Computer Science
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000m%26ps...35..733f&link_type=abstract
Meteoritics & Planetary Science, vol. 35, no. 4, pp. 733-742 (2000).
Computer Science
3
Scientific paper
La Villa is an unshocked H4 chondrite. Chemical compositions require crystallization at temperatures higher than 1250 deg C for enstatite, and higher than 1211 deg C for augite. Widespread (100) polysynthetic twins and (001) contraction cracks in enstatite indicate crystallization as protoenstatite, inverted to ortho- and/or clinoenstatite on cooling. HRTEM shows a range of ortho-clinoenstatite intergrowths: heavily faulted clinoenstatite in radial and poikilitic chondrules, almost regular orthoenstatite in a microgranular chondrule and in the matrix. In the former, the clinoenstatite lamellae are both even or odd multiples of the 9 Å periodicity, a few unit cells thick, twinned and interleaved with minor orthoenstatite. In the latter, orthoenstatite lamellae are regularly stacked for more than 2000 Å. Localized annealing effects, reversing clinoenstatite to orthoenstatite, are revealed by "U-shaped" and "Z-shaped" terminations. The variable microstructures suggest different cooling rates for the different chondrule types, soon after the liquidus-to-solidus transition (1200 to 1300 deg C) but prior to accretion. In particular, clinoenstatite-rich crystals from a radial and poikilitic chondrules give cooling rates in the order of 100 and 10 deg C/hr. Comparisons with previous works on dynamic crystallization experiments and orthopyroxene Fe-Mg cation ordering indicates a nonlinear cooling path from the high chondrule formation temperatures to a post-accretionary low-temperature (340-480 deg C) evolution.
Folco Luigi
Mellini Marcello
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