial form with a curved outline differed at each temperature. The deep etch pits grew rapidly at higher temperature, but showed at least two different retreat rates for the (001) plane at each temperature, indicating the development of the pits along different dislocations (screw and edge dislocations).
The activation energies (62–74 kJ/mol) for the step and face retreats in this study were significantly higher than those reported in earlier studies. Recalculations performed using only data obtained under similar conditions in previous studies led to activation energies of 66–79 kJ/mol. These results and the earlier report showing that the form of the deep etch pits changed from angular to bow-shaped at about 60 °C may indicate that the activation energy of barite dissolution in water is higher at lower temperatures as compared with higher temperatures, thus changing the rate-limiting step. Whether the vertical and lateral retreat rates of the barite (001) plane differ in dependence of temperature remains unclear; however, the activation energies of the retreat of the (001) face in deep etch pits tended to be slightly higher than that of the lateral retreat rates of steps or other faces in deep etch pits..
6. Yoshihiro Kuwahara, In situ Atomic Force Microscopy study of dissolution of the barite (001) surface in water at 30°C, Geochimica et Cosmochimica Acta, 10.1016/j.gca.2010.10.003, 75, 41-51, 2011.01.
7. Kuwahara, Y., Masudome, Y., Paudel, M.R., Fujii, R., Hayashi, T., Mampuku, M., Sakai, H., Controlling weathering and erosion intensity on the southern slope of the Central Himalaya by the Indian summer monsoon during the last glacial, Global and Planetary Change, 71, 1-2, 73-84, Vol. 71, 73-84., 2010.03.
8. Yoshihiro Kuwahara and Seiichiro Uehara, AFM Study on Surface Microtopography, Morphology and Crystal Growth of Hydrothermal Illite in Izumiyama Pottery Stone from Arita, Saga Prefecture, Japan, The Open Mineralogy Journal, Vol. 2, 34-47., 2008.12.
9. Yoshihiro Kuwahara, In situ observations of muscovite dissolution under alkaline conditions at 25-50°C by AFM with an air/fluid heater system, American Mineralogist, Vol.93, 1028-1033, 2008.08.
10. Yoshihiro Kuwahara, In-situ AFM study of smectite dissolution under alkaline conditions at room temperature, American Mineralogist, Vol. 91, 1142-1149., 2006.07.
11. Yoshihiro Kuwahara, In-situ,real time AFM study of smectite dissolution under high pH condtions at 25°-50°C., Clay Science, Vol. 12, Supplement 2, 57-62., 2006.04.
12. Yoshihiro KUWAHARA, Seiichiro UEHARA, and Yoshikazu AOKI, Atomic force microscopy study of hydrothermal illite in Izumiyama pottery stone from Arita, Saga prefecture, Japan, Clays and Clay Minerals, 10.1346/CCMN.2001.0490404, 49, 4, 300-309, Vol.49, 300-309, 2001.04.
13. Yoshihiro KUWAHARA, Comparison of the surface structure of the tetrahedral sheets of muscovite and phlogopite by AFM, Physics and Chemistry of Minerals, 10.1007/s002690000126, 28, 1, 1-8, Vol.28, 1-8, 2001.01.
14. Yoshihiro KUWAHARA and Yoshikazu AOKI, Dissolution kinetics of phlogopite under acid conditions, Clay Science, Vol.11, 31-45, 1999.01.
15. Yoshihiro KUWAHARA and Yoshikazu AOKI, Dissolution process of phlogopite in acid solutions, Clays and Clay Minerals, 10.1346/CCMN.1995.0430105, 43, 1, 39-50, Vol.43, 39-50, 1995.01.
