5 M3 ferriphlogopite from the Ruiz Peak (New Mexico, USA): First occurrence of a mica polytype with coexistence of M 1- and M 2-layers
Pignatelli, Isabella; Nespolo, Massimo
European Journal of Mineralogy Volume 23 Number 5 (2011), p. 703 - 715
published: Oct 1, 2011
ArtNo. ESP147052305001, Price: 29.00 €
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 , b = 92.031(4) . Its stacking sequence has been determined by the PID analysis of the single-crystal X-ray diffraction pattern and corresponds to 5M 3 in Ramsdell notation. The polytype structure refines in C 2, 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 M 2, both with symmetry C 12(1); the third layer is of type M 1 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.