Reflection twin
From Online Dictionary of Crystallography
Macle par reflexion (Fr). Ebenenzwilling, Reflektionszwilling (Ge). Geminato per riflessione (It). Macla de reflexión (Sp).
Two domains of one crystal form a reflection twin when the twin operation is a reflection across a lattice plane (twin plane).
1. Rational twin mirror plane normal to an irrational line. This lattice plane is common to both twin partners.
2. Irrational twin mirror plane normal to a rational lattice row. This lattice row [uvw] is common to both twin partners.