¹Ai-Cheng Zhang et al. (>10)
American Mineralogist 104, 150-157 Link to Article [https://doi.org/10.2138/am-2019-6597]
¹State Key Laboratory for Mineral Deposits Research and School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
Copyright: The Mineralogical Society of America

Crystallization is one of the most fundamental processes for both solid inorganic and organic materials in nature. The classical crystallization model mainly involves the monomer-by-monomer addition of simple chemical species. Recently, nanoparticle attachment has been realized as an important mechanism of crystallization in comparatively low-temperature aqueous natural and synthetic systems. However, no evidence of crystallization by particle attachment has been reported in petrologically important melts. In this study, we described spherical (Mg,Fe)-oxides with a protrusion surface in a shock-induced melt pocket from the martian meteorite Northwest Africa 7755. Transmission electron microscopic observations demonstrate that the (Mg,Fe)-oxides are structure-coherent intergrowth of ferropericlase and magnesioferrite. The magnesioferrite is mainly present adjacent to the interface between (Mg,Fe)-oxides spherules and surrounding silicate glass, but not in direct contact with the silicate glass. Thermodynamic and kinetic considerations suggest that development of the spherical (Mg,Fe)-oxides can be best interpreted with crystallization by particle attachment and subsequent Ostwald ripening. This indicates that crystallization by particle attachment can also take place in high-temperature melts and has potential implications for understanding the nucleation and growth of early-stage crystals in high-temperature melts, such as chondrules in the solar nebula, erupted volcanic melts, and probably even intrusive magmas.

^1,2Wolf Uwe Reimold, ³Alvaro Penteado Crósta, ^2,4Maximilian Hasch, ^2,5Astrid Kowitz, ¹Natalia Hauser,⁶Joana Paula Sanchez, ⁷Luiz Sergio Amarante Simões, ^2,3Grace Juliana de Oliveira, ²Patrice T. Zaag
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13233]
¹Geochronology Laboratory, Institute of Geosciences, University of Brasilia, , Brasilia, DF, CEP 70910 900 Brazil
²Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, , 10115 Berlin, Germany
³ Institute, State University of Campinas, UNICAMP, , Campinas, SP, Brazil
⁴ of Geosciences, University of the Witwatersrand, , Johannesburg, 2050 South Africa
⁵Bundesanstalt für Materialforschung und –prüfung (BAM), , 12205 Berlin, Germany
⁶ of Science and Technology, Goiás Federal University, , Goiânia, GO, Brazil
⁷Department of Petrology and Metallogeny, São Paulo State University, , Rio Claro, SP, Brazil
Published by arrangement with John Wiley & Sons

Cerro do Jarau is a conspicuous, circular morpho‐structural feature in Rio Grande do Sul State (Brazil), with a central elevated core in the otherwise flat “Pampas” terrain typical for the border regions between Brazil and Uruguay. The structure has a diameter of approximately 13.5 km. It is centered at 30^o12′S and 56^o32′W and was formed on basaltic flows of the Cretaceous Serra Geral Formation, which is part of the Paraná‐Etendeka Large Igneous Province (LIP), and in sandstones of the Botucatu and Guará formations. The structure was first spotted on aerial photographs in the 1960s. Ever since, its origin has been debated, sometimes in terms of an endogenous (igneous) origin, sometimes as the result of an exogenous (meteorite impact) event. In recent years, a number of studies have been conducted in order to investigate its nature and origin. Although the results have indicated a possible impact origin, no conclusive evidence could be produced. The interpretation of an impact origin was mostly based on the morphological characteristics of the structure; geophysical data; as well as the occurrence of different breccia types; extensive deformation/silicification of the rocks within the structure, in particular the sandstones; and also on the widespread occurrence of low‐pressure deformation features, including some planar fractures (PFs). A detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with PFs and feather features (FFs), as well as a number of quartz grains exhibiting planar deformation features (PDFs). While most of these latter grains only carry a single set of PDFs, we have observed several with two sets, and one grain with three sets of PDFs. Consequently, we here propose Cerro do Jarau as the seventh confirmed impact structure in Brazil. Cerro do Jarau, together with Vargeão Dome (Santa Catalina state) and Vista Alegre (Paraná State), is one of very few impact structures on Earth formed in basaltic rocks.