Experimental

2.1. Materials

Non-silicate samples (NBS18, IAEA-601, and -602) were used as calibration standards (Table 1). NBS28 and SWy-1, for which δ18O values have already been reported, were used as representative silicate mineral samples. NBS28 is quartz, which is a SiO2 standard distributed by the IAEA. The Wyoming Na-montmorillonite SWy-1 is distributed in the source clay repository of the US Clay Mineral Society. SWy-1 has been widely used in several experiments as a standard clay mineral. It consists of 90–95% montmorillonite and 5–10% quartz. In some experiments (described in the next section) for checking only reactivity rather than δ18O values, quartz powder with a particle size of 4 μm (99.9% purity, Kojundo Chemical Laboratory, Co., Ltd.) was also used instead of NBS28 to avoid consuming NBS28. This quartz sample will be referred as the Qz sample from now on. Furthermore, samples of clay from locations other than SWy-1 were employed. From the Tsukinuno bentonite mine in Yamagata Prefecture , Japan, three montmorillonite samples were acquired 14. They were chosen for this study because of their high montmorillonite content and as an example of natural ores obtained from clay deposits.Table 1. Samples and calibration standards