G. Artioli et G. Davoli, LOW-CA PYROXENES FROM LL-GROUP CHONDRITIC METEORITES - CRYSTAL STRUCTURAL STUDIES AND IMPLICATIONS FOR THEIR THERMAL HISTORIES, Earth and planetary science letters, 128(3-4), 1994, pp. 469-478
Crystal structural refinements of one orthorhombic (Pbca) and two mono
clinic (P2(1)/c) single crystals, from chondrules of low-Ca pyroxenes
from unequilibrated chondritic meteorites of the LL group, were carrie
d out. The intracrystalline Fe-Mg distribution between the M1 and M2 c
rystallographic sites of the Parnallee (LL-3) orthoenstatite is sugges
tive of very rapid cooling, whereas both the structural state and intr
acrystalline 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 temperature
s. These results imply that thermal metamorphism in the parent body, i
f 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. Thi
s evidence is consistent with a rubble pile model for the parent body
accretional history, or with an onion shell model with very low therma
l peak metamorphism, as is assumed for a very small object.