Shocked quartz
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Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz will be deformed along planes inside the crystal. These planes show up as lines under a microscope, which are called shock lamellae.
Discovery
Shocked quartz was discovered after underground nuclear bomb testing, which caused the intense pressures required to form shocked quartz. Eugene Shoemaker showed that shocked quartz also is found inside of craters, such as the Barringer Crater. The presence of shocked quartz proved that these craters were formed by an impact: a volcano would not generate the pressure required.
Prevalence
Shocked quartz is also found worldwide, in a thin layer at the boundary between Cretaceous and Tertiary rocks. This is further evidence (in addition to iridium enrichment) that the transition between the two geological eras was caused by a large impact.
Structure
Shocked quartz is associated with two pressure polymorphs of silicon dioxide: coesite and stishovite. These polymorphs have a different molecular structure than standard quartz. Again, this structure can only be formed by intense pressure, but moderate temperatures. High temperatures would anneal the quartz back to its standard form. Coesite and stishovite are thus also indicative of impact (or nuclear explosion).
External links and sources
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