Composition and structural substitution of meteoritic plagioclases (I)

• Main Authors: Yasunori Miura, Takeshi Tomisaka
• Format: Report
• Language: English
• Published: Department of Mineralogical Science and Geology, Faculty of Science, Yamaguchi University 1984
• Subjects: Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=1725; http://id.nii.ac.jp/1291/00001725/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=1725&item_no=1&attribute_id=18&file_no=1

Significant amounts of iron, magnesium and Na (and/or Ca) vacancy are found in meteoritic plagioclases of the Holbrook (L6), Willard (L6), Plainview (H5), ALH-77307,85 (C3) and Allende chondrites and the Juvinas eucrite. Seven components are used in this study, six of which were used already for lunar plagioclases; that is, KAl(Si)_3O_8,NaAl(Si)_3O_8,Ca(Al)_2(Si)_2O_8,(Fe, Mg)(Al)_2(Si)_2O_8,Ca(Fe, Mg)(Al)_3O_8,[ ](Si)_4O_8 and M(MT)O_8,where [ ] means Na and/or Ca vacancy, M=K, Na, Ca, and T=Al, Si. The amounts of components Ca(Fe, Mg)(Si)_3O_8 and (Fe, Mg)(Al)_2(Si)_2O_8 significantly decrease with ordinary An-content. The highest contents of components Ca(Fe, Mg)(Si)_3O_8 and (Fe, Mg)(Al)_2(Si)_2O_8 in this study are obtained as 29(mol%) in the ALH-77307,85 (C3) and 11(mol%) in the Allende (CV3) chondrites, respectively. The largest values of components [ ](Si)_4O_8 and M (MT)O_8 are found in this study as 8(mol%) in the Juvinas eucrite and 12(mol%) in the Allende (CV3) chondrite, respectively. Lower Fe/(Fe+Mg) ratio is characteristic of chondrule in meteorite. The Plainview chondrite showing unique behavior of a considerable variation of the components may indicate a mixture of various parent materials. Small amounts of excess M cations (i.e. M(MT)O_8) are observed mainly in the exsolved regions of plagioclase crystal in meteorite (esp. the Juvinas eucrite and the Holbrook chondrite). A plot of each end-member abundance vs. Ca(Al)_2(Si)_2O_8 may be considered a useful indicator of the formation process of the meteorite.