An X-ray and optical study of over 30 single-crystal transformations of KNO3(II) ⇌ KNO3(I) involving the aragonite-type (II) and disordered calcite-type (I) structures, has resulted in the following observations: (a) at slow heating rates small crystals transform with preservation of the single-crystal condition, including minute internal and external detail, (b) the 110 plane of the calcite-type crystal parallels the 010 plane of the original aragonite-type crystal, (c) the c-axes of the two structures are not parallel but differ by an angle of 26 degrees, both axes lying in the plane normal to the  (I) and  (II) directions common to the two structures, and (d) a pronounced incubation period exists for the transformation for all specimens studied; this period averages 4.1 minutes for all specimens.
The structural relationships given above are completely reversible and independent of crystal shape, size, or perfection, indicating a memory effect that must be structurally controlled. Ideally, the c-axis of phase II parallels rows of closest potassium atom spacing of the phase I structure as viewed along the  projection, and the 001 plane of phase II departs by only 4 degrees from the 1.1·10 plane of phase I, a fact verified by the singlecrystal experiments.
The volume change for the transformation at the phase boundary is only 0.71 percent whereas that for the calcite-aragonite transformation of CaCO3 is 6.4 percent at 375°C and 1 atm. Single crystals involved in the latter transformation polygonize to the point that similar transition data are inconclusive.