Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..457w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 457
Computer Science
Acfer 182, Allende, Ca-Dialuminate, Cais, Carbonaceous Chondrites, X-Ray Techniques
Scientific paper
After the first discovery of Ca-dialuminate (CaAl4O7) in Allende [1], in recent years this phase has been found in several carbonaceous chondrites. Ca- dialuminate is a major phase in Ca,Al-rich inclusions from ALH85085 (e.g., [2]) and a dominating phase in CAIs from Acfer 182 ([3,4]). X-ray data on Ca-dialuminate are known from synthetic (e.g., [5-8]; cell constants) and terrestrial CaAl4O7 ([9]; only d-spacings), but are not available from extraterrestrial Ca-dialuminate. We report here the results of the first X-ray study of extraterrestrial Ca- dialuminate. The data (Table 1) were obtained by microdiffraction using a Rigaku PSPC microdiffractometer at the Bayerisches Geoinstitut. Ni-filtered Cr radiation was used with a direct beam diameter of about 50 micrometers. This powder diffraction method allows in situ measurement of polycrystalline Ca- dialuminate in a thin section. The CaAl4O7-rich inclusion 022/9 described in [4], consisting of a ~200-micrometer-sized core of Ca-dialuminate surrounded by layers of melilite and Ca-pyroxene, was chosen for analysis. The polycrystalline core contains only a small number of tiny inclusions (especially perovskite) and is therefore an excellent candidate for an X-ray study. For determination of the d-spacings of Ca-dialuminate an external standard (Ag6Ge10P12) was used for detector calibration. A large number of reflections could be indexed based upon comparison with the X-ray pattern of synthetic CaAl4O7 available in the JCPDS compilation [7]. The comparison was simplified because of the high purity of CaAl4O7 in inclusion 022/9 [4], and suggests the same structure for synthetic and extraterrestrial Ca-dialuminate. For determination of lattice parameters (cell constants, cell volume) refinement calculations were made based on 14 reflections (Table 1). The data for extraterrestrial CaAl4O7 shown in Table 1 indicate a close similarity to those obtained for synthetic CaAl4O7. The cell constants a, b, and therefore the cell volume, are slightly higher in Ca-dialuminate from Acfer 182 than from synthetic CaAl4O7. This may be due to the incorporation of traces of refractory elements (REE) with large ionic radii, which were analyzed within inclusion 022/9 [10]. With the determination of the cell constants of natural Ca-dialuminate combined with data on synthetic CaAl4O7, sufficient X-ray data should be available required to nominate this mineral. References: [1] Christophe Michel-Levy M. et al. (1982) EPSL, 61, 13-22. [2] Kimura M. et al. (1993) GCA, in press. [3] Weber D. and Bischoff A. (1992) Meteoritics, 27, 304-305. [4] Weber D. and Bischoff A. (1993) GCA, submitted. [5] Boyko E. R. and Wisnyi L. G. (1958) Acta Cryst., 11, 444-445. [6] Goodwin D. W. and Lindop A. J. (1970) Acta Cryst., B26, 1230-1235. [7] Baldock P. J. et al. (1970) J. Appl. Cryst., 3, 188-191. [8] Geiger C. A. et al. (1988) GCA, 52, 1729-1736. [9] Gross S. (1977) Geol. Surv. Israel Bull. 70, 1-80. [10] Bischoff A. et al. (1992) Meteoritics, 27, 204. Table 1, which appears in the hard copy, shows unit-cell constants of Ca- dialuminate (monocline; space group C2/c) and X-ray powder diffraction data (CrK-alpha (Ni-beta), 45 kV, 30 mA) on extraterrestrial CaAl4O7 in comparison to JCPDS data [7]. Numbers in parentheses are uncertainties in last significant figures.
Bischoff Addi
Ross Charles R. II
Weber Darrell
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