Pyroxene
Pyroxenes are single chain silicates that are linked laterally by cations such as Mg,
Fe, Ca, and others. Pyroxenes are grouped by their chemical compositions into Mg–
Fe pyroxene, Ca pyroxene, Na pyroxene, and others. Pyroxenes are also grouped
into orthopyroxenes and clinopyroxenes according to their crystal system. As major
pyroxenes, Mg–Fe pyroxene (enstatite–ferrosilite) of the orthopyroxenes and augite
of the clinopyroxenes are introduced. According to its grouping by chemical composition, augite is one of the Ca pyroxenes. Deer et al. (1997b, 2013) described
details of the pyroxenes.
Panels (f), (g), and (h) of Fig. 2.5 show the chemical compositions of the
orthopyroxene members having the maximum, intermediate, and minimum number
of Mg atoms, respectively, from among the 73 examples listed by Deer et al.
(1997b). The vertical axis shows the number of cations per 6 oxygens. The maximum Mg member contains a small number of Fe atoms. The minimum Mg member
contains a large number of Fe atoms and a small number of Ca atoms. The example
shown in Fig. 2.5c corresponds to an orthopyroxene member between that of
Fig. 2.5f and that of Fig. 2.5g. The powder X-ray diffraction pattern of the
orthopyroxene sample is close to the reference, 31–634 (Hypersthene) of Joint
committee on powder diffraction standards (1986).
The orthopyroxene shown in Fig. 2.5c was separated from the 2–0.25 mm
fraction of the Tarumae-a (Ta-a) tephra sampled at Oiwake (Iburi Subprefecture,
Hokkaido, Japan) (Mizuno et al. 2008) near the pedon site shown in Fig. 2.5a. The
Ta-a tephra, erupted in 1739 from Mt. Tarumae, corresponds to the C horizon
labeled as H5–2 of the soil profile shown in Fig. 2.5a. The soil color of the H5–2
horizon is whitish and the soil texture is sand, indicating that weathering is weak.
Figure 2.5b shows not only light brown orthopyroxene but also light green augite,
beige pumice-like volcanic glass, whitish feldspar, and some other grains from the
tephra. Many of the crystalline minerals are partly or almost wholly covered with
colorless volcanic glass.
Figure 2.6a shows augite, one of the clinopyroxenes, separated from the same
fraction of the Ta-a tephra as the above-mentioned orthopyroxene. The color of the
augite is light-green, and the grains are partly covered with colorless volcanic glass.
The EDX spectra-mimic graphs Fig. 2.6d–f show the number of cations per 6 oxygen
atoms of the augite members having the maximum, intermediate, and minimum
number of Mg atoms, respectively, from among the 101 samples shown by Deer
et al. (1997b). With a decrease in the Mg concentration, the concentration of Fe tends
to increase. The number of Ca ions does not change very much compared with those
of Mg and Fe among these members. The EDX spectrum (Fig. 2.6c) is close to
Fig. 2.6e.
2.3 Silicate and Silica Minerals
17
Pyroxenes are single chain silicates that are linked laterally by cations such as Mg,
Fe, Ca, and others. Pyroxenes are grouped by their chemical compositions into Mg–
Fe pyroxene, Ca pyroxene, Na pyroxene, and others. Pyroxenes are also grouped
into orthopyroxenes and clinopyroxenes according to their crystal system. As major
pyroxenes, Mg–Fe pyroxene (enstatite–ferrosilite) of the orthopyroxenes and augite
of the clinopyroxenes are introduced. According to its grouping by chemical composition, augite is one of the Ca pyroxenes. Deer et al. (1997b, 2013) described
details of the pyroxenes.
Panels (f), (g), and (h) of Fig. 2.5 show the chemical compositions of the
orthopyroxene members having the maximum, intermediate, and minimum number
of Mg atoms, respectively, from among the 73 examples listed by Deer et al.
(1997b). The vertical axis shows the number of cations per 6 oxygens. The maximum Mg member contains a small number of Fe atoms. The minimum Mg member
contains a large number of Fe atoms and a small number of Ca atoms. The example
shown in Fig. 2.5c corresponds to an orthopyroxene member between that of
Fig. 2.5f and that of Fig. 2.5g. The powder X-ray diffraction pattern of the
orthopyroxene sample is close to the reference, 31–634 (Hypersthene) of Joint
committee on powder diffraction standards (1986).
The orthopyroxene shown in Fig. 2.5c was separated from the 2–0.25 mm
fraction of the Tarumae-a (Ta-a) tephra sampled at Oiwake (Iburi Subprefecture,
Hokkaido, Japan) (Mizuno et al. 2008) near the pedon site shown in Fig. 2.5a. The
Ta-a tephra, erupted in 1739 from Mt. Tarumae, corresponds to the C horizon
labeled as H5–2 of the soil profile shown in Fig. 2.5a. The soil color of the H5–2
horizon is whitish and the soil texture is sand, indicating that weathering is weak.
Figure 2.5b shows not only light brown orthopyroxene but also light green augite,
beige pumice-like volcanic glass, whitish feldspar, and some other grains from the
tephra. Many of the crystalline minerals are partly or almost wholly covered with
colorless volcanic glass.
Figure 2.6a shows augite, one of the clinopyroxenes, separated from the same
fraction of the Ta-a tephra as the above-mentioned orthopyroxene. The color of the
augite is light-green, and the grains are partly covered with colorless volcanic glass.
The EDX spectra-mimic graphs Fig. 2.6d–f show the number of cations per 6 oxygen
atoms of the augite members having the maximum, intermediate, and minimum
number of Mg atoms, respectively, from among the 101 samples shown by Deer
et al. (1997b). With a decrease in the Mg concentration, the concentration of Fe tends
to increase. The number of Ca ions does not change very much compared with those
of Mg and Fe among these members. The EDX spectrum (Fig. 2.6c) is close to
Fig. 2.6e.
2.3 Silicate and Silica Minerals
17