| Popis: |
This dissertation is a multipart study of the crystal-chemistry and structure of the mineral beryl. The H₂O and Na content of beryl are quantified and related for emerald and each major beryl variety using single-crystal X-ray diffraction methods. The crystal-chemistry of beryl is shown to have differences across beryl varieties. A model that accurately determines the permutations in the beryl crystal structure using measured chemistry is created and is used to explore the limitations of the mineral. Beryl from the Prof Pegmatite near Revelstoke, British Columbia, is examined, using knowledge gained during this research. Beryl (Be₃Al₂Si₆O₁₈) is a well-known mineral, most famously in its vivid green form of emerald, but also in a range of colors. Prominent varieties of beryl aside from emerald include aquamarine, red beryl, heliodor, goshenite, and morganite. Emerald is studied first to remedy problems in prior research and to prove that this methodology is reliable. A relation between Na and H₂O content in beryl is determined that is consistent for beryl with significant Na content. Natural red beryl is regularly anhydrous, and heliodor has too low a Na content to reliably determine H₂O content from measured Na. A complete crystal structure of beryl can be calculated using chemical composition by utilizing the average ionic radii of the cations within the Al- and Be-sites. The structure is predictable using the Al-site average ionic radius for octahedrally trending beryl or the Be-site average ionic radius for tetrahedrally trending beryl. This model makes it possible to explore limitations on the beryl structure and the possibility of unusual cation substitutions. It is robust for true beryl up to a high limit of cation substitutions. The beryl studied from the Prof Pegmatite demonstrates an application of the crystal-chemical models to a newly described locality. This research will help guide future beryl studies in classifying beryl variety by chemistry and structure and allow for the calculation of H₂O content in a range of beryl varieties from easily measured Na content. Future beryl studies will be able to determine crystal structures during standard chemical analyses using the predictive structural model. |
