The Rio do Pires Chondrite: petrographic and mineral chemistry features

Abstract

The Rio do Pires meteorite is a L6 chondrite found in Bahia, Brazil, at an unknown date. It was registered at the Meteoritical Bulletin in 1994, through a simplified analysis required for this procedure, presented by Adrian Brearley, University of New Mexico, USA. This paper aims to broaden the existing data on this meteorite through detailed petrographic, chemical and mineralogical analysis. Such analyses were performed on three polished thin sections and a hand sample, using a petrographic microscope and stereomicroscopic petrographic magnifying glass, electron microprobe (EPMA) and scanning electron microscope (MEV). The petrographic analysis shows the existence of shock veins caused by an impact event of the parent body that originated this meteorite. Approximately 93% of the minerals that compose this rock are transparent, predominating crystals of olivine, pyroxene and plagioclase, in this order. Opaque minerals are represented by Fe-Ni metal grains, troilite, chromite, and whitlockite (merrilite). Olivine is predominantly magnesian (Fa ~ 25%). The pyroxene is the enstatite and the plagioclase is oligoclase. The observed mineral chemistry is similar to that of the well-studied Suizhou meteorite, a chondrite type L6. The matrix shows signs of high re-crystallization and presence of maskelynite, an important indicator of shock, being significant for understanding the evolutionary history of Rio do Pires. The new data indicate a S4 to S5 shock grade for this meteorite, instead of the S6 grade proposed when it was submitted to the Meteoritical Bulletin.