What this page contains: This is one of a group of several pages dedicated to communicating the nature of diagnostic evidence for impact crater identification and the specific tools and techniques used in this science. This page addresses planar fractures, planar deformation features, mosaicism, and the indexing of planar deformation features. If you are new to impact crater science, you might want to start by reading [Crater Identification] and [What Makes a Confirmed Crater?] before returning to this and other specific topic pages. Please note that this website is perpetually under construction in an ongoing effort to make it more understandable and more useful.
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)
PFs are basically repeating cleavage of quartz grains along crystallographic axis. Quartz is a tough mineral. It does not normally cleave. Instead, when subjected to stresses, it will typically crush or fracture along a conchoidal trajectory. If you are unfamiliar with conchoidal fracture, do a google image search for the term. When subjected to very high and instantaneous levels of stress associated with a shock front from a large meteorite impact, quartz breaks differently; it cleaves along crystallographic axes. These axes are determined by the moelcular structure of the material. These fractures are called planar fractures (PFs). At even higher levels of stress, planar deformation features (PDFs), will form along these same rational crystal planes. PDFs aren't exactly fractures. Instead of cleaving, the quartz changes to very thin layers of glass along would-be cleavage planes. PDFs form very close together and can be difficult to resolve as individual lines at less than 100x magnification. Quartz grains in thin section will also sometimes take on a slightly 'toasted' appearance when PDFs are present. PFs are individually more distinct, wider, and smaller in number than PDFs. They also generally extent to the edge of the quartz grain, while PDF do not necessarily do so.
PDFs are slso called shock lamellae or planar lamellae in earlier papers. More recent work typically uses Planar Deformation Features (PDF), and distinguishes these from Planar Fractures (PF), which can form at lower levels of stress.
Above: 2 sets of planar fractures (PFs) in a quartz grain. This grain also showed very strong mosaicism in extinction. When working with research thin sections, bear in mind that most examples do not look as pretty as the ones that we choose for publications or for websites.
Above: PDFs are not as easy to visually resolve with a petrographic microscope. The individual PDFs are both much thinner and much more closely spaced than planar fractures. In aggregate, when viewed through a microscope at 100x to 400x magnification, they appear as a distinct criss-crossed brush pattern, most often in two or three directions (though 1 to 5 sets are possible). It takes a pretty good microscope and camera to get a good picture of PDFs.
How good are they as impact evidence?
PDFs - A published record of PDFs in quartz associated with a clearly identified structure or rock unit is typically considered to be adequate unambiguous evidence of a crater forming meteorite impact event.
PFs - Considered with due diligence to understanding their abundance within the context of surrounding rocks and soil, PFs, particularly when present in more than one set within grains, may serve as a strong indicator of an impact origin.
There are several crystallographic axes along which PFs and PDFs may form. Some directions of cleavage in quartz are easier to 'break' than others. PFs and PDFs will form in the easiest directions first, and will only develop parallel to successively more difficult planes with increasing shock pressure. PFs and PDFs formed along different planes are called 'sets.' By recording which sets have formed in quarz grains, a researcher may get a sense of specific shock pressure. In this way, PDFs may serve as a shock barometer. The process of cataloging and recording PDFs and their abundance along particular axes is called indexing.
Planar Deformation Features are identifiable in petrographic thin section, but a researcher must have a somewhat less accessible universal stage and significant training and skill in order to accomplish indexing them.
It is probably preferable that PDFs be indexed in order to be published as a absolutely definitive evidence of an impact, but the vast majority of impacts thus far cataloged on earth have not had PDFs indexed for practical reasons. Universal (spindel) microscope stages are uncommon, and most institutions do not have one, and indexing PDFs also requires knowledge and skill that is challenging to acquire. Nevertheless, its a worthwhile thing to do if possible. Indexing PDFs not only acts as a shock barometer, establishing a specific range of shock pressure to which the grain was subjected, it also confirms that the features that one is observing and reporting are oriented parallel to meaningful crystallographic planes, significantly reducing the possibility of misidentification.