1. This paper reports the results of clay mineral analysis (the amount of clay fraction, clay mineral assemblages, illite crystallinity) of samples collected from a drilled core (Rabibhawan (RB) core) located in the westcentral part of the Kathmandu Basin on the southern slope of the Central Himalaya. The amount of clay fraction in the core sediments between 12 m and 45 m depth (corresponding to ca. 17–76 ka), which belong to the Kalimati Formation, is variable and shows three clay-poor zones (19–31 ka, 44–51 ka, and 66–75 ka). The variations correspond with those of illite crystallinity index (Lanson index (LI) and modified Lanson index (MLI)) and kaolinite/illite ratio as well as the fossil pollen and diatom records reported by previous workers. These data reveal the following transformations occurring during the weathering process in this area:

micas (mainly muscovite)→illite(→illite−smectitemixedlayer mineral(R = 1))→kaolinite

The sedimentation rate (~50 cm/kyr) of clay-poor zones that correspond to dry climate intervals is only half that of clay-rich zones (~120 cm/kyr) that correspond to wet climate intervals, indicating weakened chemical weathering and erosion and low suspended discharge during dry climate intervals. The clay-poor zones commonly show unique laminite beds with very fine, authigenic calcite, which was probably precipitated under calm and high calcite concentration conditions caused by low precipitation and run-off. The variations between dry and wet conditions in this area as deduced from clay minerals appear to follow the Indian Summer Monsoon Index (ISMI) (30°N–30°S, 1 July) and northern hemisphere summer insolation (NHSI) signals (30°N) at 1 July, especially during the dry climate zones, whereas the wet maxima of the wet climate zones somewhat deviate from the strongest NHSI. On the other hand, the dry–wet records lead markedly the SPECMAP stack (by about 5000 years). These results suggest that the Indian summer monsoon precipitation was strongly controlled by the NHSI or summer insolation difference between the Himalayan–Tibetan Plateau and the subtropical Indian Ocean, showing a major fluctuation on the 23,000 years precessional cycle, and that it was not driven by changes in high-latitude ice volume, although the records of clay mineral indices during the wet intervals leave a question that other factors, in addition to insolation forcing, may play important roles in weathering, erosion, and sedimentation processes..
2. The dissolution behavior of the barite (001) surface in pure water at 30°C was investigated using in situ AFM, to better understand the dissolution mechanism and the microtopographical changes that occur during the dissolution. The dissolution of the barite (001) surface could be divided into three stages. The first stage of the dissolution was characterized only by the retreat of various steps that were formed mechanically by the initial cleaving. During the second stage, the change in the retreat behavior (from a step with one layer to a “f” step with an upper half-unit cell layer and a “s” step with a lower half-unit cell layer) and the formation of etch pits were observed. The last stage of the dissolution was characterized by an increase of the stable steps parallel to the [010] direction, followed by a decrease of steps parallel to , and the development of angular deep etch pits..
3. In-situ AFM observations on muscovite dissolution at 20-50C under alkaline conditions.
4. Mineral behavior in Paleoclimate and paleoenvironment changes in the Kathmandu Basin sediments, Central Nepal Himaraya during the last 50kyr.
5. In situ AFM study on smectite and muscovite dissolutions under high pH conditions.
6. The dissolution behaviors of muscovite and smectite under alkaline conditions at 25°C to 50°C were investigated using in-situ atomic force microscopy (AFM) with an air/fluid heater system, in order to understand the dissolution mechanism and to derive reliable dissolution rates of these minerals.The both minerals showed the same dissolution behavior in which the reactive surface was only the edge surfaces and any new etch pits were not formed on the basal surfaces within the experimental durations. The dissolution rates normalized to the edge surface area (ESA) of these minerals were also approximately the same. In addition, the effects of temperature on the ESA-normalized dissolution rates were very similar each other. These results suggest that the dissolution mechanisms (e.g., reactive surface, the rate-limiting step) of muscovite and smectite are identical each other. The dissolution rates normalized to the total surface area (TSA) of initial particle size (smectite) or initial etch pit size (muscovite) varied with the particle or etch pit size, respectively..
7. Variations of paleoclimate and paleoenvironment during the last 50 kyr in the Kathmandu Valley, Central Nepal, Himalayas: From the results of clay mineral study.
Membership in Academic Society
  • Mineralogical Society of America
Other Educational Activities
  • 2024.01.
  • 2023.04.
  • 2018.07.
  • 2019.07.
  • 2018.08.
  • 2018.02.