Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R5Mg5Fe4AlxSi18-x (x approximate to 12). These phases have a new structure type in tetragonal space group P4/mmm (a = 11.655(2) angstrom, c = 4.0668(8) angstrom, Z = 1 and R-1 = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and Al-27 and Si-29 MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at T-N = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field.