Formation of Allophane and Imogolite in an Andosol Profile as Determined by Solid-state 29Si-NMR.

Allophane and imogolite are meta-stable aluminosilicate clay minerals frequently observed in the weathering process of volcanic ash. The presence of allophane and imogolite has been determined by a selective dissolution technique with acid-oxalate, but it has been pointed out that the acid-oxalate dissolves other soil components and therefore may result error. Solid-state nuclear magnetic resonance (NMR) can detect allophane and imogolite directly and may provide more reliable results. In the present study, the solid-state NMR technique was applied to determine allophane and imogolite in a cumulative Andosol profile in Miyakonojyo basin, Miyazaki, Japan. The soil samples were collected every 5 to 10 cm in the range of 0 to 400 cm of depth, resulting in 69 soil samples. Finely ground soil sample (ca. 200 mg) was put into a high-speed spinning tube (6 mm i.d.), and NMR signals for ²⁷Al and ²⁹Si of the sample were recorded at 78.20 and 59.60 MHz at a spinning rate of 6 to 8 kHz. ²⁷Al-NMR can detect transformation of tetrahedral Al to octahedral Al, which is a good indicator of chemical weathering of volcanic glass. ²⁹Si-NMR can detect presence of allophane and imogolite specifically at -78 ppm (Q³3Al). In the upper soil horizon of 0 to 63 cm depth, which is affected by Taisho tephra (AD1941) and Bunmei tephra (AD1471), no allophane and imogolite was detectable and transformation of tetrahedral Al to octahedral Al was poor. Just under the horizon, which is affected by Miike tephra (4,600 cal BP), presence of allophane and imogolite was clearly detected by ²⁹Si-NMR and transformation of tetrahedral Al to octahedral Al became obvious. Therefore, we concluded that the formation of allophane and imogolite needs at least 500 years and it corresponds with the transformation of tetrahedral Al to octahedral Al in this soil profile.