A polycrystalline sample synthesized through the thermal transformation of zeolite Na-LTA and originally described as standard nepheline was re-investigated. Transmission electron microscopy (TEM) proved the presence of a structure with a tripled c unit-cell parameter compared to that observed in classical nepheline. Consequently, the theoretical X-ray powder diagrams of all known trinepheline compounds were compared with the experimental powder pattern, leading to the conclusion that the material is actually a mixture of two phases. Rietveld refinement against XRPD data using structural models of (a) classical Na-nepheline (S. G. P63, a = 9.971(3), c = 8.349(2) Å) and (b) monoclinic trinepheline (space group P1121, a = 9.995(4), b = 9.963(4), c = 24.988(7) Å, γ = 119.75(3) °) resulted in a satisfying agreement between calculated and observed step intensities (χ2 = 1.96, Rwp = 11.3, Rp = 8.77). The additional quantitative analyses of the samples obtained in a temperature range of stability of nepheline (900–1100 °C) showed the presence of both structures, and also a gradual increase of the weight fraction of the monoclinic trinepheline phase with increasing temperature.