Shock metamorphism of some minerals: Basic introduction and microstructural observations


Authors: Langenhorst F

Published in: Bulletin of Geosciences, volume 77, issue 4; pages: 265 - 282; Received 30 July 2002; Accepted in revised form 2 November 2002;

Keywords: impact features, shock metamorphism, shock waves, minerals, crystal structure, defects, experimental study,

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Minerals show a unique behaviour when subjected to shock waves. The ultradynamic loading to high pressures and temperatures causes deformation, transformation and decomposition phenomena in minerals that are unequivocal indicators of impact events. This paper introduces into the basics of shock compression, required to understand the formation and experimental calibration of these shock effects in minerals, and particularly focuses on the recent advances in the field of shock metamorphism achieved by the application of transmission electron microscopy (TEM). TEM studies underline that the way minerals respond to shock compression largely depends on their crystal structures and chemical compositions, as is illustrated here on the basis of four minerals: quartz, olivine, graphite and calcite.
The crystal structure of a mineral exerts an important control on the shock-induced deformation phenomena, comprising one- to two-dimensional lattice defects, such as dislocations, mechanical twins, planar fractures, and amorphous planar deformation lamellae. For example, dislocations cannot be activated in quartz due to the strong covalent bonding, whereas the island silicate olivine easily deforms by dislocation glide.
Transformation phenomena include phase transitions to (diaplectic) glass and/or high-pressure polymorphs. TEM studies reveal that high-pressure polymorphs such as coesite, stishovite and ringwoodite are liquidus phases, which form upon decompression by crystallization from high-pressure melts. The graphite-to-diamond transition is however a rare example for a solid-state transformation, taking place by a martensitic shear mechanism.
Shock-induced decomposition reactions are typical of volatile-bearing minerals and liberate toxic gases that, in case of large impacts, may affect Earth's climate. Shock experiments show that degassing of calcite does not take place under high pressure but can massively occur after decompression if the post-shock temperature is sufficiently high. A recombination reaction happens however if CaO and CO2 are not physically separated.