The first reliable structural refinement of a non-MDO mica polytype from single-crystal X-ray diffraction data is presented. The sample is a 4-layer ferriphlogopite from the Ruiz Peak volcano, New Mexico, with cell parameters a = 5.3166(2) Å, b = 9.2080(2) Å, c = 39.814(1) Å, β = 92.550(3)°. Its stacking sequence, determined by the periodic-intensity-distribution (PID) analysis, is expressed by the RTW symbols 222̅2̅: it corresponds to 4M3 in Ramsdell notation. Structural refinement in the space group C2/c converges to R(obs) = 6.3 %. This is the highest-symmetry space group compatible with the observed stacking sequence and means that no desymmetrisation occurs in this polytype. The peaks of electron density in the Fourier map show that the polytype is meso-octahedral and contains two independent layers in the asymmetric unit, one with symmetry C12/m(1), the other C12(1), which however are both of type M1. The meso-octahedral nature of the layers is confirmed by the Charge Distribution analysis. The OD symbol for the polytype is |1.1 3.3 5.5 3.3|. The formation of this polytype by spiral growth can be interpreted either as the result of the occurrence of a stacking fault in the exposed ledge of a 3T polytype or as a cell-twin formed by two 2M1 substructures. In both interpretations the 4M3 structure is decomposed in two basic structures (3T and 1M or two 2M1); the formation of a non-MDO polytype from two substructures corresponding to MDO polytypes explains why the second and fourth layers of the polytype are meso-octahedral even if the space-group symmetry does not force the cis sites to be equivalent.