Cation and anion ordering in synthetic lepidolites and lithian muscovites: influence of the OH ∕ F and Li ∕ Al ratios on the mica formation studied by NMR (nuclear magnetic resonance) spectroscopy and X-ray diffraction
Authors: Sulcek Lara , Marler Bernd , Fechtelkord Michael 
Published: March 2023

Abstract. A large number of lepidolites
K(LixAl3−x)[Si2xAl4−2xO10](OH)yF2−y
and Li-muscovites K(LixAl2-x/3□1-2x/3)[Si3AlO10](OH)yF2−y were synthesised by a gelling method in combination with hydrothermal
syntheses at a pressure of 2 kbar and a temperature of 873 K. The nominal
composition ranged between 0.0≤x≤2.0 and 0.0≤y≤2.0, i.e. from polylithionite
K[Li2.0Al][Si4.0O10](OH)yF2−y over
trilithionite
K[Li1.5Al1.5][AlSi3.0O10](OH)yF2−y to muscovite K[Al2.0□][AlSi3.0O10](OH)yF2−y. 1H, 19F,
29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS
NMR) and 27Al multiple-quantum magic-angle spinning (MQMAS) NMR
spectroscopy has been performed to investigate the order and/or disorder state of
Si and Al in the tetrahedral layers and of Li, Al, OH and F in the
octahedral layer. The synthetic mica crystals are very small, ranging from
0.1 to 5 µm. With increasing Al content, the crystal sizes
decrease. Rietveld structure analyses on 12 samples showed that nearly all
samples consist of two mica polytypes (1M and 2M1) of varying
proportions. In the case of lepidolites, the 1M / 2M1 ratio depends on
the Li/Al ratio of the reaction mixture. The refinement of the occupancy
factors of octahedral sites shows that lepidolites (1.5≤x≤2.0)
represent a solid solution series with polylithionite and trilithionite as
the endmembers. In the case of the Li-muscovites (0.0≤x≤1.5),
the 1M / 2M1 ratio depends on the number of impurity phases like
eucryptite or sanidine depleting the reaction mixture of Li or Al. There is
no solid solution between trilithionite and muscovite; instead, the
Li-muscovite crystals consist of domains differing in the relative
proportions of muscovite and trilithionite. The overall composition of the synthesised micas which consist of two
polytypes can be characterised by 29Si, 1H and 19F MAS NMR
spectroscopy. The Si/Al ratio in the tetrahedral layers and thus the content
of [4]Al were calculated by analysing the signal intensities of the
29Si MAS NMR experiments. The Li content xest was calculated from
the measured tetrahedral Si/Al ratio of the 29Si MAS NMR signals. The
calculated Li contents xest of samples between polylithionite and
trilithionite agree with the expected values. The F-rich samples show slightly
increased values and the OH samples lower values. Lepidolites with only F
(x = 1.5 to 2.0, y = 0.0), but not lepidolites with only OH (x = 1.5 to 2.0
and y = 2.0), were observed after synthesis. With decreasing Li content, x≤1.2, Li-muscovites containing mostly hydroxyl (y>1.0) are
formed. It was possible to synthesise fluorine containing micas with a
Li content as low as 0.3 and y = 0.2 to 1.8. The 19F and 1H MAS NMR
experiments reveal that F and OH are not distributed statistically but local
structural preferences exist. F is attracted by Li-rich and OH by Al-rich
environments. The quadrupolar coupling constant which represents the
anisotropy of the Al coordination is low for polylithionite with CQ=1.5 MHz and increases to CQ=3.8 MHz for trilithionite. For
tetrahedral Al a smaller increase of CQ from 1.7 to 2.8 MHz is
observed. Advancing from trilithionite to muscovite both quadrupolar
coupling constants decrease to 2.5 MHz for octahedral and 1.5 MHz for
tetrahedral Al. In polylithionite there is the most isotropic environment
for octahedral Al; there are only Li2Al sites coordinated by F in the
octahedral sheets and O from the tetrahedral sheets which are regular,
containing only Si. The distortion and anisotropy for Al in tetrahedral
as well as octahedral sheets increases with rising Al content. The most anisotropic
environment can be found in trilithionite, especially for octahedral Al.