Computer simulation of anomalous composition of Mg-Fe plagioclase in meteorite

• Main Author: Yasunori Miura
• Format: Report
• Language: English
• Published: Department of Mineralogical Science and Geology, Faculty of Science, Yamaguchi University 1984
• Subjects: The ''Y''; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=1726; http://id.nii.ac.jp/1291/00001726/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=1726&item_no=1&attribute_id=18&file_no=1

The chemical anomalies of meteoritic plagioclase which appear also in lunar plagioclase, result in the difference between An=100・Ca/(K+Na+Ca) and An(Al, Si)=(300・(Al/Si)-100)/(1+Al/Si) caused by the various types of substitution which are dealt with computer simulation. Almost all average data are plotted close to the substitution vectors of Fe and Mg cations into T or M sites. Fe-Mg plagioclases are crystallized as Mg-rich plagioclases (An_<67-91>) in C3 or CV3 chondrites, whereas Fe-rich plagioclases (An_<1-14>) are predominantly crystallized in H6,L6 or LL6 chondrites. The maximum contents of Mg and Fe are 0.343 (per formula unit in O=8) in Allende (CV3) and 0.126 in Plainview (H5) chondrites, respectively, though maskelynite has much more Mg and Fe contents with excess total cations. Chemical heterogeneity of plagioclase in eight chondrites and Juvinas eucrite can be shown by the number of regions in An-content vs. Fe/(Fe+Mg) diagram; that is, three distinct regions of Willard (L6), ALH-769,75 (L6) and Plainview (H5); two regions of Holbrook (L6), Y-74640,81 (H6), Allende (CV3) and ALH-77307,85 (C3); and single region of Y-75258,97 (LL6). Wide variation of Fe/(Fe+Mg) in Juvinas with much more Fe- and Mg-substitutions indicates a mixture of various parent materials. The different values of Fe/(Fe+Mg) between chondrule and matrix with a nearly constant An-content in the Y-75135,93 (L5 (67%)) chondrite suggest that chemical heterogeneity of plagioclase might have occurred in the formation of the chondrite.