EFFECT OF REDUCING CONDITIONS ON THE WEATHERING OF Fe3+-RICH BIOTITE IN THE NEW LAHAR DEPOSIT FROM MT. PINATUBO, PHILIPPINES

Abstract

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 K 2 O 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 Na 2 S 2 O 4 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 Fe 3+ because the amount of K + released from two reference biotites having a Fe 3+ /total Fe ratio of < 0.1 was smaller after the Na 2 S 2 O 4 treatment than after the NaCl treatment. Fe 3+ at the edge site of the P-Bt might react with the reducing agent, and released Fe 2+ and other polyvalent cations might replace the interlayer K + because a 1.4-nm reflection peak appeared after the Na 2 S 2 O 4 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 Fe 2+ /total Fe ratio of the P-Bt was 0.47, and its average structural formula was K 0.74 ,Na 0.09 (Si 2.80 ,Al 1.20 )(Al 0.06 ,Fe 3+ 0.50 ,Fe 2+ 0.45 ,Mg 1.48 , Ti 0.21 )O 10 (OH) 2 .