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 (a pulse) of extremely high pressure.

Shortly after the first atomic bombs were tested in the 1940's, American scientists discovered that, underneath the heat-fused silica glass that lined the bomb crater was a layer of quartz sand that looked different when viewed under a microscope than did "normal" quartz. Small parallel lines that intersect other parallel lines criss-crossed the face of the sand grains (Fig. 1). These intersecting lines give the impression of fractures in the quartz, but X-ray diffraction analysis of these grains revealed 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").
Scientists subsequently found shocked quartz grains at other locations around the world.


Fig. 1

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. As the compression wave from the blast passes through the sand grains, planes of atoms in the quartz get "shifted" slightly to the side relative to adjacent planes of atoms. These latice offsets create 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!

Outside of a laboratory, shocked quartz occurs in two environments:

Craters made by nuclear bomb explosions.

Meteorite impact craters.

To learn about experimental creation of shocked quartz, see Stoffler and Langenhorst (1994) (see references below).

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). As was the case with quartz, these minerals were not created by the shockwave. They were preexisting minerals that were subsequently altered by the shockwave.

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 have to make modifications to the microscope.
A petrographic microscope is used to identify and study shocked quartz. A petrographic microscope, commonly used in geology, is similar to a standard biological microscope except that it uses polarized light and other optical accessories that help to identify various minerals. To prepare the sample for viewing under a petrographic microscope, a chip from a rock or from a soil layer is cemented onto a glass slide. The chip is then ground down so that its mineral constituents are thinner than the width of a human hair, which allows light to pass through the sample. The slide is placed under a petrographic microscope and viewed under cross-polarized light, which is transmitted through the slide from below. It is the crossed-polarized illumination that allows the worker 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 a fine-grained ejecta blanket that was sent high into the stratosphere by the impact. Some of this ejecta has been hypothesized to have gone suborbital or possibly into orbit! (Melosh, 1990; Robertson et al., 2004). It is believed that when this material eventually returned to the atmosphere, stratospheric winds transported clay-, silt- and sand-sized sediment grains all over the planet (or, at least, to whereever those 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.

Hell Creek Life © 1997-2010 Phillip Bigelow
Revised 3/12/2010