Crystals are composed of repeating molecular groups which can be composed of independent ions or molecules or clumps of ions or molecules which are bound together in units by a defined molecular bonding pattern. Crystals grow by a three-dimensional repeat of these molecular units. If only a few of these units can be aligned into a 3-D array with good molecular bonding, the crystals stay small, but if the structures allow for extended good quality molecular interactions between these units, the crystals can grow to significant proportions. If we discuss only the hydroxylapatite lattice as a basic calcium phosphate structure for simplicity, the crystalline structure is termed disordered due to the varying crystallographic position of the hydroxyl ion in the structure which literally poisons the molecular bonding structure as the number of repeating units increases. The growth of these crystals is literally limited by the very nature of the chemistry and molecular bonding in the structure.
Apatite crystals are always small and poorly formed.
For the calcium oxalates, the monohydrate structure is well ordered and can form significantly dimensioned crystals, but the dihydrate structure is structurally disordered due to variations in the amount of water of crystallization throughout the crystalline lattice. The disordered water structure can have an effect similar to the hydroxyl ion disorder in the apatite structure in limiting the dimension and quality of the crystals. In our laboratory, the dihydrate structure is described as being composed of ordered monohydrate structure surrounding a disordered water pocket. Dr. Khan's data definitely highlights the presence of matrix throughout the stone fragments or stone slices he has studied. These observations are consistent with the disordered nature of the apatite lattice and the observation that apatite is molecularly sticky for biologic macromolecules and is therefore routinely used in chromatographic columns for macromolecule separation protocols.