2.4 Other Minerals in Soil
2.4.1 Titanomagnetite and Ilmenite
Iron-titanium (Fe-Ti) oxides are accessary minerals in many soils. The Fe-Ti oxides
include magnetite [Fe 3 O 4 ], titanomagnetite [(1-x)Fe 3 O 4 .xFe 2 TiO 4 ] and ilmenite
[FeTiO 3 ] (Allen and Hajek 1989; Milnes and Fitzpatrick 1989). Chemical compositions of these oxides are conveniently displayed on the TiO 2 -FeO-1/2Fe 2 O 3 ternary
diagram (Butler 1992).
Figure 2.15a is an optical micrograph of magnetite particles collected from the
lahar deposit from Mt. Pinatubo (Fig. 2.7) using a hand magnet, suggesting that these
particles are rich in titanomagnetite. Titanomagnetite has dark color and is not
transparent. Figure 2.15b is the SEM image of the dashed part of Fig. 2.15a, and
the dashed square in Fig. 2.15b yielded the EDX spectrum shown in Fig. 2.15c,
which suggests the elemental composition of ferrian ilmenite (Imai et al. 1996) or
titanohematite (Butler 1992). An EDX spectrum with smaller amount of Ti,
(Fig. 2.15d), was obtained from other part of the same particle lacking a glass
coating. The powder XRD pattern of the crushed magnetic particles is shown as
Fig. 2.14 Quartz from the 0.2–0.5 mm fraction of the 1991 Pinatubo tephra taken from the lahar
deposit (Fig. 2.7). (a) Optical micrograph, (b) SEM image, (c) EDX spectrum acquired from the
dashed area shown in (b), (d) powder XRD pattern of quartz grains, (e) reference powder XRD
pattern (Brindley and Brown 1980)
2.4 Other Minerals in Soil
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