The position of the rhombic section has been calculated for triclinic feldspars of various chemical compositions, of various degrees of disorder, and of various degrees of mechanical lattice deformation. The relative high frequency of crystals of 'diclinic' symmetry is due to the ease with which polysynthetic twinning on a unit cell scale takes place in triclinic crystals having the ? or the ? interaxial angle close to 90°.
The attitude of the rhombic section in triclinic feldspars with a note on "diclinic" crystals.