Variable-temperature synchrotron x-ray powder diffraction data from dehydrated Na-6[Al6Si6O24] reveal a structural phase transition that involves both the commensurate ordering of the extra-framework Na cations and a one-dimensional incommensurate modulation of the framework. Peak splittings and superlattice reflections implicate an orthorhombically-distorted volume-doubled supercell at room temperature (a=12.9432 Angstrom, b=12.8403 Angstrom, and c=9.1372 Angstrom) similar to(root(2) over bara x root(2) over bar axa). These data also included additional superlattice peaks associated with an incommensurate long-period modulation with tau=(3/2 1/2 1)/8.9. All unique orderings of the 12 Na cations among the 16 available sites of the commensurate supercell are enumerated. A unique solution is identified that minimizes the structural energy and accounts for the observed superlattice peaks. Rietveld analysis reveals a significant Pauling "partial-collapse" tilt angle in the cation-ordered phase, that appears to decrease with increasing temperature.