Nanocracks at Destruction of Nepheline

Fracture process in crystals begin from the formation of tiny (“primary”) cracks. Larger cracks are formed when these primary cracks unite. To register primary cracks that appear on the surface of a nepheline crystal during the destruction by diamond microcrystals, the fractoluminescence method is used. The fractoluminescence spectrum consists of three bands: 1.4, 1.68, and 1.98 eV. The 1.98-eV band corresponds to excited free radicals ≡Si–O^•, 1.68 eV corresponds to excited Fe^3+• ions, and the 1.4-eV band appears when empty traps are filled with electrons from the conduction band. These radicals, ions, and traps appear during the fracture of nepheline lattice cells and are located on the surface of “primary” cracks. The time dependences of the fractoluminescence signals are sets of separate signals with a duration of about 86 ns. The interval between the signals varies from 0.1 to 1 μs. The nepheline crystal has a hexagonal system and six systems of dislocation slip planes. At the intersection of these planes, six barriers are formed, which prevent the movement of dislocations. The breaking of each barrier causes the appearance of a primary crack and the formation of a peak in the fractoluminescence signal. When six barriers are broken, clusters are formed from the same number of primary cracks. Therefore, fractoluminescence signals contain six maxima. First, the largest crack appears. Its dimensions range from approximately 9 to 17 nm. The growth time of such crack is about 16 ns. The remaining (smaller) cracks have sizes 1.7 to 3.0 times smaller. The size distribution of cracks follows a power law with an exponent equal to 6.