Tl. Daulton et al., GENESIS OF PRESOLAR DIAMONDS - COMPARATIVE HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY STUDY OF METEORITIC AND TERRESTRIAL NANO-DIAMONDS, Geochimica et cosmochimica acta, 60(23), 1996, pp. 4853-4872
Nano-diamonds isolated from acid dissolution residues of primitive car
bonaceous meteorites (Allende and Murchison) were studied using high-r
esolution transmission electron microscopy. To discriminate among thei
r most likely formation mechanisms, high-pressure shock-induced metamo
rphism or low-pressure vapor condensation, the microstructures of pres
olar diamond crystallites were compared to those of (terrestrial) synt
hesized nano-diamonds. The synthesized diamonds used for comparison in
this study were produced by high-pressure shock waves generated in co
ntrolled detonations and by direct nucleation and homoepitaxial growth
from the vapor phase in low-pressure chemical vapor deposition (CVD)-
type processes. Microstructural features were identified that appear u
nique to shock metamorphism and to nucleation from the vapor phase, re
spectively. A comparison of these features to the microstructures foun
d in presolar diamonds indicates that the predominant mechanism for pr
esolar diamond formation is a vapor deposition process, suggesting a c
ircumstellar condensation origin. A new presolar grain component has a
lso been identified in the meteoritic residues, the (2H) hexagonal pol
ytype of diamond (lonsdaleite).