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

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

Evaluation of Lindsley pyroxene thermometer for chondrites

第6回極域科学シンポジウム[OA] 南極隕石11月16日（月）　国立国語研究所 2階 講

Yamato-86789: A heated CM-like carbonaceous chondrite

We have studied the mineralogy and petrology of the Yamato (Y)-86789 meteorite and concluded that this meteorite is a new member of heated carbonaceous chondrites with CI-CM affinities. Y-86789 contains 14vol% of translucent chondrules which mainly consist of fibrous phyllosilicate-like materials. The high abundance of the phyllosilicate-like minerals in chondrules suggests severe aqueous alteration. Major parts of the matrix are also composed of the phyllosilicate-like materials. The phyllosilicate-like materials have compositions of a mixture of serpentine and saponite. However, the phyllosilicate-like materials show consistently high analytical totals; thus, they are dehydrated. X-ray diffraction measurements of the phyllosilicate-like materials reveal that they are mainly composed of olivine. Diffractions from serpentine and saponite are absent from the X-ray powder patterns. These results suggest that Y-86789 has experienced heating and the phyllosilicate minerals which were once present in Y-86789 were dehydrated to from anhydrous materials such as olivine. Y-86789 has textural, mineralogical, and compositional characteristics similar to those of Y-86720,which is one of the three unusual Antarctic carbonaceous chondrites with CI-CM affinities that show evidence of dehydration. Chondrules are completely replaced by the phyllosilicate-like materials and are surrounded by fine-grained rims. Some chondrules contain Ca-carbonates. There is an abundance of troilite in the matrix, but magnetite and PCP, which are common in CM chondrites, are absent. Some large troilite grains have a euhedral lath-like morphology. The bulk chemical composition of Y-86789 is also similar to that of Y-86720. On the basis of these similarities, we propose that Y-86789 and Y-86720 are paired meteorites

General characterization of Antarctic micrometeorites collected by the 39th Japanese Antarctic Research Expedition: Consortium studies of JARE AMMs (III)

From November 1998 to January 1999,the 39th Japanese Antarctic Research Expedition (JARE-39) undertook Japanese first large-scale collection of Antarctic micrometeorites (AMMs), with sizes larger than 10μm, at the Meteorite Ice Field around the Yamato Mountains in Antarctica (at three different locations, for a total of 24 collection sites). The number of collected AMMs larger than 40μm is estimated to be about 5000. Here we present the general characterization (i.e., micro-morphology and surface chemical composition using SEM/EDS) of ∿810 AMMs chosen from 5 of the 24 sites. Additionally, the mineral composition of 61 out of 810 AMMs was determined by Synchrotron X-ray radiation. Preliminary results on mineralogical and chemical compositions show similarities with that of previous studies, even though a pronounced alteration of some AMMs is noticed. A correlation is found between the Mg/Si ratio at the sample\u27s surfaces of unmelted AMMs and the age of snow/ice in which the AMMs are embedded