Shocked Quartz

WHAT IS "SHOCKED QUARTZ"?
Phillip Bigelow
No, shocked quartz isn't a pyschologically-distressed rock. It is actually a structurally-altered form of quartz that was created by a sudden application of extremely high pressure.

Shortly after the first atomic bombs were detonated in the late 1940's, American scientists discovered that the quartz sand from the soil in the crater looked different under a microscope than "normal" quartz. Small parallel lines that intersect other parallel lines criss-crossed the face of the grains. These intersecting lines give the impression of fractures in the quartz, but a structural analysis (X-ray diffraction analysis) of these grains showed that the crystalline lattice is only slightly deformed, and it isn't fractured. These microscopic lines (actually, they are planar features that are viewed edge-on) were named "shock lamellae" (lamellae is pronounced "la-MEL-ee").

To visualize this type of deformation, imagine a perfectly vertically stacked deck of playing cards. Now slant the stack by pushing the upper part of the deck a little to the side. This is a rough analogy of what happens when quartz goes through a lattice offset. Planes of atoms in the quartz grain get "shifted" slightly to the side relative to adjacent planes of atoms. These latice offsets cause zones of optical interference in the sand grain which, under a microscope, show up as two or more groups of dark lines that intersect each other. Some grains may contain as many as nine different sets of intersecting shock lamellae!

Scientists have discovered that shocked quartz occurs only in two environments on earth:

•Craters made by nuclear bomb explosions.

•Meteorite impact craters.

Some shock lamellae may be filled with a glassy phase (silica glass). A rare high-pressure silica mineral, called stishovite, is sometimes present in the shocked quartz from the Hell Creek Formation (Bohor et al., 1984). Shocked metaquartzite, shocked microcline, shocked oligoclase, and shocked zircon grains also occur with the shocked quartz (Izett and Bohor, 1986; Kamo and Krogh, 1995).

Can a regular biological microscope be used to view shocked quartz?

A standard biological microscope can be used to see the quartz grains, but to distinguish shocked quartz from normal quartz, you would need to make some modifications to the microscope.
A petrographic microscope is used to identify and study shocked quartz. A petrographic microscope is similar to a standard biological microscope except that it uses polarized light and other optical accessories that help to identify the various minerals. To prepare the sample for viewing under a petrographic microscope, a chip from a rock sample is cemented onto a glass slide. The chip is then ground down so that it is thinner than a human hair, which allows light to pass through the sample. The sample is then placed under the petrographic microscope and viewed under cross-polarized light, which will allow you to distinguish the shocked quartz from the regular quartz.

How is shocked quartz distributed around the Earth? Is it only found near craters?

So, you may ask, how did shocked quartz grains get deposited in the Hell Creek Formation, which is thousands of kilometers away from the Chicxulub impact crater in Mexico? Geologists now believe that the quartz was part of the fine-grained ejecta blanket that was sent high into the stratosphere by the impact (some of this ejecta has been hypothesized to have temporarily gone into orbit! (Melosh, 1990; Robertson et al., 2004). Stratospheric winds can potentially transport silt- and sand-sized sediment grains all over the planet (or, at least, to whereever the winds were blowing). The largest shock-metamorphosed single grains in the impact bed of the Hell Creek Formation are 0.58 mm in diameter, and are "four times larger than the largest shocked grains in K-T boundary sediments elsewhere in the world." (Izett and Bohor, 1986). Grains of 0.58 mm are classified as "coarse sand"! Transportation of coarse sand from the Chicxulub impact site in the Yucatan peninsula of Mexico to what is now Montana and the Dakotas would require extremely high surface winds, or stratospheric or orbital/reentry transport.

To learn more about pressure-shocked sedimentary minerals in the Hell Creek Formation K-T boundary bed and at other localities around the world, see the following references:

•Alvarez, W., P. Claeys, and S.W. Kieffer. 1995. Emplacement of Cretaceous-Tertiary boundary shocked quartz from Chicxulub Crater. Science 269: 930-935.

•Bohor, B.F., W.J. Betterton, and T.E. Krogh. 1993. Impact-shocked zircons: discovery of shock-induced textures reflecting increasing degrees of shock metamorphism. Earth and Planetary Science Letters 119: 419-424.

•Bohor, B.F., E.E. Foord, P.J. Modreski, and D.M. Triplehorn. 1984. Mineralogic Evidence for an impact event at the Cretaceous-Tertiary boundary. Science 224: 867-869.

•Bohor, B.F., P.J. Modreski, and E.E. Foord. 1987. Shocked quartz in the Cretaceous-Tertiary clays: Evidence for a global distribution. Science 236: 705-709.

•Chao, E.C.T., O.B. James, J.A. Minkin, J.A. Boreman, E.D. Jackson, and C.B. Raleigh. 1970. Petrology of unshocked crystalline rocks and evidence of impact metamorphism in Apollo 11 returned lunar sample, in Levinson, A.A., ed., Proceedings of Apollo 11 Lunar Science Conference. Geochimica et Cosmochimica Acta Suppliment I, volume 1, pages 287-314.

•Glass, B.P., and J. Wu. 1993. Coesite and shocked quartz discovered in the Australasian and North American microtektite layers. Geology 21: 435-438.

•Goltrant, O., H. Leroux H., J-C. Doukhan, and P. Cordier. 1992. Formation mechanisms of planar deformation features in naturally shocked quartz. Phys. Earth Planet Inter. 74: 219-240.

•Grieve, R.A.F., V.L. Sharpton, and D. Stoffler. 1990. Shocked minerals and the K/T controversy. EOS Transactions AGU 71: 1792.

•Izett, G.A. 1990. The Cretaceous/Tertiary boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals: Evidence relevant to the K/T boundary impact theory. Geological Society of America Special Paper 249. 100 p.

•Izett, G.A., and Bohor, B.F. 1986. Microstratigraphy of continental sedimentary rocks in the Cretaceous-Tertiary boundary interval in the western interior of North America. Geological Society of America Abstracts with Programs, page 644.

•Kamo, S.L., and T.E. Krogh. 1995. Chicxulub crater source for shocked zircon crystals from the Cretaceous-Tertiary boundary layer, Saskatchewan: Evidence from new U-Pb data. Geology 23: 281-284.

•Krogh, R.E., S.L. Kamo, and B.F. Bohor. 1993. Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejecta. Earth and Planetary Science Letters 119: 424-429.

•Krogh, T.E., S.L. Kamo, V.L. Sharpton, L.E. Marin, and A.R. Hildebrand. 1993. U-Pb ages of single shocked zircons linking distal K/T ejecta to the Chicxulub crater. Nature 366: 731-734.

•Melosh, H. J. 1990. Reentry of fast ejecta: The global effects of large impacts. EOS (Transactions, American Geophysical Union) 71:1429. [Abstract].

•Robertson, D. S., M. C. McKenna, O. B. Toon, S. Hope, and J. A. Lillegraven. 2004. Survival in the first hours of the Cenozoic. Geological Society of America Bulletin 116:760-768.

•Stoffler, D., and F. Langenhorst. 1994. Shock metamorphism of quartz in nature and experiment. 1. Basic observation and theory. Meteoritics 29: 155-181.