United States Meteorite Impact Craters

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 crater-forming meteorite impacts 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 distinguish impact craters from 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.

Loring Coes Jr. (1915-1973) discovered coesite in 1953, in an article titled 'A new dense crystalline silica.'  Coesite is a high pressure polymorph of the common mineral, quartz.  He immediately recognized its potential as a geological barometer - a record of the pressure to which rocks had been exposed.  Coes pointed out that Coesite "may provide a means by which the conditions attendant on the crystallization of some deep seated rocks can be more closely estimated.  It's absence from these rocks provideds a maximum pressure above which they could not have formed."

This provided a key tool in the identification of large, old meteorite impact structures.

Coesite was first identified outside of the lab in the Coconino sandstone at Barringer Crater (Meteor Crater), in Arizona, in 1960.  The initial publication of the discovery (Chao et al., 1960) observed that coesite might provide a means of distinguishing impact craters from structures with similar morphology but a different origin.  Within a year, it had been identified at Ries crater, in Germany, at the <100 meter Wabar Crater(s), in Saudi Arabia, at Bosumtwi Crater, in Ghana, and at the Teapot Ess nuclear test crater.

Cohen et al. (1961) associated coesite with shatter cones at the Kentland and Serpent Mound impact craters, offering it as proof of a meteoritic origin for the structures, and contributing to an emerging understanding that a wider set of complex 'cryptovolcanic' or 'cryptoexplosive' structures were larger, older structures of impact origin.  This work also intersected with work from the same era by Dietz (see chapter on shatter cones), providing strong evidence that shatter cones might also be used in a similar diagnostic fashion to distinguish these kinds of impacts.

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).  But such rock units are uncommon.  It is now understood that 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 concerning stishovite are so obscure that they may be neglected in most reasonable considerations.)

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, and is relatively chemically inert, showing less solubility and chemical reactivity than ordinary (alpha) quartz (Coes, 1953).  [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.]

In non-impact environments of static pressure, coesite can form at pressures as low as 2 gigapascals (GPa).  In sedimentary rocks in impact settings, however, it occurs at a minimum of 5.5 GPa, and is common at 10 GPa (Ferrier and Osinski, 2012, citing Kieffer et al., 1976).  Stishovite forms at 8 to 10 gigapascals of pressure (or so).

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.

The percentage of coesite and stishovite, relative to (ordinary) alpha quartz in impacted quartz is very low, and may not be detectable by XRD without concentration.  Coesite and stishovite are much more resistant to being dissolved by hydrofluoric acid than ordinary (alpha) quartz, so finely seived samples are treated in the acid to leave a concentrate of the high pressure polymorph grains before XRD analysis is done.  <Note: Hydrofluoric acid is extremely dangerous.  It will eat through ordinary containers, and even a tiny exposure can cause death.>

Fahey J.J. 1964, Am. Mineral, 49, 1643 reports 100 grams of Coconino rock flour from Barringer produces 3 grams coesite and .42 grams stishovite after concentration by dissolution of alpha quartz with hydrochloric and hydrofluoric acid. (note: again, hydrofluoric acid is <extremely> dangerous.)  Using a similar technique, hydrochloric acid to remove carbonates and hydrofluoric to concentrating the coesite quartz fraction, Cohen et al. (1961) found about 10 parts per million of coesite in shatter cones from Serpent Mound.

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.

http://onlinelibrary.wiley.com/doi/10.1111/maps.12218/suppinfo

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 Bibliography and References:

Chao, E. C. T., Shoemaker, E. M., and Madsen  B. M. First Natural Occurence of Coesite. Science 22 July 1960, Volume 132, Issue 3421, pages 220-222.

http://www.sciencemag.org/content/132/3421/220.short

[Coesite first identified in nature, in Coconino sandstone at Meteor Crater (Barringer Crater), Arizona.] 

Coes L. ( 1953) A new dense crystalline silica. Science, Volume 118, page 131.

[Announced the initial discovery of coesite.]

Cohen A. J., Bunch T. E., Reid A. M. 1961. Coesite Discoveries Establish Cryptovolcanics as Fossil Meterorite Craters. Science, 17 Nov 1961, Volume 134, Issue 3490, pages 1624-1625.

http://science.sciencemag.org/content/134/3490/1624

[Coesite found in association with shatter cones at Kentland and Serpent Mound.]

Fahey, J.J. (1964) Recovery of coesite and stishovite from Coconino sandstone of Meteor Crater, Arizona. American Mineralogist, v. 49, pp. 1643-1647.

http://www.minsocam.org/ammin/AM49/AM49_1643.pdf

Stöffler, D. (1971), Coesite and stishovite in shocked crystalline rocks, J. Geophys. Res., 76(23),5474–5488

http://onlinelibrary.wiley.com/doi/10.1029/JB076i023p05474/abstract

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

http://onlinelibrary.wiley.com/doi/10.1029/JB076i023p05541/abstract

Stoffler, D. and Langenhorst, F. (1993) Meteoritics, 29, 155-181.

Spectra and such:

http://rruff.info/Coesite/R070565

http://rruff.info/coesite/display=default/X050094

http://rruff.info/stishovite/display=default/R070183

http://rruff.info/stishovite/display=default/R080042

(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.

http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2003.tb00307.x/pdf

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.

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

http://adsabs.harvard.edu/full/1994Metic..29..155S

Grieve, R. A. F., Langenhorst, F., & Stöffler, D. 1996. Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience. Meteoritics & Planetary Science, Volume 31, Issue 1, pages 6-35.

http://adsabs.harvard.edu/full/1996M%26PS...31....6G