A discussion of the application of the universal stage to the specific task of indexing PDFs is found at the bottom of Langenhorst F., 2002, Shock metamorphism of some minerals: Basic introduction and microstructural observations, Bulletin of the Czech Geological Survey, Vol. 77, No. 4, 265–282. The document is at: http://www.geology.cz/bulletin/fulltext/03langenhorstfinal.pdf
Other key references associated with the process and meterials include:
Stöffler, D. and Langenhorst, F. (1994), Shock metamorphism of quartz in nature and experiment: I. Basic observation and theory. Meteoritics, 29: 155–181. doi: 10.1111/j.1945-5100.1994.tb00670.x http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.1994.tb00670.x/abstract or free at http://adsabs.harvard.edu/full/1994Metic..29..155S
Grieve, R. A. F., Langenhorst, F. and Stöffler, D. (1996), Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience. Meteoritics & Planetary Science, 31: 6–35. doi: 10.1111/j.1945-5100.1996.tb02049.x http://0-onlinelibrary.wiley.com.library.uark.edu/doi/10.1111/j.1945-5100.1996.tb02049.x/abstract or free at http://articles.adsabs.harvard.edu//full/1996M%26PS...31....6G/0000006.000.html
Wilcox, R.E., 1959, Use of the spindle stage for determination of principal indices of refraction of crystal fragments: American Mineralogist, v. 44, no. 11–12, p. 1272–1293.
Grieve and Robertson, 1976, describes a classification scheme for progressively shocked rocks based on which sets of PDFs are present. http://link.springer.com/article/10.1007%2FBF00384743
An excellent discussion of PFs, PDFs, and structures with which they might be confused, as well as pictures of each, can be found in Reimold et al., 2014. The text of this article also points the researcher towards a significant body of related literature and introduces the relevance of each article:
Reimold, W. U., Ferrière, L., Deutsch, A. and Koeberl, C. (2014), Impact controversies: Impact recognition criteria and related issues. Meteoritics & Planetary Science, 49: 723–731. doi: 10.1111/maps.12284
PDFs and PFs Can be Confused With
Structures known as Bohm Lamellae or Boehm Lamellae can be confused for PDFs, but the distinction is pretty straightforward with the accumulation of modest experience and a good set of photos of each. Boehm lamellae can be formed from static stresses to quartz grains. They occur in only one set (direction) within a grain.
Durability of Evidence
Quartz is one of the most stable minerals within earth's surface-normal temperature and pressure and chemical environment. Quartz grains can easily preserve PFs and PDFs for a billion years or more. Even when subject to substantial heat and pressure, shy of the destruction of the quartz grain itself, the signatures may remain - though they do become less distinct. Within the few hundred million years range of time that encompasses most discussion of impact craters on earth's continental surfaces, PFs and PDFs provide excellent durability as impact indicators.
Toasting and Mosaicism
Most quartz grains that contain PFs or PDFs will also exhibit a feature called mosaicism. When a quartz grain is rotated on the stage under a polarizing microscope, it will go from white to black (extinction) and back again. You can see this here: http://www.youtube.com/watch?v=MguAnzie-tg If a quartz grain has experienced strain at some point its history, it may go into extinction unevenly; the dark region sort of 'rolls' across the grain as it is rotated. This is called undulose extinction, and it is not a sign of impact alteration. You can see this here, particularly in the center of the grain: http://www.youtube.com/watch?v=HpTENXNj8es In a shocked quartz grain that has developed PFs or PDFs, the extinction can become very uneven and patchy. This is called mosaicism. Some heavily shocked grains also develop a substantiall overall darkening, which has been described as a toasted appearance. The grain shown earlier on this page to illustrate PDFs is also slightly 'toasted' in patches. (Though not visible in the image, this grain also showed strong mosaicism.) Toasting is discussed here: http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1751.pdf and here http://www.researchgate.net/publication/249520848_Origin_of_toasted_quartz_in_terrestrial_impact_structures?ev=pub_cit
Above: Mosaicism - patchy, uneven extinction common in quartz grains that exhibit PFs or PDFs.
Gaps in this page
I have not discussed PFs or PDFs in minerals other than quartz, such as feldspar. The literature on this subject is not nearly as well developed, but is far enough along for a future mention here.
A nice discussion of PDFs, along with some remarkable photomicrographs, can be found in: 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-extinction theory: Geological Society of America Special Paper 249, 100 p. A freely available variation of this paper, containing excellent discussion and photomicrographs of PDFs (beginning on p. 66) can be found at: http://pubs.usgs.gov/of/1987/0606/report.pdf
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 http://www.univie.ac.at/geochemistry/koeberl/impact/
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.