X-ray study of PCP from the Murchison CM carbonaceous chondrite

Abstract

We have performed X-ray diffraction measurements and electron-microprobe analyses of PCP particles in the Murchison CM carbonaceous chondrite and determined the relative abundances of minerals in PCP. Type-I PCP, which is rich in Fe and S and occurs as rounded, massive forms, shows high abundance of tochilinite and variable amounts of cronstedtite and magnetite. Tochilinite shows very strong 002 reflection at 5.4A and very weak 001 reflection at 10.8A, indicating a higher Fe content in octahedral sites. Comparison of the observed X-ray diffraction pattern of type-I PCP with the calculated X-ray patterns of tochilinite in various compositions shows that Fe occupies more than 90% of the octahedral sites in tochilinite in PCP, which is quite different from terrestrial tochilinite. Type-II PCP, which occurs mainly in the matrix as fibrous needle clusters, is composed mainly of cronstedtite with a disordered stacking sequence along the c-axis. The other major component of type-II PCP differs between non-clastic and clastic portions. In the former, it is tochilinite, whereas in the latter, it is a mixed-layer mineral made of cronstedtite and tochilinite layers. Relative abundances of minerals in type-II PCP indicate that the proportion of the mixed-layer mineral increases with a decrease of tochilinite; thus the mixed-layer mineral is presumed to be a secondary alteration product of tochilinite. Therefore, the enrichment of the mixed-layer mineral in type-II PCP in clastic portion suggests that aqueous alteration in the clastic portion has advanced relative to that in the non-clastic portion and that dynamic processes such as mixing and fragmentation of the rocks might have assisted to activate the aqueous alteration reactions. Our experiments reveal that Murchison is a mixture of multiple lithic phases that have experienced different degrees of aqueous alteration and the alteration might have been enhanced by cataclastic effects probably induced by impacts on the meteorite parent body