The composition and structural state of feldspar from chondritic meteorites

Abstract

Electron microprobe analysis and X-ray diffraction techniques were used to determine the chemical compositions and structural states of feldspar in recrystallized (Type 6) chondritic meteorites from the L, LL, H, and E Groups. The feldspar in these meteorites is a secondary reerystallization product and is commonly partially or wholly converted to maskelynite. The feldspars are rich in the albite (Ab) end-member, containing small amounts of anorthite (An) and orthoclase (Or). Their compositions cluster near Ab 82An 12Or 6 for H Group chondrites, Ab 84An 10Or 6 for L Group chondrites, and Ab 81An 15Or 4 for Enstatite chondrites. The compositional clusters for these three groups are essentially discrete and non-overlapping; ±1 mole per cent ranges on the An and Or contents will include most chondrites belonging to any given group. This compositional uniformity of feldspar within a single group reflects the bulk chemical uniformity of chondrites within a group, and the differences in feldspar composition from group to group are probably due to primary elemental fractionations. The feldspar from LL Group chondrites also has a narrow range of An content (10.0–10.5 mole per cent), but has a range in Or content from 0.9 to 5.2 mole per cent that is a consequence of varying bulk potassium contents within this group. X-ray diffraction patterns of crystalline feldspar from chondrites yield values of [2θ(131) –2θ(1 3 ̄ 1)] = 1.75 ± 0.10° indicating a high-intermediate structural state. Unit cell refinements were carried out for some samples, and consideration of the cell angle γ puts the structural state in the high to high-intermediate range. Although such structural states are indicative of high equilibration temperatures (650–850°C), the fact that chondritic feldspars are still in the high-temperature state does not preclude cooling of these meteorites over 10 6-10 8 yr within asteroidal sized bodies. Finally, all of our observations and data are consistent with earlier suggestions that maskelynite is produced by shock.