Biotite in the new lahar (volcanic mud flow) deposit from Mt. Pinatubo, Philippines (hereafter referred to as P-Bt) was easily converted to vermiculite during lowland rice-cultivation in pots. The K2O content of the P-Bt gradually decreased, and the peak intensity of the X-ray diffraction (XRD) at 1.4 nm increased during seven cultivations of the paddy rice. The amount of K+ release from the P-Bt with Na2S2O4 treatment was about 1.2 times greater than that released with a NaCl treatment when they were compared at the same pH and Na+ concentration. This result was considered to be a characteristics of biotite containing a large amount of Fe3+ because the amount of K+ released from two reference biotites having a Fe3+/total Fe ratio of < 0.1 was smaller after the Na2S2O4 treatment than after the NaCl treatment. Fe3+ at the edge site of the P-Bt might react with the reducing agent, and released Fe2+ and other polyvalent cations might replace the interlayer K+ because a 1.4-nm reflection peak appeared after the Na2S2O4 treatment. The reducing conditions during lowland rice cultivation could facilitate vermiculitization of the P-Bt and contribute to the K+ supply for the rice plants. The characteristics of the P-Bt were confirmed by XRD, infrared absorption (IR) spectroscopy, and elemental analysis. The trioctahedral type was also confirmed from the (060) reflection peak at 0.153 nm. IR bands of P-Bt were observed in the 400-1400-cm-1 and 3400-3700-cm-1 region. The IR bands of P-Bt from 400 to 1400 cm-1 were very similar to those for the two reference biotites whereas the IR bands in the 3400 to 3700 cm-1 region of the P-Bt differed from those for the reference biotites. The atomic Mg/Fe ratio of the P-Bt was 1.56, which was in the range of biotite. The atomic Fe2+/total Fe ratio of the P-Bt was 0.47, and its average structural formula was K0.74, Na0.09 (Si2.80 Al1.20)(Al0.06, Fe0.50/3+ Fe0.45/2+ Mg1.48, Ti0.21)O10(OH)2..
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