A CRYSTALLOGRAPHICALLY SUPPORTED EQUATION FOR CALCULATING WATER IN EMERALD FROM THE SODIUM CONTENT

Emerald is the most well-recognized beryl variety (Be₃Al₂Si₆O₁₈), and although it has been regularly studied, a satisfactory way of quantifying the water content within the structural channels of the crystal lattice has yet to be proposed. Water is frequently present in the structural channels of beryl and can occur in two orientations (Type I and Type II). While spectroscopic methods are ideal for determining the orientation of the water molecules, measuring the overall water content often requires expensive or destructive analytical techniques. Although Na⁺ is necessary to charge balance divalent cation substitutions at the Al³⁺ site of beryl, it is also correlated with the H₂O in the structural channels, which typically occurs as Type II water. In this study, we present equations that can be used to easily calculate the H₂O content of an emerald beryl with significant Na+ content based on either Na⁺ apfu, or Na₂O weight percent. Unlike previous work, these equations are derived from single crystal X-ray diffraction data which is capable of measuring both the Na⁺ and H₂O content accurately. We checked the validity of the data using electron probe micro analyses for elements heavy enough to be detected. We compared the results with hypothetical scenarios in which different cation substitutions are prevalent, as weight percentages are variable based on the elemental contents. Our results indicate that Na⁺ apfu or Na₂O weight percent can be used to calculate H₂O content in emerald beryl with reasonable accuracy, which will allow future researchers to use a simple calculation instead of expensive or destructive techniques when determining H₂O content in emeralds.