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

2. Measurements

2.1. Sample preparation

Pyroxenes have the general formula, . In the Ca-Mg-Fe pyroxenes, the M site is occupied mainly by Mg, Fe, and Ca, and T by Si (Al can also be incorporated into M and T sites as a minor element). These pyroxenes are divided into sub divisions (Morimoto et al. 1988); the Mg-Fe pyroxenes (or Ca-poor pyroxenes) with the end members of enstatite (En) and ferrosilite (Fs), and the Ca pyroxenes (Ca-rich pyroxenes) with the end members of diopside (Di) and hedenburgite (Hd). Their chemical composition are expressed by using the end components of En, Fs and Wo (wollastonite ).

The crystalline and amorphous pyroxenes used in the present measurements are listed in Table 1. The crystalline samples are one Ca-rich (diopside) and three Ca-poor pyroxenes (orthoenstatite, clinoenstatite, and orthopyroxene). The orthoenstatite () crystals were synthesized by the flux method, i.e. single crystals of orthoenstatite were grown from a melt in the system (Ozima 1982). The contamination of the flux components LiO₂, MoO₃ and V₂O₅ is negligible ( 0.5 wt.%) and does not affect the infrared spectrum as we compared the spectra of the flux with those of the samples. The clinoenstatite crystal () was converted from the orthoenstatite crystal; the orthoenstatite crystal was heated to 1100-1200 , above the protoenstatite-orthoenstatite transition point of 985 , for a few hours in air and afterward quenched into water. The product became clinoenstatite crystals with polysynthetic twinning, which showed a typical texture formed by rapid transformation from protoenstatite. Another Ca-poor pyroxene used is natural orthopyroxene from Bambel, Norway. The composition has been determined by an EPMA (electron probe microanalyzer) (JEOL733) at Osaka University and is ()

(, or ().

The diopside crystal was synthesized by the CZ (Czochralski) method, i.e. a single crystal of diopside grown from its melt by pulling a seed crystal (Takei et al. 1982). The crystal is slightly nonstoichiometric, and has the chemical formula , that is, ().

Two of the amorphous samples have an enstatite and one has a diopside composition. The enstatite glass () and the diopside glass () were synthesized by melting mixtures of reagent-grade , and (the carbonate was decarbonated during heating) in air and quenched into water. The samples are almost stoichiometric in composition. Another amorphous enstatite (enstatite gel) was formed by a gelling method (Hamilton & Henderson 1968). No crystalline feature was detected in X-ray diffraction patterns.

2.2. The procedure of measurements

The bulk samples were crushed and ground in an agate mortar. Large size particles were removed by sedimentation in alcohol. The size of the particles, measured with the SEM (scanning electron microscope) is smaller than 0.5  and 1 in the mid-infrared region and far-infrared region respectively. These fine particles were dispersed in KBr pellets and polyethylene (PE) sheets. The transmission of the pellets and sheets were measured with the Fourier transform infrared spectrometer JASCO FT/IR-350 (resolution; 0.5 ) in the mid- and far-infrared regions, and with the BOMEM DA3 (resolution; 1.0 ) at the Institute of Space and Astronautical Science (ISAS) (at Sagamihara, Kanagawa, Japan) in the far infrared region.

The mass extinction coefficient, , was derived from the transmittance, T, of pellets and sheets as follows (Koike et al. 1989):

where S is the surface area of a KBr pellet or polyethylene sheet, and M is the mass of the sample embedded in the sample pellet or sheet.

The alteration of the amorphous samples (enstatite glass, enstatite gel and diopside glass) due to hydration was examined by infrared spectroscopy up to 40 . The hydration proceeded through the following steps. The KBr pellet containing sample particles was dissolved into water and kept at room temperature for 48 hr. After that, the solution containing sample particles was heated at 90  for 7 hr in order to evaporate water. The residue (KBr and sample particles) was reheated at 120  for 19 hr in order to dry up thoroughly. The recovered mixture of KBr and sample particles was ground down and re-pressed into a pellet.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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