Green transparent single crystals of alpha -Ca-3[Al2N4] (monoclinic, P2(1)/c, No. 14, a = 957.2(3) pm, b = 580.2(3) pm, c 956.3(5) pm, beta = 111.62(3)degrees; Z = 4) were obtained from reactions of mixtures of the representative metals with nitrogen above temperatures of 1000 degreesC. beta -Ca-3[Al2N4] (monoclinic, C2/c, No. 15, a = 1060.6(2) pm, b = 826.0(2) pm, c = 551.7(1) pm, beta 92.1(1)degrees; Z = 4) was formed as a byproduct of a reaction of calcium with alumina under nitrogen at T = 930 degreesC in form of colorless crystals. The crystal structures of the two polymorphs contain edge- and corner-sharing AlN4 tetrahedra, leading to different layered anionic partial structures: (2)(infinity)[(AlN2/2N2/3)(2)(AlNN2/2N1/3)(6/3)(12-)] in the alpha -phase and (2)(infinity)[Al2N2N4/26-] in the beta -polymorph.