Lithium in the interlayer space of synthetic trioctahedral micas

Abstract This paper gives several new and strong arguments in favour of the possibility of fixation of anhydrous lithium in the interlayer space of trioctahedral potassium micas. From the chemical viewpoint Li + can replace K + , but is located out of the alkaline cation site; it enters pseudo-octahedral cavities limited by the triangular bases of two aluminous tetrahedra of two consecutive sheets. The solubility limit of Li + in the interlayer is a function of the Al IV content of the mica. It is given by the relation ( Li Li + K ) max = 2[( Al Si + Al ) IV ] 2 . In both micas investigated — phlogopite, KMg 3 (Si 3 Al)O 10 (OH) 2 , and eastonite, K(Mg 2.5 Al 1.5 )(Si 2.5 Al 1.5 )O 10 (OH) 2 — there is a remarkable agreement between the calculated values of the solubility limits and those measured by exchange reactions with hydrothermal solutions, at 600°C, 2 kbar. In high-Al micas, the interlayer Li content can be very important, with about one-third of K + replaced by Li + . The fixation of Li + according to this model provokes a strong flattening of the interlayer (strong decrease of the reticular distance d 005 ) and a slight increase of the reticular distance d 060 . Infra-red (IR) absorption spectrometry shows that vacant K + sites are created when Li + enters the interlayer; one observes low-frequency OH stretching bands attributed to OH dipoles lying towards these empty sites. Fixation of Li + does not provoke any modification of the IR spectra in the region 1200-300 cm −1 , indicating that Li + is really out of the sheet. For both cell dimensions and IR spectra, a comparison is made with “ordinary” lepidolites, having Li + in the octahedral sheet; it provides a guide for the distinction between the two species of Li-bearing micas.