Effect of reducing conditions on the weathering of Fe3+-rich biotite in the new lahar deposit from Mt. Pinatubo, Philippines

Biotite in the new lahar (volcanic mud flow) deposit from Mt. Pinatubo, Phi lippines (hereafter referred to as P-Bt) was easily converted to vermiculit e during lowland rice-cultivation in Dots. The K2O content of the P-Bt grad ually 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 sa me pH and Na+ concentration. This result was considered to be a characteris tics of biotite containing a large amount of Fe3+ because the amount of Kreleased from two reference biotites having a Fe3+/total Fe ratio of < 0.1 was smaller after the Na2S2O2 treatment than after the NaCl treatment. Fe3 at the edge site of the P-Bt might react with the reducing agent, and rele ased Fe2+ and other polyvalent cations might replace the interlayer K+ beca use a 1.4-nm reflection peak appeared after the Na2S2O2 treatment. The redu cing conditions during lowland rice cultivation could facilitate vermiculit ization 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 LR bands of P-Bt from 400 to 1400 cm(-1) were very similar to those for the two refe rence biotites whereas the IR bands in the 3400 to 3700 cm(-1) region of th e P-Bt differed from those for the reference biotites. The atomic Mg/Fe rat io 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 wa s K-0.74,Na-0.09 (Si-2.80,Al-1.20)(Al-0.06,Fe-0.50(3+),Fe-0.45(2+),Mg-1.48, Ti-0.21)O-10(OH)(2).