Shocked quartz is a form ofquartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), thecrystalline structure of quartz is deformed along planes inside the crystal. These planes, which show up as lines under a microscope, are calledplanar deformation features (PDFs), or shock lamellae.
Shocked quartz was discovered followingunderground nuclear weapons testing, which generated the intense pressures required to alter the quartz lattice.Eugene Shoemaker showed that shocked quartz is also found insidecraters created bymeteor impact, such as theBarringer Crater andChicxulub crater.[1] The presence of shocked quartz supports that such craters were formed by impact, because a volcanic eruption would not generate the required pressure.[2]
Lightning is now known to contribute to the surface record of shocked quartz grains, complicating identification ofhypervelocity impact features.[3]
Shocked quartz is usually associated in nature with two high-pressurepolymorphs ofsilicon dioxide:coesite andstishovite. These polymorphs have a crystal structure different from standard quartz. This structure can be formed only by intense pressure (more than 2gigapascals), but at moderate temperatures. Coesite and stishovite are usually viewed as indicative ofimpact events oreclogite facies metamorphism (ornuclear explosion), but are also found in sediments prone tolightning strikes and infulgurites.[4][3]
Shocked quartz is found worldwide, and occurs in the thinCretaceous–Paleogene boundary layer, which occurs at the contact betweenCretaceous andPaleogene rocks. This is further evidence (in addition toiridium enrichment) that the transition between the two geologic periods was caused by a large impact.[5]
Lightning also generatesplanar deformation features in quartz and is capable of propagating appropriate pressure/temperature gradients in rocks and sediments alike.[6] This very common mechanism may significantly contribute to the accumulation of shocked quartz in the geologic record. Mantle xenoliths and sediments derived from them may contain coesite or stishovite.[7]
Though shocked quartz is only recently recognized,Eugene Shoemaker discovered it prior to its crystallographic description in building stones in theBavarian town ofNördlingen, derived fromshock-metamorphic rocks, such asbreccia andpseudotachylite, ofRies crater.[8][9]
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