Why quartz crystals twinned after Japan Law take flattened morphology and grow larger than the co-existing single crystals is explained based on the results of previous morphological, X-ray topographic and re-growth experimental data. It has been concluded that Japanese twin is a contact twin, and the characteristic V shape represents the upper half of a Y shape and not opposing half of an X shape. In a new locality, Japanese twins of X shape or doublet and triplet of X shapes are found, which opens a question to the validity of the above model. In this locality, unusually high proportion of quartz crystals, more than 90%, are twinned. Investigation of the samples from this locality revealed that the apparent X shape is a result of a combination of two Y shapes, indicating that the above model is valid in this case as well.

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