Astron. Astrophys. 343, 933-938 (1999)

2. The extraction procedure

Two fractions of 26.8 g (sample I ) and 10.0 g (sample II ) from an originally large (100 g) piece of Murchison (a CM2 chondrite) was used for the extraction of meteoritic SiC grains. The originally large piece was crushed with a steel mortar, to obtain smaller fractions.

To obtain a large and clean sample of the SiC grains for the spectroscopic measurement, it is necessary to isolate the presolar grains from the rest of the meteoritic material. A physical, non-destructive separation does not work well because the grains are tiny and cling to the much larger amounts of fine-grained clay minerals and kerogen (macromolecular organic matter). Therefore, a destructive chemical separation in which undesirable minerals are dissolved by appropriate reagents is necessary. The chemical separation procedures used in this study were variants of those developed by Tang & Anders (1988) and Amari et al. (1994), which mainly consist of a set of progressively more corrosive "selective solvents" to remove the mineral phases one by one, but also includes density separations.

Extracting presolar SiC from meteorites is much more complicated than extracting the presolar nano-diamonds, because the SiC (6 ppm) is much rarer than the nano-diamonds (500 ppm) (Amari et al. 1994), and because the diamonds can be density separated out of the meteoritic sample without having to remove all mineral phases first. It is therefore much harder to obtain a clean sample of presolar SiC grains than of presolar diamonds. There will in most cases always be smaller amounts of corundum, hibonite, spinel and chromite grains present, which are very hard to get rid of without also losing the SiC grains.

The extraction treatment was primarily designed to result in a fraction of very clean presolar nano-diamonds (Braatz in prep.). The individual extraction steps for the two samples were almost identical, except that for the first two steps sample I was treated in a sealed Teflon bomb at 180^oC while sample II had a more gentle treatment without the Teflon bomb and at 80^oC. The extraction steps were the following: 1) solution of concentrated HCl (32%) to dissolve metals and sulfides; 2) alternating treatment with concentrated HF (48%) and concentrated HCl to dissolve silicates; 3) extraction of precipitated sulfur with CS₂; 4) oxidation with 0.5 N Na₂Cr₂O₇ + 2 N H₂SO₄ at 80^oC; 5) extraction of the nano-diamonds as a colloid with a solution of bidistilled water and NH₃ (pH 9-10). This step results in a precipitate at the bottom of the container containing mainly residual spinels and SiC. The nano-diamonds were used for other studies (Braatz in prep.). The spinel-SiC residue was further treated by; 6) boiling in 70% HClO₄ (200^oC) to remove residual organic material and graphite; 7) boiling in H₂SO₄ (180 ^oC) to remove the spinels (spinel MgAl₂O₄ and chromite FeCr₂O₄) and 8) another treatment with HF/HCl (60^oC) to remove remaining silicon bearing grains other than SiC.

Most of the treatments were carried out in Teflon containers. Between all steps intensive washing was carried out, with diluted HCl (pH 2) and/or bidistilled water, which is important in order to remove possible coatings that impede further reactions (Amari et al. 1994). Each time the supernatant liquid was pipetted into separate discard tubes for the two samples and all solids which settled down in the discard tubes were returned to the main sample.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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