Ammonium nitrate crystalline modifications

The effects of transformation between ammonium nitrate crystalline MODIFICATIONS (AFTER Hendricks, Posnjak, Kracek [3a])... 

Ammonium nitrate exists in the form of crystals, melting at 169.6°C. It occurs under five crystalline modifications distinguished by the transition temperatures ... 

Suspended Transformation and Supersaturation.— As has already been learned, the transformation of the one crystalline form into the other does not necessarily take place immediately the transition point has been passed and it has therefore been found possible in a number of cases to follow the solubility curve of a given crystalline form beyond the point at which it ceases to be the most stable modification. Now, it will be evident from Fig. 71 that if the solubility curves for rhombohedral and regular ammonium nitrate be prolonged beyond the point of intersection, the solubility of the less stable form is greater than that of the more stable. Above 125°, for example, rhombohedral ammonium nitrate (metastable) is more soluble than regular ammonium nitrate (stable). A solution, therefore, which is saturated with respect to the less stable form, i.e. which is in equilibrium with that form, is supersaturated with respect to the more stable modification. If, therefore, a small quantity of the more stable form is introduced into the solution, the latter must deposit such an amount of the more stable form that the concentration of the solution corresponds to the solubility of the stable form at the particular temperature. Since, however, the solution is now unsaturated with respect to the less stable variety, the latter, if present, must pass into solution and... 

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