The coexistence of two types of layer (M1 and M2, differing for the origin in the octahedral sheet) in a mica polytype is confirmed for the first time in this work. The sample analysed is a 5-layer ferriphlogopite from the Ruiz Peak volcano, with cell parameters a = 5.3146(2) Å, b = 9.2063(3) Å, c = 49.730(2) Å, β =92.031(4)°. Its stacking sequence has been determined by the PID analysis of the single-crystal X-ray diffraction pattern and corresponds to 5M3 in Ramsdell notation. The polytype structure refines in C2, which is the maximal space-group type compatible with the stacking sequence; the cation distribution in the octahedral sites has been estimated from the analysis of peaks in the Fourier map. The refinement results show that the polytype is composed of three independent layers: the first two are of type M2, both with symmetry C12(1); the third layer is of type M1 and has symmetry C12/m(1): the OD symbol for the 5-layer polytype is |1.5 5.3 5.5 1.1 3.1|. The coexistence of two types of layer confirms that structural adjustments during the crystal growth may play a fundamental role in the formation mechanism, as foreseen by the perturbative theory of polytypism.

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