  The domain structure in K2Cd2xMn2(1-x)(SO4)3 (x=0.5, 0.7 and 0.9) langbeinite
crystals is studied with the aid of optical polarization microscopy. It is
shown that the domain walls in ferro-elastic langbeinites separate
enantiomorphous orientation states. These orientation states appear in the
phase with the symmetry 23 in connection with the hypothetic phase transition .
In the phase with the symmetry 222, these domain walls are transformed to those
separating ferroelastic domains with the opposite signs of enantiomorphism. It
is revealed that one enantiomorphous domain can only transform to the other via
a thick layer of the parent phase with symmetry. The results for the volume
thermal expansion are presented. It is shown that the isolated point at the
x,T-phase diagram of K2Cd2xMn2(1-x)(SO4)3 solid solutions corresponds to the
concentration x=0.6.
