Astron. Astrophys. 360, 777-788 (2000)

3. Characterization of the samples

In this section, we discuss physical characteristics of our samples of small particles (magnesium rich olivine and a ground piece of Allende meteorite), in particular their chemical composition, size distribution, complex refractive index and morphological characterization.

3.1. Olivine

The olivine was obtained from a Norwegian dunite rock with a composition of . In Table 1, we present the chemical composition of the rock. The original rock was prepared so that the measurements could be repeated for different size distributions. The sample was ball milled and first sieved with a 125 µm sieve. The portion of the sample that passed through the sieve (particles smaller than 125 µm in diameter) was subsequently sieved in water through a sieve of 65 µm. Again the smallest particles (smaller than 65 µm) were subsequently sieved through a sieve of 20 µm. In such a way we produced four different size distributions designated as XL (m), L (m), M (m) and S (m), where d is the diameter of the sieving grid.

Table 1. Chemical analyses (in by weight) of our Olivine sample and Carbonaceous chondrite meteorite type III, which is the group to which Allende meteorite belongs (Mason 1971).

3.2. Allende meteorite

The group of carbonaceous chondrite meteorites, type III (Mason 1971) to which the Allende meteorite belongs, has a composition close to that of the Sun (see e.g. Beatty & Chaikin 1990). The only exception relates to volatile elements. Hydrogen, carbon, nitrogen, oxygen and the noble gases are so volatile, or form compounds so volatile, that they are incapable of condensing in the inner solar system. This supports the theory that the carbonaceous chondrite meteorites condensed from the primitive solar nebula and have undergone little subsequent chemical modification. In Table 1 we present results of chemical analyses of this type of meteorites.

3.3. Particle sizes

The projected surface area distributions of projected surface equivalent spheres have been measured by using a Fritsch laser particle sizer (Konert & Vandenberghe 1997). The results for olivine samples S, M, L and XL and for the Allende meteorite particles are presented in Fig. 3, showing the projected surface distributions S(logr) as a function of logr. Here, r is the radius of a sphere having the same projected surface area as the irregular particle has, and S(logr)dlogr gives the relative contribution by spheres with radii in the size range [logr; logr+dlogr] to the total projected surface per unit volume of space. Since for irregular particles larger than about 1 µm, the projected surface area is proportional to the scattering cross section (Hodkinson 1963), Fig. 3 gives us information about how particles of different size contribute to the scattering. According to these measurements the sieving procedure did not remove all particles with diameters smaller than 65 µm from sample XL nor particles with diameters smaller than 20 µm from sample L.

Fig. 3. Projected surface area distribution of the four olivine samples and the ground piece of Allende meteorite as a function of the radius in micrometers on a logarithmic scale.

Values of the effective radius () and variance () of each sample are given in Table 2. These two parameters are defined as follows:

where r is the radius and is the size distribution of projected surface equivalent spheres (Hansen & Travis 1974). Values of were derived from the measured projected surface distributions.

Table 2. Overview of the properties of the samples studied.

Since the olivine samples XL and L show bi-modal projected surface distributions, the and that are used to characterize them are only a first order indication of the size of the particles.

3.4. Refractive indices

The exact values of the refractive indices of our samples are unknown. According to the measured optical constants of different types of silicates published so far (e.g. Jäger et al. 1994; Dorschner et al. 1995), the imaginary part of the refractive index, k, of iron-poor silicates is very low (of the order of in the visible range). However, the absorption increases by increasing the amount of iron in the sample being higher at 442 nm than at 633 nm. Since the amount of iron in our olivine sample is quite low we expect low values of the imaginary part of the refractive index. In contrast, the amount of iron in the Allende sample is much higher than for olivine (see Table 1), so we can expect a higher imaginary part of the refractive index. This is something we can easily establish by looking at the sample: the color of the Allende sample is dark grey, whereas the olivine sample is light green.

3.5. Morphology

The morphological characterization was done by using a field emission Scanning Electron Microscope (SEM). In Fig. 4, we present SEM photographs of our samples of olivine and Allende meteorite particles. Since the four olivine samples have been produced by milling and sieving from the same original rock, we do not expect significant differences in the shape of the particles of the different olivine samples. Indeed, we see quite similar shapes for all of the olivine samples shown in Fig. 4. The shape of the particles of the Allende meteorite (Fig. 4e), is very similar to that of the olivine samples. Therefore, the possible effect of differences in shape on the scattering behavior of these five samples has not been taken into account in the discussion of the measurements (Sect. 4).

Fig. 4a-e. SEM photographs of olivine samples XL (a.1 and a.2 ), L b , M c , S d and Allende meteorite e . In each photograph the white bar denotes 10 µm.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
helpdesk.link@springer.de