Metastable growth of coesite in highly strained quartz

Coesite has been produced outside its stability field in highly strained samples of Dover flint. Large crystals grew in nine cylindrical samples compressed more than 50% at a strain rate of 10−4 sec−1, at temperatures between 450° and 900°C, and at confining pressures of 5–20 kb. Specimens deformed under similar conditions but subjected to less strain contained no coesite. Two coesite-bearing samples showed textural evidence of reversion to quartz associated with reduction of the local strain rate during deformation. A highly strained specimen annealed at high pressure after deformation contained no coesite. The explanation offered is that the high densities of dislocations developed in greatly compressed specimens increased the free energy of the deformed quartz to a value higher than that of unstrained coesite. In this specimen coesite could grow in preference to unstrained quartz in accordance with Ostawald's law of successive reactions. These observations indicate that care must be taken when natural or experimental environmental pressures are being inferred from polymorphic transitions.