Wollastonite whiskers in the Allende meteorite and their bearing on a possible post-condensation process.

Abstract

The fine-grained white inclusions in the Allende meteorite have been studied by the scanning electron microscope with non-dispersive and dispersive X-ray detector, and a transmission electron microscope. The white inclusions consist almost entirely of the aggregates of losely packed semi-euhedral crystals rich in Ca and Al on the whole. Whisker crystals 0.5-5 microns in width and 10-50 microns long are found in cavities about 0.1 mm in diameter. These whisker crystals are wollastonite (CaSiO3) whose formation process has not been fully elucidated with the equilibrium condensation. On the basis of the evidence that a portion of the wall of the wollastonite-filled cavity is rich in Al2O3, we presumed some possible formation processes of these wollastonite whiskers. In conjunction with a finding of layered rims enclosing Ca-Al rich inclusions in carbonaceous chondrites, the growth of wollastonite whiskers in the inclusions also provide an evidence that some post-condensation processes took place within the inclusions as well as at the rims.