Low-ca Pyroxenes From Ll-group Chondritic Meteorites - Crystal Structural Studies and Implications For Their Thermal Histories

Crystal structural refinements of one orthorhombic (Pbca) and two monoclinic (P2(1)/c) single crystals, from chondrules of low-Ca pyroxenes from unequilibrated chondritic meteorites of the LL group, were carried out. The intracrystalline Fe-Mg distribution between the M1 and M2 crystallographic sites of the Parnallee (LL-3) orthoenstatite is suggestive of very rapid cooling, whereas both the structural state and intracrystalline Fe-Mg distribution in the Soko Banja (LL-4) and Jolomba (LL-6) clinoenstatites indicate rapid cooling from the high temperature polymorphs, with no significant re-equilibration at lower temperatures. These results imply that thermal metamorphism in the parent body, if present, was insufficient to allow re-equilibration of the pyroxene minerals to low temperature, ordered crystal structures. The data also indicate that, assuming low or mild pressure and shock effects, there is no well defined correlation between equilibrium temperature of the mineral phases and the alleged petrologic type of the meteorites. This evidence is consistent with a rubble pile model for the parent body accretional history, or with an onion shell model with very low thermal peak metamorphism, as is assumed for a very small object.