soil water. The formation of smectite from volcanic glass is also reported under
hydrothermal conditions in the laboratory (Tomita et al. 1993; Cuadros et al. 1999).
Although humus-rich horizons are generally dark-colored, highly black colored
A horizons are often found beneath grass vegetation. The grass vegetation was
maintained even under humid climates through burning by ancient people. Under
forest vegetation, the color of A horizons is dark brown. The difference in color
between these two types of A horizons is due to the differences in their humic acid
types. The humic acid, under grass vegetation is A-type, which is highly humified
and rich in aromatic groups whereas that under forest vegetation is either P-type or
B-type. The A-type humic acid is separated from other types of humic acid by
having a melanic index lower than 1.7, whereas the other types of humic acid have
melanic indices of 1.7 or higher (Shoji et al. 1993).
Andisols are characterized by high active Al and Fe contents. The active Al
materials are allophane, imogolite, and Al-humus, and the active Fe material is
ferrihydrite. These materials are mainly formed from volcanic glass. The typical
Si:Al atomic ratios of non-colored and colored volcanic glasses are 5.0 and 2.4,
respectively. These values are significantly higher than those for soil formation
products such as kaolin minerals (often halloysite), allophane, imogolite, and
Al-humus, as shown in Table 4.2. Morphological changes of volcanic glasses
accompanying these changes in elemental composition were examined using
polished sections of new pumice particles, partially weathered pumice particles
and soil clods from Udands.
4.4.1 Fresh Pumice Particle
A sample of fresh pumice was obtained from the 1991 Mt. Pinatubo tephra.
Figure 4.15a shows an optical micrograph of the polished section. The white part
is sponge-like volcanic glass with inclusion of feldspar, quartz, and other particles.
Figure 4.15b shows an EDX spectrum of a glassy area of the particle shown by a
dashed square (b) in Fig. 4.15a, and it is close to the typical non-colored volcanic
glass (Fig. 4.2c). Magnifying the rim of the pumice, fine particles of 10 μm or less are
found in the open cavities, and these are also non-colored volcanic glasses as shown
by the similar EDX spectrum (Fig. 4.15e) to that for Fig. 4.15b. Figure 4.15e may
correspond to the start of the micromass coating formation (Stoops 2007). Highly
vesicular characteristics similar to Fig. 4.2b are seen inside the pumice.
Figure 4.15f, g are the element maps of Al and Si, respectively. The cyan color
used to indicate Si displays its second highest intensity for the major and glassy part
of the pumice particle. The highest color intensity of Si may indicate that some
quartz was included in the pumice. The magenta color chosen for Al is strongest for
feldspar particles, and the intensity of the magenta color for the volcanic glass is
lower than the cyan color intensity as suggested from the EDX spectra shown in
Fig. 4.15b, e.
76
4 Non-crystalline Inorganic Constituents of Soil
hydrothermal conditions in the laboratory (Tomita et al. 1993; Cuadros et al. 1999).
Although humus-rich horizons are generally dark-colored, highly black colored
A horizons are often found beneath grass vegetation. The grass vegetation was
maintained even under humid climates through burning by ancient people. Under
forest vegetation, the color of A horizons is dark brown. The difference in color
between these two types of A horizons is due to the differences in their humic acid
types. The humic acid, under grass vegetation is A-type, which is highly humified
and rich in aromatic groups whereas that under forest vegetation is either P-type or
B-type. The A-type humic acid is separated from other types of humic acid by
having a melanic index lower than 1.7, whereas the other types of humic acid have
melanic indices of 1.7 or higher (Shoji et al. 1993).
Andisols are characterized by high active Al and Fe contents. The active Al
materials are allophane, imogolite, and Al-humus, and the active Fe material is
ferrihydrite. These materials are mainly formed from volcanic glass. The typical
Si:Al atomic ratios of non-colored and colored volcanic glasses are 5.0 and 2.4,
respectively. These values are significantly higher than those for soil formation
products such as kaolin minerals (often halloysite), allophane, imogolite, and
Al-humus, as shown in Table 4.2. Morphological changes of volcanic glasses
accompanying these changes in elemental composition were examined using
polished sections of new pumice particles, partially weathered pumice particles
and soil clods from Udands.
4.4.1 Fresh Pumice Particle
A sample of fresh pumice was obtained from the 1991 Mt. Pinatubo tephra.
Figure 4.15a shows an optical micrograph of the polished section. The white part
is sponge-like volcanic glass with inclusion of feldspar, quartz, and other particles.
Figure 4.15b shows an EDX spectrum of a glassy area of the particle shown by a
dashed square (b) in Fig. 4.15a, and it is close to the typical non-colored volcanic
glass (Fig. 4.2c). Magnifying the rim of the pumice, fine particles of 10 μm or less are
found in the open cavities, and these are also non-colored volcanic glasses as shown
by the similar EDX spectrum (Fig. 4.15e) to that for Fig. 4.15b. Figure 4.15e may
correspond to the start of the micromass coating formation (Stoops 2007). Highly
vesicular characteristics similar to Fig. 4.2b are seen inside the pumice.
Figure 4.15f, g are the element maps of Al and Si, respectively. The cyan color
used to indicate Si displays its second highest intensity for the major and glassy part
of the pumice particle. The highest color intensity of Si may indicate that some
quartz was included in the pumice. The magenta color chosen for Al is strongest for
feldspar particles, and the intensity of the magenta color for the volcanic glass is
lower than the cyan color intensity as suggested from the EDX spectra shown in
Fig. 4.15b, e.
76
4 Non-crystalline Inorganic Constituents of Soil