Carbonaceous Nanospheres in Chondrites

Introduction: In the 1960s, prior to the widespread use of TEMs, several papers described rounded, micron-sized organic particles from carbonaceous chondrite (CC) meteorites [1-3]. Some of these particles were hollow. Except for two early TEM studies [4, 5], the rounded structures were essentially neglected until similar material was discovered in the Tagish Lake meteorite [6]. We subsequently examined such nanospheres in a broad range of CI and CM meteorites [7, 8]. Here we report on their occurrence in additional CC meteorites, suggest they are widespread constituents of carbonaceous meteorites, and review their physical and chemical properties. Materials and Methods: We found carbonaceous nanospheres in all CCs that we examined, which Acfer 214 (CH) meteorites. This is the first report of their occurrence in almost half these meteorites. Samples were examined as: disaggregated submillimeter-pieces, the insoluble organic material (IOM) in HF-HCl acid residues, and in an ion-beam-thinned section. Material for TEM and EELS were gently disaggregated and dispersed onto lacey TEM grids. Images were acquired with a Topcon 002B TEM operated at 200 kV. EELS data were acquired with a GATAN 766 DigiPEELS spectrometer attached to a Philips 400-ST field-emission-gun (FEG) TEM operated at an accelerating voltage of 100 kV. Results: The nanospheres are recognized by their rounded outlines and low contrast (Fig. 1). They constitute up to 10% of the residues. Nanospheres occur both singly and clustered. Hollow cores occur in nanospheres with diameters >100 nm and range from single and spherical to complex and vesicular. Hollow nanospheres are spherical to subrounded and <1000 nm in diameter. Micron-sized clusters of nanospheres occur in the Tagish Lake and GRA95229 (CR2) meteorites. GRA95229 is distinct in that the IOM contains a higher proportion of large solid nanospheres (Fig. 1d). HRTEM images and electron diffraction patterns of the nanospheres show that they are poorly ordered, and this lack of crystalline structure suggests that they had not been significantly heated. The nanospheres are dominated by C, with subordinate amounts of S, N, and O. Hydrogen is excluded from our analyses since EELS cannot detect bonded H. The average compositions from the different meteorites are similar, with ca. 90 at% C, although there are significant particle-to-particle variations. The C K edges from the nanospheres are intermediate in shape to those from amorphous C and graphite, but distinct from structurally dissimilar C-bearing materials in the meteorites. The C K edges of all nanospheres exhibit similar shapes, …