Boracite single crystals with the general formula M3B7O13X (where M is a divalent cation, typically Mg, Mn, Ni Cu Fe etc., and X is Cl~, Br~ or I~) have recently excited interest for their potential applications to pyroelectric detectors, particularly for use as pyroelectric vidicon targets. The following discussion describes the way in which a thermodynamic analysis of the gaseous phases present during the growth of NiBr and FeI boracites has been used to develop a technique for the restriction of the density of spontaneous nucleation and to permit the growth of crystals onto previously-introduced seeds.