Three-dimensional, scintillation-counter measured, X-ray diffraction data have been used in least-squares refinement of the atomic parameters and cation-sile occupancy factors for the crystal-structtire of glaucophane from Tiburon Peninsula, California. By assuming that each of the four cation sites in amphiboles is fully occupied by no more than two atomic species, preferably of the same valence state, occupancy factors are obtained from the X-ray diffraction data refinement as follows: M(l), 0.84Mg+0.16Fe2+; M(2), 0.91Al+0.09Fe3+; M(3), 0.71Mg+0.29Fe2+; M(4), 0.98Na+0.02Ca. These values lead to the chemical composition (Na1.95Ca0.04)(Mg2.39Fe0.612+Fe0.183+Al1.82)Si5O22(OH)2, comparing reasonably well with the composition obtained from the bulk chemical analysis, (Na1.84 Ca0.20)(Mg2.38Fe0.702+Fe0.303+Al1.58Ti0.06Mn0.01) (Si7.92Al0.08(O2- OH-, F-, Cl-)24. The location of the atomic species in the four sites agrees with the crystal-chemical proposal by Whittaker that trivalent cations would occupy M(2) when univalent cations are located in M(4). The ordered occupancies are in accord with the suggestion by Ernst that the natural material, which he called the high-pressure, low-temperature polymorph, glaucophane II, would have an ordered cation distribution.