Astron. Astrophys. 363, 1115-1122 (2000)

3. Results

3.1. Crystalline pyroxenes

The spectra of the pyroxenes are shown in Fig. 1. The absorption spectra of the present samples vary considerable and differ from those of the two natural pyroxenes from Ichinome-gata previously measured (Koike et al. 1993). Based on the chemical compositions of pyroxenes, we divided these spectra into three groups, namely pure enstatite (synthetic orthoenstatite and synthetic clinoenstatite; the Mg end member of pyroxene), orthopyroxene (orthopyroxene from Bambel and from Ichinome-gata; Mg-rich but contains small amounts of Fe, Al and Ca) and Ca-rich pyroxene (synthetic-diopside and natural-augite from Ichinome-gata; Ca0.9-1.0 and Mg0.9-1.0 with O = 6). In the mid infrared region, the peak positions are blue shifted for about 0.1  compared to the data measured by the previous low resolution spectrometer Shimadzu IR-27G (Koike & Shibai 1998). Although the peak wavelengths of the corresponding features are different, the spectrum of each group has similar features. In the pure enstatite group, the spectra are not so much dependent on the type of crystal structures (ortho or clino) in the mid infrared region, but the peaks of the clinoenstatite in the mid infrared region become about 30% stronger than those of the orthoenstatite. In the far infrared region the peaks become different. In the case of Ca-rich pyroxenes, the three strong peaks in the 10  region are very similar for the synthetic and the natural samples. Diopside has a strong and broad peak at about 66 . This peak is about 50% stronger than the peak of natural augite. Natural augite has a degenerate band at 33  and only a very weak peak at 45  compared to diopside. As for the case of the natural orthopyroxenes, the two spectra are similar. However, the two peaks at 70  are very strong in the Norway sample compared to the very weak band for the Ichinome-gata sample.

Fig. 1a-d. The mass extinction coefficients of present crystalline pyroxenes; a orthoenstatite, b orthopyroxene from Bambel, Norway, c clinoenstatite, and d diopside.

Many sharp peaks appear not only in the mid-infrared region, but also in the far-infrared region. The peak wavelengths of the present samples are listed in Table 2, together with those of the two previous samples (Koike et al. 1993). The enstatite group shows sharp peaks compared with the orthopyroxenes, which has slightly broader or degenerate bands.

Three strong absorption peaks appear in the 9-12  region for each sample, although the strength of the peaks depends on the sample as is shown in Fig. 1. The 9.3  feature is detected in all pyroxenes. The 20  feature also varies in the shape, width and peak position. The peak positions of the clino- and ortho-enstatite samples are about 19.7-20 . The synthetic diopside and natural orthopyroxene show double peaks at 20 and 21 .

The synthetic orthoenstatite and natural orthopyroxene from Bambel show many strong peaks in the far-infrared region compared with the two natural pyroxenes from Ichinoeme-gata, and interestingly the bands of the orthoenstatite at 42, 50 and 70  clearly appear as double bands.

As for the 32.8  peak of the orthoenstatite, the present data shows a weaker peak, about 2.5 times, than the data of the orthoenstatite (Koike & Shibai 1998) in spite of the fact that it is the same material. This might be due to differences in the shape distribution for each sample, although the exact reason is not clear.

In summary, the sharp peaks of pyroxene appear very distinctly at 25, 28-29, 33-36, 40-45, 50, and 66-75 .

3.2. Amorphous pyroxenes

The three samples show two broad bands near 10 and 20  as is shown in Fig. 2.

Fig. 2. The mass extinction coefficients of the amorphous pyroxenes; diopside glass, enstatite gel, and enstatite glass, which are multiplied by 1, 10, and respectively. The dotted lines represent the spectra after hydration.

The 20  band of the present samples is changed due to hydration and shifts to 21.7  (thin dotted lines in Fig. 2); the 20  band of amorphous olivine was also changed by hydration and annealing (Koike et al. 1992). The peak positions of the present samples before and after hydration are listed in Table 3. The extinction coefficients of the enstatite glass and enstatite gel follow a power law with and , respectively, in the wavelength range of 30-100 , and those of the diopside glass have a power law with in the wavelength range of 45-100 . The enstatite glass shows a sharp decrease, which is proportional to , in the far-infrared wavelength region above 140 .

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
helpdesk@link.springer.de