Liquid limit tests were conducted on Mexico City clay samples prepared at different pore water salinities. The results demonstrate that the liquid limit decreases almost linearly from 340% to 275% as salinity decreases from 54 to 3 g/L, and then exhibits a sharp decrease for salinities below 3 g/L. X-ray analyses indicated that smectite is the main clay mineral in the clay, and a sediment volume test on clay fractions confirmed that the smectite is of a high-swelling type. The contents of Al, Si, and Fe extracted with oxalate were 26.9, 37.8, and 246 g/kg, respectively. Despite the fact that the smectite in the Mexico clay is of a high-swelling type, the change in the liquid limit due to pore water salinity was opposite to that for bentonite and paddy soil composed of high-swelling smectite but was similar to that for marine clays composed of low-swelling clay minerals. This characteristic was ascribed to a dominant role of poorly crystalline aluminosilicates and iron (ferrihydrite) with low-swelling character over the high-swelling smectite in determining the liquid limit of the Mexico clay as the pore water salinity is changed.

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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℃ in air or 〜4℃ 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 BaSO_4 solution at reduced temperature (25℃ to 15℃) using the new AFM method.

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The generation of gold nanoparticles (NPs) on the montmorillonite-type clay surface by a deposition reduction method was examined. The generated gold NPs on the clay surface were observed by TEM. The generated gold particles on the clay surface is turned out to be small, uniform, and no aggregated (average particle size=3.0 nm, σ=0.8 at gold precursor=5.9 wt%) without protective reagents. Even when the amount of gold was increased (10.3 wt%), the uniform dispersion of gold NPs was almost kept. It is turned out that clay minearls is one of the suitable host materials for gold NPs formation.

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