PREDICTING THE CRYSTAL STRUCTURE OF BERYL FROM CHEMICAL ANALYSES

A crystal structure model has not previously been developed that uses the chemical composition of a known mineral to determine the complete crystal structure, including all major bond lengths and angles, atomic coordinates, polyhedral volumes and distortions, and unit cell parameters. The mineral beryl is used to create such a model.

Beryl (Be₃Al₂Si₆O₁₈) is an ideal mineral to show that predicting the crystal structure using chemistry is possible: the base structure is known, it has two cation sites that experience substitutions, and these only minimally occur simultaneously. Ideally vacant channel sites are involved in coupled substitutions allowing alkali cations (typically Na⁺) to enter the structure, and the channel regularly contains molecular H₂O. This research uses single-crystal X-ray diffraction and electron-probe micro-analyses of 80 samples to create a structure prediction model which can determine all major structural parameters. The model’s validity is tested with 33 samples to verify that the measured structures fall within the predicted boundaries.

The results indicate that a complete crystal structure of beryl can be calculated accurately using chemical composition by utilizing the average ionic radii of the measured cations at the Al- and Be-sites. The full structure is predictable using the Al-site average ionic radius (Al-SAIR) for octahedrally trending (OT) beryl, or the Be-site average ionic radius (Be-SAIR) for tetrahedrally trending (TT) beryl, which separates beryl into two groups. Beryl for which Al-SAIR > (0.45 × Be-SAIR) + 0.414 is considered octahedrally trending and beryl for which Al-SAIR ≤ (0.45 × Be-SAIR) + 0.414 is considered tetrahedrally trending. Red beryl (differentiated by high Fe and Mn) has a slightly different trend, forming a subset of OT beryl. There is an upper limit to the predictable range of beryl structures of 0.604 Å Al-SAIR or 0.326 Å Be-SAIR. This model enables exploration of the beryl structure and the possibility of unusual cation substitutions, or conversely to compute the structure of a hypothetical pure beryl. It is robust for true beryl up to a high limit of substitutions, but not for other beryl group minerals including stoppaniite, bazzite, avdeevite, and johnkoivulaite.

This research is beneficial for future beryl studies as it enables the creation of an extensive beryl database, aids comparisons of natural beryl to synthetics, and helps provide further guidance on provenance studies. It also invites future crystal structure prediction research.