Rutile crystal structure, molecular model. Rutile (titanium dioxide, TiO2) is a common mineral found in metamorphic and igneous rocks. It has a tetragonal structure, with its unit cell (repeating crystal region, outlined in red) being based on a rectangular prism. A titanium ion (blue) is at each vertex of the prism, with another at the prism's centre, and each is surrounded by an octahedron of oxygen ions (silver).