  We present a novel method to predict the fracture surface energy, {\gamma},
of isochemically crystallizing silicate glasses using readily available
crystallographic structure data of their crystalline counterpart and tabled
diatomic chemical bond energies, D0. The method assumes that {\gamma} equals
the fracture surface energy of the most likely cleavage plane of the crystal.
Calculated values were in excellent agreement with those calculated from glass
density, network connectivity and D0 data in earlier work. This finding
demonstrates a remarkable equivalence between crystal cleavage planes and glass
fracture surfaces.
