The cavities in the random close packing of spheres are re-examined with a view to considering the possible applicability of the structure as a model for the arrangement of oxygen atoms in a silicate melt. For this purpose a limit of 1.20 diameters is used to define near contacts between spheres, and this leads to substantial changes in both the numbers and the types of polyhedral cavities found. Serious limitations are deduced for the Si: O ratios of silicate structures for which the random close packing might serve directly as a model, but suggestions are made as to how the introduction of ionic charges might itself tend to modify the packing into a more appropriate form.