X-ray crystallographic data are of particular importance to the mineralogist. Beyond the considerations of structural chemistry, they provide one of the most accurate methods for phase and/or compositional determination and for obtaining the molar volumes and densities of minerals (Table 5-2). Selected data for approximately 300 minerals are tabulated in Table 5-1. These are taken from the recent literature or from unpublished sources. With minor exceptions, we have restricted ourselves to data for chemically and physically well-defined phases for which the unit cell parameters are known with an accuracy of the order of .2 per cent or better. The data are presented by mineral groups following Dana's System. Within a group, however, the order may be alphabetical, structural, or for the sulfides, approximately by increasing sulfur-metal ratio. Temperatures at which the measurements were made are given in the second column from the right. The letter r indicates the data were obtained at an unspecified room temperature and may be taken as 25° ± 5° C. The number of gram formula weights per unit cell is given in the column labeled Z. Compounds denoted by an asterisk indicate the measurements were made on natural specimens which may have deviated slightly from the listed formula. Substances of rhombohedral symmetry are denoted by the symbol hex-R to distinguish them from materials of true hexagonal symmetry. The space group is given along with its number in the 1952 International Tables for X-ray Crystallography (Henry and Lonsdale, 1952). All cell dimensions are given in Angstrom