Author: Friedrich Frey

Abstract
Apart from lunar samples which became available after the Apollo missions at the end of the sixties, all extraterrestric material is due to meteoritic falls. Among the numerous meteorites there are around 30 pieces which are identified to be from planet mars, some others, such as eucrites or howardites, are of asteroidal origin. The meteoritic samples contain crystalline phases, such as pyroxenes or feldspars, which belong to the most abundant mineral (silicate) phases of the earth crust, and also glassy phases. Meteorites are therefore messengers from the space. A study of phase ratios, the structures and microstructures of the crystals, textures, and intergrowth structures may help to gain insight into the prior history of the meteorites, in particular into the rock forming temperature and pressure conditions of the host planet, of the impact event at the planet, about cooling rates, and also on other temperature-time dependent phenomena.
X-ray (synchrotron) experiments carried out on μm-sized pyroxene crystals from martian meteorites revealed cooling rates via an analysis of the cationic distribution. Neutron powder work carried out at SPODI/FRMII allowed for a determination of the augite/pigeonite phase ratio indicating the exsolution history. As larger single crystalline specimens are typically highly distorted, the cationic distribution, e.g. Fe/Mn/Mg or Si/Al, can only be revealed by neutron powder work. A combined neutron and x-ray diffraction analysis of microstructures via careful analysis of diffuse phenomena may help to understand the “cosmic attacks” on the material. In-situ high-temperature neutron work seems to be promising under the aspect of calibration which is necessary for geothermometry. Ex-situ annealing experiments should be done with “larger” samples which may be investigated by neutrons. Textural studies of the intergrowth of crystalline and glassy phases, (e.g. “maskelynite”) store a considerable amount of information on the diaplectic glass formation during shock events. Summarizing, there is a wide field of interest which could be covered by neutron (diffraction) methods, and (almost) nothing is done so far.