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[marco campos-venuti]

The photo shows a 13.6 cm Japan twin quartz from Minas Gerais, Brazil.
Japan twin quartz crystals are considered typical contact twins. By definition, in a contact twin, the two individuals meet on a plane, unlike interpenetration twins, in which the two individuals occupy a common space. Japan twin quartz crystals usually exhibit a butterfly shape, but only when they are standing upright on the matrix. In the case of floater crystals where the quartz is double-terminated, the twin's shape forms an X and therefore should be better described as interpenetration. I have read that the twin boundary between the two individuals in a Japan twin is actually undulating (Lenart et al., 2012), which is strange for a contact twin.
I found an article by Sunagawa on the subject in which he claims that X-shaped twins are the result of a combination of two Y-shaped forms (not V-shaped). Since I couldn't find the article online, I don't understand this statement taken from the abstract. What do you think?

Lenart, A., Samardžija, Z., Godec, M., Mirtič, B., & Šturm, S. (2012). Twin-boundary formation in Japan-law twinned quartz crystals. European Journal of Mineralogy, 24(3), 509-517.
Sunagawa, I., Imai, H., Takada, M., & Hoshino, Y. (2004). Morphogenesis of quartz crystals twinned after Japan Law. European Journal of Mineralogy, 16(1), 91-97.