CRYSTAL-CHEMICAL OBSERVATIONS AND THE RELATION BETWEEN SODIUM AND WATER IN DIFFERENT BERYL VARIETIES

Beryl (Be₃Al₂Si₆O₁₈) is a well-known mineral, most famously in its vivid green form of emerald, but also as a range of colors. Prominent varieties of beryl aside from emerald include aquamarine, red beryl, heliodor, goshenite, and morganite. There has not been a significant amount of research dedicated to comparing the crystal-chemical differences among the varieties of beryl except in determining chromophoric cations. While the water content of emerald has been explored, and the water content of individual beryl specimens has been studied, there has not yet been a study to compare the water content systematically across beryl varieties. In this study we consider single-crystal X-ray diffraction data and electron probe microanalyses of dozens of beryl specimens, of six primary varieties, to compare and contrast their crystal chemistry. Beryl cation substitutions are dominantly coupled substitutions that require Na to enter a structural channel site. The results indicate with increasing Na content beryl varieties diverge into two groups, characterized by substitutions at octahedral or tetrahedral sites, and that the dominant overall cation substitutions in each beryl variety tend to be different in more than just their chromophores. We find that the relation between Na and H₂O content in beryl is consistent for beryl with significant Na content, but not among beryl with low Na content. Natural red beryl is found to be anhydrous, and heliodor has too low of Na content to reliably determine H₂O content from measured Na. We determined equations and recommendations to relate the Na and H₂O content in emerald, aquamarine, goshenite, and morganite from a crystallographic perspective that is applicable to beryl chemistry measured by other means. This research will help guide future beryl studies in classifying beryl variety by chemistry and structure, and allow the calculation of H₂O content in a range of beryl varieties from easily measured Na content instead of requiring use of expensive or destructive methods.