Two borides are known: ThB₄(gray) and ThB₆(deep red). More attention has been paid to the tetraboride, the properties of which are melting point, >2,200 degrees C (but oxidizes slowly above 1,000 degrees C); thermal expansion, 5.9X10_-6(20 to 1,000 degrees C); sp gr, 8.45 g/mL; modulus of rupture (20 degrees C), 20,000 psi (138 MPa) . Some properties of ThB₆are melting point, 2,200 degrees C; sp gr, 7.1.