In case of the [w.sub.9]

symmetry element failure the number of achievable nulls are eight, while due to the [w.sub.7]

symmetry element failure only six nulls are achieved.

The acoustopolariscopy method allows one to determine spatial orientation of the symmetry element projections, to distinguish the quasi-transverse-isotropic elastic symmetry type from the orthorhombic type.

Sharp minimums of the amplitudes on the VC acoustopolarigrams allow one to determine the directions of elastic symmetry elements (axes and planes).

One can draw straight lines connecting the minimal amplitude of the signal passing through the sample almost everywhere on the VC acoustopolarigrams and thus determine the spatial location of the projections of elastic symmetry elements. The VP acoustopolarigrams of some samples have a distinctly flattened shape.

The presence of the symmetry elements on the acoustopolarigrams of samples 24996s, 28184s point to near orthorhombic symmetry type, while those of samples 30020, 24947--to transversal-isotropic.

The acoustopolarigrams obtained at the VC position allow one to determine the number and orientation of the projections of elastic symmetry elements of the anisotropic sample.

The VC acoustopolarigrams for some samples are minimal and occupy an area close to the coordinate origin, so it is impossible to determine the direction of the symmetry elements. At the VC position small acoustopolarigrams have been registered except the samples P-13-31-1 (faces 2-2' and 3-3'), P-13-34-1 (faces 1-1' and 3-3') and P-13-36-1 (faces 2-2' and 3-3').

The projection orientation of the elastic symmetry elements was determined on the three faces of the cube by the VC acoustopolarigrams.

The collection of symmetry elements that describes the symmetry of a crystalline material is summarized by the crystallographic space group.

Point group (b) (Hermann-Mauguin Space group (a) notation) Hexagonal ice (ice [I.sub.h]) [P6.sub.3]/mmc 6/mmm (#194) Cubic ice (ice [I.sub.c]) Fd[bar.3]m (#227) m[bar.3]m Stacking-disordered ice P3ml (#156) 3ml (ice [I.sub.sd]) Point group (b) (Schonflies Crystal notation) system [c] Hexagonal ice (ice [I.sub.h]) [D.sub.6h] Hexagonal Cubic ice (ice [I.sub.c]) [O.sub.h] Cubic Stacking-disordered ice [C.sub.3v] Trigonal (ice [I.sub.sd]) (a) The space group describes all the symmetry elements of a crystal lattice.

For the first time the existence of a well-ordered homogeneous (not twinned !) structure having symmetry elements incompatible with translational periodicity was shown.

Symmetry elements lost in each step of the sequence determine the possibilities for variants of the low symmetry phase and domains with characteristic interfaces that can be present in the microstructure.