Hawthorne, Frank C. Department of Geological Sciences, University of Manitoba, Winnipeg, Canada.
Last reviewed:October 2019
- Structural classification
- Mixed-anion borate minerals
- Industrial uses
- Related Primary Literature
- Additional Reading
A large group of minerals in which boron is chemically bonded to oxygen. Boron is a fairly rare element. However, because of its chemical character, it is very susceptible to fractionation in Earth processes and can become concentrated to a degree not found in other elements of similar abundance. Boron is symbolized B, has the atomic number 5, and has the ground-state electronic structure [He]2s22p1. The very high ionization potentials for boron mean that the total energy required to produce the B3+ ion is greater than the compensating structure energy of the resulting ionic solid, and hence bond formation involves covalent (rather than ionic) mechanisms. However, boron has only three electrons to contribute to covalent bonding involving four orbitals, (s, px , py , pz ). This results in boron being a strong electron-pair acceptor (that is, a strong Lewis acid) with a very high affinity for oxygen. The structural chemistry of boron and silicon (Si), when associated with oxygen (O), is quite similar. The BO3, BO4, and SiO4 groups have a marked tendency to polymerize in the solid state, and this aspect of their behavior gives rise to the structural complexity of both groups. However, subtle differences in chemical bonding do give rise to differences in the character of this polymerization, particularly when H2O is also involved. These differences result in the very different properties of the resultant minerals and their very different behavior in Earth processes. See also: Boron
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