Computer Science
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33...75g&link_type=abstract
Meteoritics & Planetary Science, vol. 33, no. 1, pages 75-87.
Computer Science
4
Scientific paper
A transmission electron microscope study of three coarse-grained Type A Ca,Al-rich inclusions (CAIs) from Allende, Acfer 082 and Acfer 086 (all CV3 chondrites) was performed in order to decipher their origin and effects of possible metamorphism. The constituent minerals of the CAIs are found to exhibit very similar microstructural characteristics in each of the inclusions studied. In general, the minerals show a well-developed equilibrium texture with typical 120 triple junctions. Melilites are clearly considerably strained and characterized by high dislocation densities up to 3 x 1011 cm-2. The dislocations have Burgers vectors of [001], [110] or [011] and often form subgrain boundaries subparallel {100}. Melilite in the Allende CAI additionally contains thin amorphous lamellae mostly oriented parallel to {001}. Fassaite (Al-Ti-diopside) is almost featureless even on the TEM scale. Only a few sub-planar dislocation walls composed of dislocations with Burgers vectors [001] and 1/2 [110] were detected. Although enclosed within the highly strained melilites, the euhedral spinels contain only low dislocation densities (<2 x 104 cm-2). In the Allende CAI, spinels were found twinned on {111}. Perovskite is also characterized by a low number of linear lattice defects. All grains possess orthorhombic symmetry and are commonly twinned according to a 90 rotation around [101]. Many crystals exhibit typical domain structures as well as curved twin walls where two orthogonal sets intersect. In addition to the mineral phases described above, tiny inclusions of the simple oxides CaO and TiO2 were found within melilite (CaO), spinel (CaO, TiO2) and perovskite (CaO, TiO2). Based on these observations it is assumed that at the beginning of the formation of the CAIs a condensed solid precursor was present. Euhedral spinels poikilitically enclosed within melilites suggest that this solid aggregate was then molten. If the pure oxides represent relict condensates, their presence proves that this melting was incomplete. While still plastic, the CAIs were shocked by micro-impacts causing the high dislocation densities in melilite as well as diaplectic melilite glass and twinned spinels in the Allende CAI. In Acfer 082 and 086 the deformation took place at elevated temperatures, preventing the solid phase transition and mechanical twinning. The absence of linear lattice defects in spinel, fassaite and perovskite most probably reflects inhomogeneous pressure distribution in the polycrystalline CAI as well as the different strengths of the minerals. According to cooling-rate experiments on perovskite by Keller and Buseck (1994), the dominating (101) twins in the CAI perovskites point to cooling rates (50 C/min. Finally, after crystallization of the CAI was complete, mild thermal metamorphism caused the formation of subgrain boundaries, 120 triple junctions and chemical homogenization of the melilites.
Bischoff Addi
Greshake Ansgar
Putnis Andrew
No associations
LandOfFree
TEM study of compact Type A Ca,Al-rich inclusions from CV3 chondrites: Clues to their origin does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with TEM study of compact Type A Ca,Al-rich inclusions from CV3 chondrites: Clues to their origin, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and TEM study of compact Type A Ca,Al-rich inclusions from CV3 chondrites: Clues to their origin will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1611674