Summary: An examination of the structure of zircon, ZrSiQ₄, has been made by the ionization spectrometer and by a Debye-Scherrer powder photograph. The lattice upon which the structure is based corresponds to the space-group D4h19, possessing the symmetry of the ditetragonal-bipyramidal class. The unit cell, containing eight molecules ZrSiQ₄, has the dimensions a = 9·80, c = 5·98 Å.

The silicon and zirconium atoms form two interpenetrating face-centred lattices. Eight digonal axes pass vertically through each unit cell and on each axis is situated one silicon and one zirconium lying at a distance c/2 apart. Symmetry considerations limit the assembling of oxygen atoms to tetrahedra, each enclosing a silicon atom. All the atoms in the structure are situated on reflection planes.

The structure resembles in some ways that of anhydrite, CaSO₄, an account of which appears in another paper. The analogous dispositions of atoms in the planes (110) in zircon and in the planes (001) in anhydrite is most interesting. The distribution of atoms about the c-axis in zircon is similar to that about the a-axis in anhydrite.