Re-examination of “P21/a coesite”

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Abstract
Kirfel, Will, and Arndt (1979) reported a quenchable new phase of SiO2, synthesized in aqueous solution at 44 – 49 kbar and 610°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/c coesite. Therefore, a distinct P21/a phase probably does not exist. The evidence is: (1) The intensity distributions in the diffraction patterns of the (100)-twinned C2/c coesite are similar to those calculated using the reported P21/a structure. (2) The least-squares refinements using the (100)-twinned model yielded results similar to those reported for the P21/a phase. (3) The cell dimensions reported for the P21/a phase, when transformed to a C2/c cell, are not significantly different from those reported by other authors for normal coesite. (4) The P21/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 C2/c coesite crystals.