Abstract

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 [110] (I) and [010] (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 [110] 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.

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