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Lara Sulcek, Bernd Marler, Michael Fechtelkord

• A large number of lepidolites \(K(Li_{x}Al_{3−x})\)\([Si_{2x}Al_{4−2x}O_{10}]\)\((OH)_{y}F_{2−y}\) and Li-muscovites \(K(Li_{x}Al_{2-x/3}◻_{1-2x/3})\)\([Si_{3}AlO_{10}]\)\((OH)_{y}\)\(F_{2-y}\) 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 \(\leq\) \(\it x\) \(\leq\) 2.0 and 0.0 \(\leq\) \(\it y\) \(\leq\) 2.0, i.e. from polylithionite \(K[Li_{2.0}Al][Si_{4.0}O_{10}]\)\((OH)_{y}F_{2-y}\) over trilithionite \(K[Li_{1.5}Al_{1.5}][AlSi_{3.0}O_{10}](OH)_{y}F_{2-y}\) to muscovite \(K[AL_{2.0}□][AlSi_{3.0}O_{10}](OH)_{y}F_{2-y}\). \(^{1}\)H, \(^{19}\)F, \(^{29}\)Si and \(^{27}\)Al magic-angle spinning nuclear magnetic resonance (MAS NMR) and \(^{27}\)Al 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 \(\mu\)m. 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 \(2M_{1}\)) of varying proportions. In the case of lepidolites, the 1M  \(2M_{1}\) ratio depends on the ratio of the reaction mixture. The refinement of the occupancy factors of octahedral sites shows that lepidolites (1.5 \(\leq\) \(\it x\) \(\leq\) 2.0) represent a solid solution series with polylithionite and trilithionite as the endmembers. In the case of the Li-muscovites (0.0 \(\leq\) \(\it x\) \(\leq\) 1.5), the 1M  \(2M_{1}\) 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 \(^{29}\)Si, \(^{1}\)H and \(^{19}\)F MAS NMR spectroscopy. The ratio in the tetrahedral layers and thus the content of [4]Al were calculated by analysing the signal intensities of the \(^{29}\)Si MAS NMR experiments. The Li content \(x_{est}\) was calculated from the measured tetrahedral ratio of the \(^{29}\)Si MAS NMR signals. The calculated Li contents \(x_{est}\) 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 (\(\it x\) = 1.5 to 2.0, \(\it y\) = 0.0), but not lepidolites with only OH (\(\it x\) = 1.5 to 2.0 and \(\it y\) = 2.0), were observed after synthesis. With decreasing Li content, \(\it x\) \(\leq\) 1.2, Li-muscovites containing mostly hydroxyl (\(\it y\)>1.0) are formed. It was possible to synthesise fluorine containing micas with a Li content as low as 0.3 and \(\it y\) = 0.2 to 1.8. The \(^{19}\)F and \(^{1}\)H 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 \(C_{Q}\)=1.5 MHz and increases to \(C_{Q}\)=3.8 MHz for trilithionite. For tetrahedral Al a smaller increase of \(C_{Q}\) 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 \(Li_{2}Al\) 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.