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. 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.
Coesite, Stishovite, and Other High Pressure Mineral Polymorphs
Minerals of identical chemical composition may express different crystal structures depending upon the temperature and pressure at which they formed. These different crystal expressions of a single mineral are referred to as mineral polymorphs. The temperatures and pressures at which various mineral polymorphs form are very specific. Polymorphs cannot form without exposure to a particular temperature and pressure environment. As a result, the existance of a particular polymorph is a diagnostic indicator that the required temperature and pressure were present at some previous time. The best known mineral polymorph pair are the carbon minerals, graphite and diamond, but there are many others. A lot of minerals have several polymorphs that reflect a variety of formation environments.
Large impact craters produce pressures that do not occur in any other environment on or within earth's crust except in laboratories and in rocks adjacent to man-made nuclear explosions. Therefore, impact craters contain mineral polymorphs that are not found in any other earth rocks, and which can thus be used to unambiguously tell the difference between impact craters and other structures that are similar in appearance. Not all mineral polymorphs remain stable when temperatures and pressures drop below their formation temperatures, but many, like diamonds, are not only stable at lower temperatures, but are durable and relatively chemically stable as well, and are thus well preserved over geologic time scales.
Quartz is arguably the single most common minerals in the earth's crustal rock. As a result, it can be found in some quantity in most if not all impact craters, as well as in easily confused structures of terrestrial origin. (Nearly all sedimentary rocks and many igneous rocks contain at least some quartz grains.) Quartz produces several high pressure and high temperature polymorphs. Two of these, coesite and stishovite, occur at pressures that are indicative of a hypervelocity impact event. Coesite is occasionally found in igneous rocks that have undergone ultrahigh-pressure metamorphism from very deep subduction in collisions between continental plates followed by re-emergence at the surface (Schreyer, 1995). Such rock units are uncommon. The presence of coesite, in most situations where such subductive metamorphism can be ruled out (virtually everywhere), is reasonably unambiguous evidence of a large meteorite impact crater. The second diagnostic quartz polymorph, stishovite, is uniquely found in impact craters. (The exceptions to this statement are so obscure that they may be neglected in most reasonable considerations.)
In non-impact environments of static pressure, coesite can form at pressures as low as 2 GPa. In sedimentary rocks in impact settings, however, it occurs at a minimum of 5.5 GPa, and is common at 10 gigapascals (GPa) (Ferrier and Osinski, 2012, citing Kieffer et al., 1976). Both of these polymorphs are technically 'metastable' at earth surface normal temperature and pressure, meaning that they will return to alpha (ordinary) quartz with time. For most intents and purposes, however, coesite is reasonably stable in impacted rocks over geologic time scales. [I would like to better express the temperature and chemical constraints on the decay of stishovite and coesite, and will attempt to do so in the future.] Stishovite forms at 8 to10 gigapascals of pressure (or so), and is similarly stable over long periods of time.
Coesite and stishovite have been identified in impacted rocks through the technique of X-Ray Powder Diffraction (XRD) (in Stoffler, 1971 and others), and also by Micro Raman Spectroscopy.
XRD looks at the crystal structure of a mineral directly, while Raman Spectroscopy produces a spectra that derives from the way that molecules twist, turn, and stretch as they vibrate, thus also providing information about the crystal structure of a mineral. Each of these techniques can thus discriminate between high pressure mineral polymorphs. Gnos et al., 2013, recently mentions having performed X-ray diffractometry both on quartz that has simply been powdered, as well as on samples that have been bathed in dilute hydrofluoric acid to dissolve the ordinary (alpha) quartz component. Ordinary quartz is significantly more soluble than coesite in weak solutions of hydrofluoric acid, so this method is sometimes used to concentrate coesite for analysis. Details for the analytic techniques, with references, are listed in the supporting documents for the online version of the paper at the link below. Note: Hydrofluoric acid is <extremely> dangerous.
Other High Pressure Polymorphs
In addition to coesite and stishovite, several other high pressure mineral polymorphs have been identified in impact crater environments. The best known is impact diamonds. Others include the common metamorphic mineral, kyanite, two high pressure forms of rutile, and reidite, a high pressure polymorph of zircon. A good discussion of these can be found in Ferriere and Osinski, 2012.
Coesite and Stishovite References:
Chao, E. C. T., Shoemaker, E. M., and Madsen B. M. First Natural Occurence of Coesite Science 22 July 1960: 132 (3421), 220-222.
Fahey, J.J. (1964) Recovery of coesite and stishovite from Coconino sandstone of Meteor Crater, Arizona. American Mineralogist, v. 49, pp. 1643-1647.
Stöffler, D. (1971), Coesite and stishovite in shocked crystalline rocks, J. Geophys. Res., 76(23),5474–5488
The presence of coesite in impacted rocks has been incorporated into a systematic classification of impact shock levels in:
Stöffler, D. (1971), Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters, J. Geophys. Res., 76(23), 5541–5551
Spectra and such:
(see additional references and links related to this topic at end of page)
References: Raman Spectroscopy and Coesite or Stishovite Recognition
Boyer H., Smith D., Chopin C., and Lasnier B. 1985. Raman microprobe determination of natural and synthetic coesite. Physics and Chemistry of Minerals 12:45–48.
Gnos, E., Hofmann, B. A., Halawani, M. A., Tarabulsi, Y., Hakeem, M., Al Shanti, M., Greber, N. D., Holm, S., Alwmark, C., Greenwood, R. C. and Ramseyer, K. (2013), The Wabar impact craters, Saudi Arabia, revisited. Meteoritics & Planetary Science, 48: 2000–2014
Gucsik, A., Koeberl, C., Brandstätter, F., Libowitzky, E. and Reimold, W. U. (2003), Scanning electron microscopy, cathodoluminescence, and Raman spectroscopy of experimentally shock-metamorphosed quartzite. Meteoritics & Planetary Science, 38: 1187–1197.
Halvorson K. and McHone J. F. 1992. Vredefort coesite confirmed with Raman spectroscopy (abstract). 23rd Lunar and Planetary Science Conference. pp. 477–478.
Lounejeva E., Ostroumov M., and Sánches-Rubio G. 2002. Micro-Raman and optical identification of coesite in suevite from Chicxulub. In Catastrophic events and mass extinctions: Impacts and beyond, edited by Koeberl C. and MacLeod K. G. Special Paper 356. Boulder: Geological Society of America. pp. 47–54.