THE CRYSTAL CHEMISTRY OF BERYLLIUM

Abstract

Beryllium is not a very abundant element in the Earth, but, being an incompatible element in common rock-forming silicate minerals, it is susceptible to concentration via fractionation in geochemical processes. Moreover, its properties are such that Be does not tend to show extensive solid-solution with other elements, and hence usually forms minerals in which it is a discrete and essential constituent. Beryllium (atomic number 4) has the ground-state electronic structure [He]2 s 2 and is the first of the group IIA elements of the periodic table (Be, Mg, Ca, Sr, Ba, Ra). The first (899 kJ/mol) and second (1757 kJ/mol) ionization enthalpies are sufficiently high that the total energy required to produce the Be2+ ion is greater than the compensating energy of the resulting ionic solid, even when the latter involves extremely electronegative elements. Hence bond formation involves covalent (rather than ionic) mechanisms. However, Be has only two electrons to contribute to covalent bonding involving four orbitals ( s , p x, p y, p z), resulting in Be being a strong Lewis acid (i.e., a strong electron-pair acceptor) with a high affinity for oxygen. The cation radius of Be2+ is 0.27 A, (Shannon 1976) and Be is known only in tetrahedral coordination in minerals, although BeO3 groups are known in synthetic compounds (e.g., Ca12Be18O29, Y2BeO4 and SrBe3O4; Harris and Yakel 1966, 1967, 1969). The BeO4 and SiO4 groups have a marked tendency to polymerize in the solid state. Although [4]Be (r = 0.27 A) and [4]Si (r = 0.26 A) are very similar in size, solid solution between Be and Si is inhibited by the difference in formal charge of the two species (Be2+ vs. …