The Isotopic Composition and Mineralogy of Silicon Nitride (Si3N4) Within Ordinary and Enstatite Chondrites
During stepped combustion studies of primitive meteorite residues, samples with high relative nitrogen abundances and delta^15N values have been encountered (e.g., Grady et al., 1986; Alexander, 1990; Russell et al., 1991). Such characteristics are consistent with the presence of nitrogen-rich minerals such as silicon nitride (Si3N4), which has recently been identified within the enstatite chondrites Indarch (Stone et al., 1991) and Qingzhen (Alexander et al., 1991). In order to explore this possibility more fully, the isotopic composition and mineralogy of acid residues from a number of ordinary, enstatite and carbonaceous chondrites has been determined. The carbon and nitrogen isotopic composition of HF/HCl, Cr(sub)20(sub)7^2-, HClO4 residues were analysed by stepped combustion and simultaneous C/N abundances measured. The delta^15N and C/N ratio over the 800 to 1200 degree C regime, the expected combustion temperature of nitrides and carbides, is shown in the figure. The carbonaceous chondrites contain isotopically light nitrogen (delta^15N = -650o/oo) and have a C/N ratio of ~30-35. This can be attributed to the combustion of SiC. In contrast, the ordinary chondrites and enstatite chondrite Indarch yield a more nitrogen-rich gas at the same temperatures, which can be attributed to the cocombustion of Si3N4 with SiC. Assuming the data in the figure represent two-component mixing and that nitrogen in SiC from all the meteorite classes has the same mean delta^15N and C/N ratio as that from carbonaceous chondrites, then an estimate can be made of the nitrogen isotopic composition of nitride grains. Thus we calculate that Si3N4 in ordinary chondrites has a delta^15N = +336 +- 13o/oo whereas that in Indarch is +12o/oo. The data for Indarch do not conflict with the ion probe study of Alexander et al., (1991). The mineralogical information concerning the acid residues was determined by pipetting samples onto perforated carbon films and examining them using a transmission electron microscope equipped with an ultrathin window X-ray detector. Silicon nitride with a low C/N ratio has so far been identified from the Indarch and Tieschitz residues. Silicon nitride is abundant in Indarch and its electron diffraction patterns are consistent with alphaSi3N4. This mineral forms elongate tabular crystals, ~1.5 micrometers to ~5.0 micrometers long by ~0.2 micrometers to ~0.7 micrometers wide. Silicon nitride grains within the Tieschitz residue are smaller than those from Indarch and ~500 to 1000 times less abundant. The crystals are needle-like in shape, ~0.4 micrometers to ~1.5 micrometers long by ~0.04 micrometers to ~0.07 micrometers wide. Most of the silicon nitride indexes as alphaSi3N4, although one large segmented crystal found was formed of both alphaSi3N4 and betaSi3N4. The contrasting isotopic and mineralogical characteristics of silicon nitrides from enstatite (Indarch) and ordinary (Tieschitz) chondrites may be due to differences in their origin. Whereas the delta^l5N of nitride in Indarch falls within the normal solar system range and could be a solar system condensate, as suggested by Alexander et al., (1991), the nitride component in ordinary chondrites is isotopically heavy and possibly represents a newly identified presolar grain. References Alexander C. M. O'D., Arden J. W., Ash R. D., and Plllinger C. T. (1990) Earth Planet. Sci. Lett. 97, 220-229. Alexander C. M. O'D., Prombo C. A., Swan P. D., and Walker R. M. (1991) Lunar Planet Sci. (abstract) 22, 5-6. Grady M. M., Wright I. P., Carr L. P., and Pillinger C. T. (1986) Geochim. Cosmochim. Acta, 50, 2799-2813. Russell S. S., Ash R. D., Pillinger C. T., and Arden J. W. (1991) Meteoritics, 26, 390. Stone J., Hutcheon I. D., Epstein S., and Wasserburg G. J. (1991) Earth Planet. Sci. Lett., 107, 570-586.
- Pub Date:
- July 1992