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Falko Langenhorst a b booster laser d D high explosive flyer plate spacing ring cover layer specimen Fe cylinder vacuum chamber lens mirrors sample steel blocks Fig. 4. Two different experimental designs to produce shock waves: (a) High-explosive device used at the Ernst-Mach-Institut, Efringen-Kirchen, Germany (modified after Langenhorst and Deutsch 1994) and (b) a laser irradiation setup with the sample being clamped into an Al block. The laser is focused on a thin Al foil, acting as flyer plate (modified after Langenhorst et al. 1999a and 2002a). minerals (e.g. French and Short 1968, Roddy et al. 1977, Asay and Shahinpoor 1993, Davison et al. 2002). Most experiments related to cratering mechanics employ spherical projectiles that produce and excavate some crater in an infinite half space medium via a spherically expanding shock wave. In contrast, most experiments related to shock metamorphism employ flat-plate projectiles that drive a planar shock wave through a similarly planar target; the thickness of the latter is typically less than projectile thickness to avoid measurable pressure decay across the sample (Barker et al. 1993). The target is usually a metallic container, encapsulating the sample in a fashion to allow its partial or complete recovery. The experiments may differ widely in the type of accelerating system (Fig. 4): powder and light-gas guns, high-explosive charges, electric discharge guns, and laser irradiation techniques have been used and tested to successfully reproduce shock effects in minerals (e.g. Milton and DeCarli 1963, Müller and Hornemann 1969, Gratz et al. 1992, Stöffler and Langenhorst 1994, Langenhorst et al. 1999a, 2002a). The principle of the electric gun is to vaporise a thin metal foil by rapid electric discharge of a capacitor. The high electrical current leads to the instantaneous vaporisation of the foil and the production of a shock wave (Langenhorst et al. 2002a). Laser irradiation experiments can be performed either with or without a projectile. In the latter case, the beam is directly focused on the sample surface (Langenhorst et al. 2002a), whereby the absorption of the laser energy generates rapidly exploding plasma that subsequently induces a shock wave. Such plasma techniques are capable to produce the highest shock pressures with an unbelievable world record of 750 Mbar (Cauble et al. 1993). However, higher shock pressures are commonly at the expense of shorter shock durations and smaller sample volumes. This is because higher impact velocities and hence higher shock pressures can only be achieved by reducing the size and weight of the projectile. Typical pressure pulses in laser irradiation, electric discharge and highexplosive experiments are approximately 1 ns, 10 ns, and 1 µs with shocked sample volumes on the order of 0.1, 1, and 100 mm 3 , respectively (Langenhorst et al. 2002a). To determine pressures in shock experiments it is necessary to measure the velocities associated with the shock waves (projectile v, particle u and shock U velocities), using electrical pin contact, optical interferometry (VISAR) or similar techniques (Hornemann and Müller 1971, Barker et al. 1993). It is then possible to calculate the pressures temperature (°C) 3000 2500 2000 1500 1000 500 quartz eclogite liquid coesite release paths stishovite porous sandstone 0 1 10 100 shock pressure (GPa) Fig. 5. Phase diagram depicting the pressure-temperature conditions reached in quartz, olivine (solid line), and porous sandstone (long dashed line) by shock compression (data after Wackerle 1962, Kieffer et al. 1976, and Holland and Ahrens 1997). The release paths hold for porous quartz, which first melts on loading and then solidifies on cooling as coesite or stishovite. The equilibrium phase boundaries between quartz, coesite, stishovite and liquid are drawn as dashed lines. The grey pressure-temperature field represents the conditions reached in regional metamorphism; the solid line within this field depicts the pressure-temperature path of a diamond-bearing gneiss (Stöckhert et al. 2001). quartz olivine 268
