Evolution of the Miscibility Gap Between Muscovite and Biotite Solid Solutions with Increasing Lithium Content: An Experimental Study in the System K2O-Li2O-MgO-FeO-Al2O3-SiO2-H2O-HF at 600°C, 2 kbar PH2O: Comparison with Natural Lithium Micas

Gilles Monier and Jean-Louis Robert
Laboratoire de Pétrologie, Université d'Orléans, 45046 Orléans Cedex, France
Centre de Recherche sur la Synthèse et la Chimie des Minéraux, G.I.S. C.N.R.S.-B.R.G.M, 1A rue de la Férollerie, 45071 Orléans Cedex 2, France

Abstract: This paper presents the results of an experimental study of the miscibility gap between trioctahedral and dioctahedral micas in the system K2O Li2O-MgO-FeO-Al2O3-SiO2-H2O-HF at 600°C under 2 kbar PH2O. The existence of this miscibility gap is known from previous experimental studies. The gap is large in the lithium-free system; its width reduces progressively with increasing Li content; for sufficient Li contents (Li > 0.6 atom per formula unit, based on 11 oxygens), a single Li-mica phase is obtained, intermediate between trioctahedral and dioctahedral micas. Any bulk composition located inside the miscibility gap gives an assemblage of two micas, one of the biotite-type and one of the muscovite-type. All the compositions located outside the gap, and, in particular, those belonging to the joins phlogopite-trilithionite and muscovite-zinnwaldite (or its magnesian equivalent) give a single mica phase, provided that the fluorine content is sufficient. The ratio Li/F ≈ 1 is a convenient suitable value. The types of micas and the evolutions of their compositions are well characterized by their interplanar distance d060. These experimental results allow the interpretation of most compositions of naturally occurring lithium micas, in the range 0 ⩽ Li ⩽ 1 a./f.u. Natural micas of biotite-type and muscovite-type are located on both sides of the miscibility gap and their compositions get closer with increasing Li content.

Keywords: biotite • muscovite • lithium mica • zinnwaldite • lepidolite • crystal-chemistry • experimental mineralogy • leucogranite

Mineralogical Magazine; December 1986 v. 50; no. 358; p. 641-651; DOI: 10.1180/minmag.1986.050.358.09
© 1986, The Mineralogical Society
Mineralogical Society (www.minersoc.org)