The crystal structure of an anorthite (An94) from a slowly cooled, coarse-grained lunar troctolite (sample 76535) was refined in space group P1 from 9393 symmetry-independent X ray intensities of which 5145 were greater than three standard deviations above background. Long-exposure X ray precession photographs indicate that all classes of diffracted intensities are equally sharp. The final weighted R-factor for all reflections is 0.042. The results indicate that Na is ordered into the A(000) and A(0i0), and that there may be up to 10% vacancy in these two sites. The vacancy is consistent with a significant deviation from stochiometry in the electron microprobe analyses. Two of the four A sites are modeled using split atom positions indicating substantial positional disorder in the primitive structure. Observed cation ordering is consistent with slow cooling above the I1¿ to P1¿ transition, and there is no conclusive evidence of cation ordering in violation of body-centering.