Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.445b&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 445
Computer Science
2
Breccia, Chemical Composition, Meteoritic Composition, Meteoritic Microstructures, Mineralogy, Pyroxenes, Chemical Analysis, Crystal Structure, Grain Size, Ilmenite, Inclusions, Temperature Effects
Scientific paper
Caldera is the second known unbrecciated noncumulate eucrite. Caldera consists mainly of pyroxene and plagioclase, with subordinate tridymite, chromite, ilmenite, troilite, and rare metal. The main pyroxene is pigeonite, which shows exsolutions in one or two directions. Most pigeonite crystals show coarse (about 100 microns) exsolution lamellae of augite. Both and pigeonite host and the augite subsequently exsolved very fine (about 1 micron) lamellae of augite and low-Ca pyroxene respectively. A few pigeonite crystals show exsolution lamellae of augite (about 5-10 microns) in two different orientations; the host pigeonite also exsolved very fine lamellae parallel to one set of the augite exsolutions. Discrete augite crystals are less common and display fine exsolution lamellae of low-Ca pyroxene. The exsolution microtextures in the pyroxene are suggestive of prolonged slow annealing at subsolidus temperature. Exsolution appears to have occurred in two stages, with the coarse lamellae forming at a higher temperature than the fine lamellae. Plagioclase (An90-91) contains abundant inclusions of pigeonite (occasionally exsolved) and tridymite with rare chromite, ilmenite, and troilite. Individual grains are homogeneous in composition, but grain-to-grain variations in Ti are quite large. Ilmenite occurs either as oriented lamellae in chromite or as granules. Magnesium and Zr partition in favor of the ilmenite relative to chromite, which is consistent with the equilibrium partitioning of these elements between the two phases. The chemical composition of Caldera is similar to that of main-group eucrites. Instrument Neutron Activation Analysis (INAA) analysis of a bulk sample of Caldera (139 mg) gave 14.43 wt% Fe, 7.14 wt% Ca, 3190 ppm Na, 326 ppm K, and 29.2 ppm Sc. The Rare Earth Element (REE) pattern is flat with no Eu anomaly; the average of the CI-enrichment factors for 12 REE is 10.68 +/- 0.26. Caldera also shows unusually low CI-enrichment factors for Th (3.5) and U (4.1). It also is more enriched in Na and K relative to Ibitira.
Boctor Nabil Z.
El Goresy Ahmed
MacPherson Glenn J.
Palme Herbert
Spettel Bernhard
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