The EDX spectrum-mimic graphs (Fig. 2.13d) show the number of cations per
32 oxygens. Four representatives were chosen from among the 7 anorthites, 10 labradorites, 10 albites, and 62 K-feldspars (sanidine, orthoclase, microcline, amazonite, etc.) listed by Deer et al. (2001). Referring to Fig. 2.13d, the EDX spectrum
shown in Fig. 2.13c is close to that of labradorite, one of the plagioclase feldspars.
Plagioclase also exists as high- and low-temperature phases. The high- and
low-temperature types of albite and oligoclase can be identified from a powder
XRD pattern, but it gradually becomes difficult with a further increase in the ratio
of anorthite (Huang 1989). The powder XRD pattern (Fig. 2.13e) appears closer to,
although not completely the same as, the pattern of natural labradorite listed as
No. 6 by Goodyear and Duffin (1955), low-temperature labradorite, than to the
synthetic one, high-temperature type.
2.3.2.2 Silica Minerals
Among the various silica minerals in soils are quartz and cristobalite, which are
grouped as tectosilicates (Table 2.1) (Drees et al. 1989; Deer et al. 2004). Mizota and
Aomine (1975) reported cristobalite in the clay fraction of volcanic ash from
Hokkaido, Japan.
Quartz, in particular, is found in the silt and sand fractions of many soils, although
clay-sized quartz does exist in some soils affected by airborne dust. Quartz grains are
colorless and transparent in many cases. There are two structure types, high- and
low-temperature types, distinguishable by their XRD patterns (Brindley and Brown
1980), and the one in soils is the low-temperature type. Cracks sometimes can be
found in quartz grains, possibly due to shrinkage as they cooled and converted from
the high-temperature type to the low-temperature type. Conchoidal fractures are also
a characteristic of quartz grains. Surface etchings due to partial dissolution are very
slight.
Figure 2.14a (optical micrograph) and (b) (SEM image) show a colorless and
transparent quartz grain separated from a volcanic ash deposit (1991) remobilized in
a lahar from Mt. Pinatubo (Fig. 2.7). A conchoidal fracture can be observed on the
left side (Fig. 2.14b). As is often the case of minerals in volcanic ash, the quartz grain
is partly covered by volcanic glass. The EDX spectrum of the exposed quartz surface
shows only Si (Fig. 2.14c). The powder XRD pattern (Fig. 2.14d) of a hand-picked
quartz grain from the volcanic ash is identical to that of the low-temperature type of
quartz (Fig. 2.14e) (Brindley and Brown 1980), although a very small reflection peak
at about 2θ ¼ 28 degrees, probably from plagioclase, is included in Fig. 2.14d.
All the quartz in soil is the low-temperature phase because inter-conversion
between high- and low-temperature quartz occurs rapidly. The powder XRD pattern
for high-temperature quartz is different from that of low-temperature quartz.
Further readings for silica minerals are Drees et al. (1989), Monger and Kelly
(2002) and Deer et al. (2004).
26
2 Primary Minerals
32 oxygens. Four representatives were chosen from among the 7 anorthites, 10 labradorites, 10 albites, and 62 K-feldspars (sanidine, orthoclase, microcline, amazonite, etc.) listed by Deer et al. (2001). Referring to Fig. 2.13d, the EDX spectrum
shown in Fig. 2.13c is close to that of labradorite, one of the plagioclase feldspars.
Plagioclase also exists as high- and low-temperature phases. The high- and
low-temperature types of albite and oligoclase can be identified from a powder
XRD pattern, but it gradually becomes difficult with a further increase in the ratio
of anorthite (Huang 1989). The powder XRD pattern (Fig. 2.13e) appears closer to,
although not completely the same as, the pattern of natural labradorite listed as
No. 6 by Goodyear and Duffin (1955), low-temperature labradorite, than to the
synthetic one, high-temperature type.
2.3.2.2 Silica Minerals
Among the various silica minerals in soils are quartz and cristobalite, which are
grouped as tectosilicates (Table 2.1) (Drees et al. 1989; Deer et al. 2004). Mizota and
Aomine (1975) reported cristobalite in the clay fraction of volcanic ash from
Hokkaido, Japan.
Quartz, in particular, is found in the silt and sand fractions of many soils, although
clay-sized quartz does exist in some soils affected by airborne dust. Quartz grains are
colorless and transparent in many cases. There are two structure types, high- and
low-temperature types, distinguishable by their XRD patterns (Brindley and Brown
1980), and the one in soils is the low-temperature type. Cracks sometimes can be
found in quartz grains, possibly due to shrinkage as they cooled and converted from
the high-temperature type to the low-temperature type. Conchoidal fractures are also
a characteristic of quartz grains. Surface etchings due to partial dissolution are very
slight.
Figure 2.14a (optical micrograph) and (b) (SEM image) show a colorless and
transparent quartz grain separated from a volcanic ash deposit (1991) remobilized in
a lahar from Mt. Pinatubo (Fig. 2.7). A conchoidal fracture can be observed on the
left side (Fig. 2.14b). As is often the case of minerals in volcanic ash, the quartz grain
is partly covered by volcanic glass. The EDX spectrum of the exposed quartz surface
shows only Si (Fig. 2.14c). The powder XRD pattern (Fig. 2.14d) of a hand-picked
quartz grain from the volcanic ash is identical to that of the low-temperature type of
quartz (Fig. 2.14e) (Brindley and Brown 1980), although a very small reflection peak
at about 2θ ¼ 28 degrees, probably from plagioclase, is included in Fig. 2.14d.
All the quartz in soil is the low-temperature phase because inter-conversion
between high- and low-temperature quartz occurs rapidly. The powder XRD pattern
for high-temperature quartz is different from that of low-temperature quartz.
Further readings for silica minerals are Drees et al. (1989), Monger and Kelly
(2002) and Deer et al. (2004).
26
2 Primary Minerals