Studies of Brazilian Meteorites Xiv. Mineralogy, Petrology, and Chemistry of the Conquista, Minas Gerais, Chondrite

Abstract

The Conquista chondrite consists of major olivine, low‐Ca pyroxene (both ortho‐ and twinned clino‐), troilite and metallic nickel‐iron; minor glassy to microcrystalline material and pigeonite; and accessory chromite, high‐Ca clinopyroxene and hydrous ferric oxides that formed by terrestrial weathering of metallic nickel‐iron. Results of microscopic, electron microprobe, and bulk chemical studies, particularly the compositions of olivine (Fa17.2) and low‐Ca pyroxene (Fs15.4); the contents of metallic nickel‐iron (18.5%) and total iron (25.83%); and the ratios of Fe°/Fetotal (0.64), Fe°/Ni° (9.59) and Fetotal/SiO2 (0.69) indicate H‐group classification. The pronounced, well‐developed chondritic texture; the slight compositional variations in constituent phases; the high Ca contents of pyroxene and the presence of pigeonite, glassy to microcrystalline interstitial material rich in alkalis and SiO2, and of twinned low‐Ca clinopyroxene suggest that Conquista is of petrologic type 4.

Analysis of ordinary chondrites using powder X-ray diffraction: 1. Modal mineral abundances

“…3). Pigeonite (Wo 5-20 ) is compositionally and structurally similar to clinoenstatite, the monoclinic form of low-Ca pyroxene, and has been identified in ordinary chondrites (primarily in type 3) in previous studies (e.g., Keil 1978;McSween and Patchen 1989). A pigeonite standard was not available at the time this study was complete, so any pigeonite present in a sample would be fitted either as low-Ca pyroxene (using the enstatite standard) or as high-Ca pyroxene (using diopside).…”