The structure of aragonite was first determined by Lawrence Bragg in 1924 in what is the now standard space-group setting Pnma (No. 62). Subsequent studies have all taken his structure as their starting points, despite Bragg's own stated doubts and some earlier etching studies which indicated that the underlying symmetry may really be polar. We have reinvestigated the structure and found that there are many reflections with significant intensity among those that should be systematically extinct in Pnma. Some of these reflections have been subjected to further experimental analysis and have been shown not to be due to Renninger effects. A possible model that satisfies these observations is one where the true structure is in space group P1; and the structure is twinned about the three axial twofold rotation axes of Pnma. The space group P1 cannot be ruled out. Evidence for these conclusions is presented. The crystal chemistry of aragonite is revisited and described in terms of the stuffed alloy CaC. The carbonate group is confirmed to be non-planar in the crystal.

AbstractA polyhedral model, based on M6F37 square‐antiprismatic clusters, is proposed to represent the structure of Ca2YbF7(Ca10Yb5F35) being the member n = 15 of the homologous series MnF2n+5.

ChemInformVolume 20, Issue 37 Physical Inorganic Chemistry ChemInform Abstract: Crystal Chemistry of Ca2YbF7 and Other Closely-Related Structures with Cuboctahedral Anion Clusters D. J. M. BEVAN, D. J. M. BEVAN Sch. Phys. Sci., Flinders Univ. South Aust., Bedford Park, South Aust. 5042Search for more papers by this authorS. E. NESS, S. E. NESS Sch. Phys. Sci., Flinders Univ. South Aust., Bedford Park, South Aust. 5042Search for more papers by this authorM. R. TAYLOR, M. R. TAYLOR Sch. Phys. Sci., Flinders Univ. South Aust., Bedford Park, South Aust. 5042Search for more papers by this author D. J. M. BEVAN, D. J. M. BEVAN Sch. Phys. Sci., Flinders Univ. South Aust., Bedford Park, South Aust. 5042Search for more papers by this authorS. E. NESS, S. E. NESS Sch. Phys. Sci., Flinders Univ. South Aust., Bedford Park, South Aust. 5042Search for more papers by this authorM. R. TAYLOR, M. R. TAYLOR Sch. Phys. Sci., Flinders Univ. South Aust., Bedford Park, South Aust. 5042Search for more papers by this author First published: September 12, 1989 https://doi.org/10.1002/chin.198937005Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume20, Issue37September 12, 1989 RelatedInformation