An electron diffraction study was done with fine aragonite particles about 0.5 to 1.0 μ in size. The specimens were obtained by mixing a boiled calcium chloride solution (0.1 n) with 0.1 n of an ammonium carbonate solution. Extra spots were observed with half-integer indices {1⁄2,1⁄2,l}. This suggests the existence of a structure that differs from that of normal aragonite. From an X-ray diffraction study of the specimen, a {1/2, 3/2, 0} diffraction was also observed. The structure for explaining these half-integer diffractions is presented as a superstructure of aragonite; it belongs to the space group of D2h15−Pbca, with cell dimensions of A=8.420 Å, B=9.385 Å, and C=5.741 Å, and it containes 8 formula units in a unit cell. A dark-image photograph shows this anomalous area to be comparatively wide and suggests that the anomalous area is dominant in a crystal. The authors conclude that rapidly-precipitated aragonite powders contain a small quantity of the repeatedly-twinned crystals mentioned above.
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Hiragi et al. (1966) studied this question.