Shock metamorphism of some minerals: Basic introduction and microstructural observations by aid of the known Hugoniot relations of the materials involved (Marsh 1980). Thus, the determination of shock pressure is fundamentally different from the approach used in static compression techniques (e.g. piston cylinder, multi-anvil or diamond anvil cell experiments), in which the pressures are calibrated via properties and/or phase transformations of reference materials (e.g. Rubie 1999). The determination of temperatures is a less precise and more difficult undertaking. So far, pyrospectrometric techniques have been used to measure both shock and postshock temperatures (e.g. Raikes and Ahrens 1979, Holland and Ahrens 1997; Fig. 5). The knowledge of temperatures is however of fundamental importance for the determination of the melting temperatures of materials at very high pressures. For example, the precise measurement of the melting curve of iron enables the temperature at the boundary between Earth’s inner (solid) and outer (liquid) core to be determined (Brown and McQueen 1986), an important fix point of the geotherm. calcite graphite olivine quartz shock effects mechanical twins PF PDF mosaicism diaplectic glass \ lechatelierite stishovite \ coesite dislocations PF and PDF mosaicism recrystallisation and staining ringwoodite kink bands diamond mechanical twins dislocations decomposition and recombination products shock pressure (GPa) 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Shock effects in minerals Fig. 6. Compilation of the pressure intervals over which certain shock effects are formed in quartz, olivine, graphite and calcite (modified after Stöffler and Langenhorst 1994 and Langenhorst and Deutsch 1998). The diagram is based on shock experiments with non-porous samples. Shock-induced physical and chemical changes in minerals are collectively called shock effects or shock-metamorphic effects. This term is relatively broad and covers any type of shock-induced change, such as formation of lattice defects, phase transformations, decomposition reactions and resultant changes in physical and chemical properties. A great diversity of natural shock effects has been described on the basis of spectroscopic techniques, X-ray diffraction, and optical microscopy (Hörz and Quaide 1973, Stöffler 1972 and 1974, Schneider 1978, Boslough et al. 1989). The physical nature of some of the effects was not completely clear and others were not even known until TEM was used to characterize the mineralogical effects at the nanometer scale. Based on TEM observations, the following simple subdivision of shock effects and processes occurring during shock compression and decompression has been proposed (Langenhorst and Deutsch 1998): (1) Deformation: formation of dislocations, planar microstructures (planar fractures and planar deformation features), mechanical twins, kink bands, and mosaicism (2) Phase transformations into high-pressure phases and diaplectic glass (3) Decomposition into a solid residue and a gaseous phase (4) Melting and vaporisation of entire mineral (subsequently quenched as shock-fused glass or polycrystalline aggregates) This simple list does not contain the post-shock thermal effects forming distinctly after the impact. In a wide sense, these effects may also be regarded as shock indicators, although they are not primary effects of shock compression and decompression and simply result from the high temperatures prevailing after an impact event. Examples of diagnostic post-shock effects are the formation of Ballen quartz and checkerboard feldspars in impact melts (Carstens 1975, Bischoff and Stöffler 1984) or the crystallization of highly oxidised Ni-rich spinels (magnesioferrites) in microtektites (Robin et al. 1992). We will focus here exclusively on the shock effects listed above. The deformation and transformation effects largely form during the compression phase of shock waves, whereas decomposition, melting and vaporisation are temperature dominated processes, which take place during the decompression phase when pressure declines to a larger extent than temperature. There is no single mineral that shows all of these effects (Fig. 6). The response of a mineral to shock compression largely depends on its crystal structure and composition. A mineral with strong three-dimensional covalent bonding between polyhedra in the crystal structure can, for example, not respond by dislocation glide. Also, minerals without volatile components cannot respond by decomposition reactions such as dehydration and decarbonation. To highlight these aspects and to illustrate the various types of shock effects in minerals we will concentrate on four minerals, differing largely in terms of crystal structure and composition. Quartz Among the rock-forming minerals, quartz (SiO 2 ) displays the widest variety of shock effects. It possesses a crystal structure, consisting of three-dimensionally linked, corner-shared SiO 4 -tetrahedra with strong covalent bonding. In the low-pressure regime (< 30 GPa), quartz can 269
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Page 4 and 5: Asteroids: Their composition and im
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Page 14 and 15: The recognition of terrestrial impa
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Page 36 and 37: Falko Langenhorst Acknowledgements.
Page 38 and 39: Falko Langenhorst Langenhorst F., S
Page 40: Falko Langenhorst M A West K H K N

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