Glossary of Terms

Swallowtail twin

Form of penetration twinning in which two monoclinic crystals twin to form a v-shaped model. This form of twinning is most frequently observed in the mineral Gypsum. A swallowtail twin is very similar to fishtail twin, but with the two edges pointing outward a bit more than a fishtail twin. The two terms are often interchanged.