Twinning is common in gypsum crystals which often results in beautiful swallowtail habit.
I saw very nice exposure of swallowtail gypsum crystals in Cyprus. The crystals are several meters long and they all seem to be twinned.
These crystals formed in a shallow hypersaline lagoon in the Messinian (the last stage of the Miocene) about 6 million years ago. I have also written about laminated gypsum that is exposed nearby and formed roughly at the same time but probably in somewhat deeper water.


good luck with that thesis!