Abstract

The French theory of twinning is not based on any genetic consideration; its predictions, notably with respect to the existence and the frequency of twinning, apply to primary and secondary twins alike. In no way can the theory be construed to mean that the age of a given twin is that of the seed crystal. Internal factors control the frequency of the twin if and when the twinning does occur, regardless of its mode of origin.

That external factors, such as thermodynamic conditions, may counteract the effect of the internal factors is recognized. Low index and small obliquity (internal factors) are the prerequisites of twinning; stress and temperature (external factors) only initiate twinning (in secondary twins). If external factors were the ultimate cause of twinning, minerals occurring in the same rock with finely twinned plagioclases would show similar polysynthetic twinning, since they have been subjected to similar conditions.

The width of lamellae in pseudomorphous twins due to replacement or to mutual orientation obviously is not controlled by the composition of the later plagioclase and thus cannot be invoked as an argument against the theoretical predictions as to frequency of twinning.

The undisputed fact that the predicted width of albite-twinning lamellae generally agrees with the observation data is additional proof that external factors do not outweigh internal ones. The effect of external factors, although real, falls short of obliterating the overall statistical picture of the twinning frequency in plagioclases.

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