High-pressure (0Ц43 GPa) single-crystal X-ray diffraction experiments were conducted on natural datolite CaBSiO4(OH). We observe polymorphic transition between 27 and 33 GPa into the high-pressure phase. The high pressure crystal structure of datolite contains Si atoms coordinated by five oxygen atoms to form a trigonal bipyramid SiO5. As a result of phase transition, two SiO4 tetrahedra across the eight-membered ring of the borosilicate layer approach each other and form dimers of edge-sharing SiO5 bipyramids. The structure symmetry (P21/c) remains unchanged.