Muscovite- and phengite-3T: crystal structure and conditions of formation
Amisano-Canesi, Alessandra; Chiari, Giacomo; Ferraris, Giovanni; Ivaldi, Gabriella; Soboleva, Svetlana V.
European Journal of Mineralogy Volume 6 Number 4 (1994), p. 489 - 496
published: Jul 27, 1994
manuscript accepted: Mar 7, 1994
manuscript received: Sep 10, 1993
ArtNo. ESP147050604010, Price: 29.00 €
Abstract The crystal structure of two 3T phengitic muscovites in schists from North Kazakhstan (KZ 6 < P < 9 kbar, T ≈ 600°C) and in the pyrope-bearing whiteschists of the Dora-Maira massif (Italy) (DM P ≈ 30 kbar, T ≈ 700°C) are refined in the space group P3\12 (single-crystal diffractometer; MoKα radiation Rw = 0.037 (KZ) and 0.034 (DM)). The compositions and cell parameters are as follows: KZ: (K0.93Na0.03)(Al1.54Fe0.25Mg0.21Ti0.04)(Si3.34Al0.66)O10(OH)2, a = 5.212(1), c = 29.804(6)Å; DM: (K0.92Na0.01)(Al1.41Mg0.60Ti0.02)(Si3.54Al0.46)O10(OH)2, a = 5.215(1), c = 29.755(5)Å. Whereas in both samples the two independent tetrahedral sites show no differences, the two independent octahedral sites differ at least geometrically, particularly in the case of DM. Assuming that (i) high pressure favours less distorted structures and (ii) the presence of two independent octahedral filled sites in 3T allows for higher flexibility, the structural features of 3T micas are consistent with a greater abundance of phengite-3T in high-pressure metamorphic rocks. This is explained in terms of easier fitting between T and O sheets in phengite-3T with respect to muscovite-3T, because of composition, and monoclinic phengite, which has only one independent filled octahedral site.