hemitrope

from The Century Dictionary.

• Half-turned: specifically applied in mineralogy to a compound or twin crystal which has two similar parts or halves, one of which is turned half round upon the other.
• noun Anything that is hemitropous in structure.
• noun A twin crystal.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Half turned round; half inverted; (Crystallog.) having a twinned structure.
• noun That which is hemitropal in construction; (Crystallog.) a twin crystal having a hemitropal structure.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun crystallography A twin crystal having a hemitropal structure.