La Villa is an unshocked H4 chondriate. Chemical compositions require crystallization at temperatures >1250 °C for enstatite and >1211 °C for augite. Widespread (100) polysynthetic twins and (001) contraction cracks in enstatite indicate crystallization as protoenstatite, inverted to either ortho- or clinoenstatite or both on cooling. High-resolution transmission electron microscopy shows a range of orthoclinoenstatite 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 A periodicity, a few unit cells thick, twinned and interleaved with minor orthoenstatite. In the latter, orthoenstatite lamellae are regularly stacked for more than 2000 A. Localized annealing effects, reversing clinoenstatite to orthoenstatite, are revealed by 'U-shaped' and 'Z-shaped' terminations. The variable microstructure suggest different cooling rates for the different chondrule types, soon after the liquidus-to-solidus transition (1200 to 1300 °C) but prior to accretion. In particular, clinoenstatite-rich crystals from radial and poikilitic chondrules give cooling rates on the order of 100 and 10 °C/h. Comparisons with previous works on dynamic crystallization experiments and orthopyroxene Fe-Mg cation ordering indicate a nonlinear cooling path from the high chondrule formation temperatures to a postaccretionary low-temperature (340-480 °C) evolution.
Enstatite chemical composition and microstructures in the La Villa H4 chondrite
FOLCO, LUIGI;
2000-01-01
Abstract
La Villa is an unshocked H4 chondriate. Chemical compositions require crystallization at temperatures >1250 °C for enstatite and >1211 °C for augite. Widespread (100) polysynthetic twins and (001) contraction cracks in enstatite indicate crystallization as protoenstatite, inverted to either ortho- or clinoenstatite or both on cooling. High-resolution transmission electron microscopy shows a range of orthoclinoenstatite 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 A periodicity, a few unit cells thick, twinned and interleaved with minor orthoenstatite. In the latter, orthoenstatite lamellae are regularly stacked for more than 2000 A. Localized annealing effects, reversing clinoenstatite to orthoenstatite, are revealed by 'U-shaped' and 'Z-shaped' terminations. The variable microstructure suggest different cooling rates for the different chondrule types, soon after the liquidus-to-solidus transition (1200 to 1300 °C) but prior to accretion. In particular, clinoenstatite-rich crystals from radial and poikilitic chondrules give cooling rates on the order of 100 and 10 °C/h. Comparisons with previous works on dynamic crystallization experiments and orthopyroxene Fe-Mg cation ordering indicate a nonlinear cooling path from the high chondrule formation temperatures to a postaccretionary low-temperature (340-480 °C) evolution.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
