United States Meteorite Impact Craters

This is a petrographic microscope, which is also sometimes referred to as a polarizing microscope.  In addition to the various components of an ordinary microscope, it has a number of unique features that enable it to be used to analyze rocks and minerals.  The simplest of these are a pair of polarizing plates and a rotating stage.

Below is a thin section of a rock from a possible impact crater.  A thin section is a microscopic slide that is prepared for mineralogical and petrological analysis with the petrographic microscope.  Thin sections are prepared by gluing a cut and finely sanded surface of a rock to a thin piece of glass (microscope slide), and then grinding away the excess rock until there is only a thin, translucent layer remaining.  The thickness of the layer is optimally about 30 microns (micron is slang for micrometer), or about 30 one-thousandths of a millimeter.  This thickness allows light to pass through the slide in a way that produces useful color (and other) distinctions when viewed through the microscope.

The petrographic microscope is not the only instrument that is used to view and analyze very small scale changes that are produced in rocks by hypervelocity impacts, but it is by far the simplest and most accessible.  With it, an investigator can identify planar fractures (PFs), planar deformation features (PDFs), microtwinning in calcite, kink banding in mica, and many other suggestive or diagnostic features.

Quality varies greatly in these instruments, however, and it requires a relatively decent system with a good quality camera to gather and communicate the necessary evidence to confirm an impact crater.

Planar Fractures (PFs) and Planar Deformation Features in Quartz (PDFs)

French B. M. and Short N. M., editors. 1968. Shock metamorphism of natural materials. Baltimore: Mono Book Corporation. 644 p.

Engelhardt W. V., Bertsch W. 1969. Shock induced planar deformation structures in quartz from the Ries crater, Germany. Contributions to Mineralogy and Petrology 20(3)203-234.

Ferriere L., Morrow J. R., Amgaa T., Koeberl C. 2009. Meteoritics & Planetary Science 44(6)925-940.

IMPACT CRATERING: AN OVERVIEW OF MINERALOGICAL AND GEOCHEMICAL ASPECTS at
Koeberl, C., 1997, Impact cratering: The mineralogical and geochemical evidence. In: Proceedings, "The Ames Structure and Similar Features", ed. K. Johnson and J. Campbell, Oklahoma Geological Survey Circular 100, 30-54.