A Crystallographically Supported Equation for Calculating Water in Emerald from the Sodium Content

Abstract

ABSTRACT Emerald is the most well-recognized beryl (Be3Al2Si6O18) variety, and although it has been extensively studied, a satisfactory method for 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. Sodium is necessary to charge-balance divalent cation substitutions at the Al site of beryl; it is also correlated with 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 can be used to accurately measure both the Na+ and H2O contents. We checked the validity of the data using electron probe microanalyses for elements heavier than O. 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+ 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.