Characteristics of the structure of single crystals of binary lithium-gadolinium borate Li₆GdB₃O₉ with mixed-type bonding (ionic and covalent) are studied. The system of cleavage planes in Li₆GdB₃O₉ single crystals is associated with the break of the Gd-O bridging bonds and the longest Li-O bonds in the Li five-vertex polyhedra. The fracture toughness in the cleavage planes is determined by microindentation. It is shown that the character of resistance to deformation and fracture toughness are determined by the orientation of the cleavage planes with respect to the faces where boron-oxygen triangles with directional covalent bonds are located.