Ansgar Greshake

The primitive matrix components of the unique carbonaceous chondrite Acfer 094: a TEM study.

The mineralogical and chemical characteristics of the fine-grained matrix (< or = 3 micrometers) of the unique primitive carbonaceous chondrite Acfer 094 have been investigated in detail by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). Generally, the fine-grained matrix represents a highly unequilibrated assemblage of an amorphous material, small forsteritic olivines (200-300 nm), low Ca-pyroxenes (300-400 nm), and Fe,Ni-sulfides (100-300 nm). The matrix is basically unaffected by secondary processes. Only minor amounts of serpentine and ferrihydrite, as products of hydrous alteration, are present. Texturally, the amorphous material acts as a groundmass to olivines, pyroxenes, and sulfides, mostly exhibiting rounded or elongated morphologies. Only very few clastic mineral grains have been found. The texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment. As such, the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites. The non-clastic matrix olivines (Fo98-99) and pyroxenes (En97-100) are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material. The formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons. Rapid cooling caused the observed intergrowths of clino/orthoenstatite in the Mg-rich matrix pyroxenes. Although some similarities exist comparing the fine-grained matrix of Acfer 094 with the matrices of the unequilibrated CO3 chondrite ALHA77307 and the unique type 3 chondrite Kakangari, Acfer 094 remains unique. Since it contains the highest measured concentrations of circumstellar SiC and the second highest of diamond (highest is Orgueil), it seems reasonable to suggested that at least parts of the amorphous material in the fine-grained matrix may be of circumstellar origin.

Pulse-Heating of Fragments from Orgueil (CI): Simulation of Atmospheric Entry Heating of Micrometeorites

Pulse-heating experiments on 100 μm-sized fragments of Orgueil (CI) were carried out in order to simulate the atmospheric entry heating of microme-teorites. The fragments were heated in a furnace in air to 700–1250°C for 10 to 60 s and allowed to cool in air. Chemical and mineralogical changes have been investigated by Proton-Induced-X-Ray-Emission (PIXE) and transmission electron microscopy, respectively. In the experiments loss of the volatile elements S, Se, Ga, Ge and Zn in times applicable for the entry heating of micrometeorites were observed. Sulphur and Se are already lost from samples heated to 700°C. Gallium, Ge and Zn losses are obvious for samples heated to 1100°C. TEM investigations have shown that the transformation of phyllosilicates to olivine and enstatite starts already at 700°C/20 s. At 800°C/20 s all phyllosilicates are replaced by olivine and enstatite.

Structural and chemical modifications of microsamples induced during PIXE analysis

Abstract Mass losses caused by proton bombardment in the course of PIXE analyses have been frequently reported in the case of biological samples but only very little information on the damage of rock targets is available. With the Heidelberg Proton Microprobe we scanned seven ≈100 μm fragments of a sheet silicate using various proton currents and doses that — after the PIXE analysis — were analysed with transmission electron microscopy (TEM). The effects of the proton bombardment range from intact lattice to a totally amorphous structure. Our results demonstrate the necessity to limit the proton current and dose to below certain values that have to be experimentally determined for a given sheet silicte in order to maintain intactness. Time resolved PIXE measurements of the alkali (Na, K) contents of feldspar and alkali glass fragments do not result in volatilisation losses which often are observed in SEM and EMPA studies.

History of the meteorite collection at the Mseum für Naturkunde, Berlin

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.