Screw dislocation mechanisms

The comparison of the Re content of a molybdenite with its structural polytype has been proposed as a method for assessing the viability of samples for Re-Os dating. 3R molybdenite has been assumed to grow via a screw dislocation mechanism triggered by the incorporation of Re during crystallisation. Low Re 3R ... 

D nucleation occurs when nuclei at the crystal surfaces act as sources of steps that allow for the further incorporation of growth units. In general, this mechanism accounts for the crystal growth observed at high supersaturations. The screw dislocation mechanism often accounts for growth at lower supersaturations. When the supersaturation is below the threshold for formation of 2-D nuclei, the presence of screw dislocations provides a source of steps for the addition of growth units in an infinite sequence of equidistant and parallel steps. 

In general, the predicted supersaturation required for 2D growth is much larger than has ever been observed. At lower (real) supersaturations the screw dislocation mechanism is much more feasible and two situations can be distinguished. If the supersaturation a is sufficiently smaller than <7i, tanh(cTi/a) becomes equal to 1 (eq. 11). On the other hand, for a>0 one has taxih 0 lo)KO la. The relationships between the growth rate and supersaturation for each case can be written respectively as ... 

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