FORMULA

The chemical formula is Be3Al2Si6O18. The weight of its chemical composition is:

Beryllium 5.03 % BeO 13.96 %
Aluminum 10.04 % Al2O3 18.97 %
Silicon 31.35 % SiO2 67.07 %
Oxygen 53.58 %  
Total Elements = 100.00 % TOTAL OXIDE = 100.00 %
<>Numerous chemical elements can enter the beryl structure, thus modifying its physical properties. The major substitutions appear in the Be tetrahedron that can be substituted by Li, B, and in the Al octahedron that can be substituted by Mg, Mn, Fe, Cr, Ti, V and Sc.
The substitution of Al and Be by ions with a lower valence results in a charge deficiency. Alkali ions such as Na, K, Cs, Rb and Ca enter the beryl channels to balance the charge deficiency along with molecules of water, carbon dioxide and, according to some authors, iron ions.

This complex crystal-chemical behavior shows variations, some of them of gemological interest. Small quantities of trace elements that enter the beryl structure can result in intense coloration. The most common formation is in granitic pegmatites.

The chemical composition of emerald is the same as beryl, with the addition of some caracteristic elements.

An analysis of an emerald sample from the Swat Valley, Pakistan shows the following.

The crystal-chemical formula includes the subdivision of the elements in different sites.

The traditional representation of chemical formula is in blue.

A breakdown of the chemical composition of a mineral, and more specifically of emerald, can be obtained through destructive and non-destructive techniques. Among the latter, the most commonly used is the electron microprobe (EMPA), that permits to determine both the quality and the quantity of all elements present, from fluorine to uranium. The precision for the major elements is ± 0.01 and ± 0.05 % for the minor ones.

The electron microprobe cannot yet determine the values of elements having an atomic number smaller than fluorine, such as Li, Be, B, C, N, and O.

Values for these, as well as trace elements, are determined through SIMS (Secondary Ion Mass Spectrometry) analysis.