Borate glasses are an enigma in that these is now increasing evidence that their structures are dominated by superstructural units, which comprise well defined arrangements of the basic BO_3 and BO_4 structural units, with to internal degrees of freedom in the form of variabel bond or torsion angles. In the present paper,it is shown that considerable insight into the structure of borate glasses can be gained from a study of the corresponding crystalline polymorphs. A simple, model is proposed to predict the fraction, x_4, of 4-fold co-ordinated boron atoms in vitreous borate networks and the topological criteria for the formation of such networks are discussed, taking into account the degrees of freedom necessary for conventional glass formation.
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