FMF - Friends of Minerals Forum, discussion and message board

[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.

[Bob Morgan]

The specimen in the photo does not appear to be a Japan Law twin. There should be parallel prism faces toward the camera that are in the same plane.
Japan Law contact surfaces are still a mystery. I have examined 'ears' or half Quartz twins where the contact surface is exposed, and complexity is revealed.
Even more a mystery is their origin. How would two crystals develop a relationship on a plane that quartz doesn't produce as a face?
You might be interested in:
Momma, Koichi et al, (2015) Growth history and textures of quartz twinned in accordance with Japan law, European Journal of Mineralogy, 27, 71-80.

[Bob Carnein]

At first, I thought it might be a grieserntal twin, but it appears that the angle between the two crystals on the left is near 90 degrees while the angle between the crystals on the right is more than 90 degrees. Can you accurately measure the angles or the angles between the front-facing edges?