Re - examination of " P 21 / a coesite "

Kirfel, Will, and Arndt (1979) reported a quenchable new phase of Si02, synthesized in aqueous solution at 4449 kbar and 6100 C, having the space group P21/a and a crystal structure closely related to coesite. However, single-crystal X-ray and TEM studies have revealed that their observations could be the result of (100)-twinning of C2/e coesite. Therefore, a distinct P2da phase probably does not exist. The evidence is: (1) The intensity distributions in the diffraction patterns of the (100)-twinned C 2/e coesite are similar to those calculated using the reported P2da structure. (2) The leastsquares refinements using the (100)-twinned model yielded results similar to those reported for the P21/a phase. (3) The cell dimensions reported for the P2da phase, when transformed to a C2/e cell, are not significantly different from those reported by other authors for normal coesite. (4) The P 21/a phase cannot be distinguished from normal coesite on the basis of the calculated powder diffraction patterns. (5) Syntheses under a wide range of conditions have produced only C 2/e coesite crystals.