Further study of the cation ordering in phengite 3T by neutron powder diffraction

Abstract

Phengite 3T (Dora-Maira massif, Italian western Alps), with chemical composition K 0.96 Na 0.01 Al 1.44 Mg 0.56 (Si 3.59 Al 0.41 )O 10 (OH) 2 has been investigated using powder neutron diffraction, at the ILL (Institut Laue-Langevin) Facility, on the D2B diffractometer. Data sets were collected at 293 and 873 K, and the present results are compared with those obtained previously using the time-of-flight (TOF) technique, on the same compound. In the octahedral sheet, Al tends to order into the M2 site, in accordance with the measurements mentioned above, whereas the Al/Si tetrahedral ordering we observe (i.e. Si fully into T1 site) is at variance with that determined previously. These discrepancies are ascribed to the presence of talc impurities in the sample used previously (~6 wt.%), which affected the results obtained.

Computer simulations of cations order‐disorder in 2:1 dioctahedral phyllosilicates using cation‐exchange potentials and monte carlo methods

“…Pavese et al [77][78][79] have conducted many experimental studies of the 2M 1 and 3T polytypes of phengite. Much of their work suggests a low degree of octahedral cation order in both polytypes, but by contrast, their most recent investigation of a different natural phengite 3T sample from that previously studied, concluded that the sample exhibited both octahedral and tetrahedral order.…”

A high-temperature Fourier transform infrared study of the interlayer and Si–O-stretching region in phengite-2M1

“…Neutron diffraction provides a powerful structural adjunct to FTIR studies of micas (e.g. Pavese et al, 2000). Here we use both in situ high-temperature FTIR and crystallographic data from refinement of the neutron diffraction experiments previously reported in Mookherjee et al (2001) to shed light on the behaviour of the interlayer and basal oxygens of mica.…”

What drives the distribution in nature of 3T vs. 2M1 polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks

“…Numerous crystallographic data have been published after Sassi et al (1994), showing that HP favors cation ordering and stabilizes 3T stacking (Pavese et al 1997(Pavese et al , 1999a(Pavese et al , 1999b(Pavese et al , 2000(Pavese et al , 2001(Pavese et al , 2003Brigatti et al 2005;Curetti et al 2006;Gatta et al 2009, and references therein), and Gatta et al (2010) find evidence supporting a better stabilization of 3T stacking than 2M 1 . However, experimental studies on phengite are limited with respect to polytype formation and crystallization pressure (Smyth et al 2000), although synthetic phengites may not be representative of equilibrium conditions (Schmidt et al 2001;Curetti et al 2008).…”