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THE dislocation theory of crystal growth1 suggests that the crystal growing at low supersaturation in the presence of a single dislocation is not composed of an infinite number of layers
stacked on each other as ideally considered, but is a helicoid with the dislocation line as its axis and the point of emergence of the dislocation at the top. The observation of a growth
hill in the form of a ‘spiral staircase’ on a number of crystals reported in recent years2 has amply substantiated the theoretical predictions. Silicon carbide crystals have afforded
evidence3 of a large variety of spiral growths, for example, monomolecular as well as microscopic, and interaction of spiral growth due to two or more dislocations of same as well as of
opposite sign. In some cases, hitherto unobserved, silicon carbide crystals exhibit spirally terraced pits. Fig. 1 shows a phase-contrast micrograph of a spirally terraced pit. It is easy to
decide with the fringes of equal chromatic order that this feature is a pit. The step height of this terraced pit as determined from high-dispersion Fizeau fringes corresponds to 27 ± 2 A.,
which is the repeat distance of this crystal identified by X-rays as 33 R (Ramsdell's symbol).
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