The study of α-quartz and α-cristobalite ballen in rocks from 16 impact structures (Bosumtwi, Chesapeake Bay, Chicxulub, Dellen, El'gygytgyn, Jänisjärvi, Lappajärvi, Logoisk, Mien, Popigai, Puchezh-Katunki, Ries, Rochechouart, Sääksjärvi, Ternovka, and Wanapitei) shows that ballen silica occurs mainly in impact melt rock and also in suevite, and more rarely in other types of impactites. Ballen α-cristobalite by itself was observed only in samples from the youngest craters studied here (at Bosumtwi and El'gygytgyn), but it occurs in association with α-quartz ballen in impactites from structures with intermediate ages (from ca. 35 to 120 Ma); thus, our observations suggest that α-cristobalite ballen are back-transformed to α-quartz with time. Transmission electron microscope observations show that α-cristobalite and α-quartz ballen have similar microtextures and are formed of several tiny angular crystals with sizes up to ~6 μm. The observation of toasted α-quartz ballen, notably at the Popigai impact structure, further supports the notion that toasting is due to vesicle formation after pressure release, at high post-shock temperatures, and, thus, represents the beginning of quartz breakdown due to heating. Our investigation increases the number of impact structures at which ballen silica has been found to 35.

Abstract The meteorite collection at the Museum für Naturkunde (Museum of Natural History), Berlin, had its beginning in 1781 at the Royal Academy of Mining. Enlarged by donations from, among others, the Russian tsar Alexander I and Alexander von Humboldt, the collection in 1810 was transferred to the Mineralogical Museum of the newly founded University of Berlin. During the directorship of C.S. Weiss and later G. Rose, the private collections of M. Klaproth and E.F.F. Chaldni were acquired, and in 1864 the meteorite collection comprised fragments from 181 of the about 230 known meteorites. Based on studies of these meteorites, Rose proposed a classification scheme in 1863 that is still valid in principle today. He also introduced the terms chondrule, mesosiderite, pallasite, howardite, eucrite, chondrite and chassignite. In 1888 the collection was moved to the new Museum of Natural History and by 1906 the number of meteorites had increased to 500. In the following 60 years the meteorite collection did not receive much attention until G. Hoppe and his successor, H.-J. Bautsch again actively acquired new samples and studied meteorites scientifically. In 1993 Bautsch was followed by D. Stöffler and the study of meteorites became one of the main research interests of the Institute of Mineralogy. Stöffler also appointed a meteorite curator for the first time in the collection's history. As a result of two major acquisitions of Saharan meteorites, and continuous classification work, the number of separate meteorites increased to 2110 at the present time, making the collection both an exceptional historical heritage and a modern research tool.