All biaxial minerals have optical symmetry equivalent to 2/m2/m2/m.  But, in each of the crystal systems, the optical directions have different correspondence to the crystallographic directions.
 

• In orthorhombic crystals the optical directions correspond to the crystallographic axes, i.e. the X direction and its corresponding refractive index, ? can be either the a, b, or c crystallographic axes, the Y direction and ? can be parallel to either a, b, or c, and the Z direction or ?, can be parallel to either a, b, or c. 
• In monoclinic crystals, one of  the X (?), Y (?), or Z (?) directions or indices is parallel to the b crystallographic axis, and the other two do not coincide with crystallographic directions. 
• In triclinic crystals none of the optical directions or indices coincide with crystallographic directions, although in some rare case one of the indices might coincide with one of the crystallographic directions.