This paper presents the results of an experimental study of muscovite solid solutions in the system K2O-M2+O-Al2O3-SiO2-H2O-(HF), with M2+ = Mg2+ or Fe2+ in the temperature range 300-700°C under 2 kbar PH2O. Muscovite solid solutions can be described, in this system, as the result of two substitutions. One is the phengitic substitution (x), which preserves the pure dioctahedral character of the mica; the second is the biotitic substitution (y), which leads to trioctahedral micas and does not change the composition of the tetrahedral layer Si3Al. The general formula of muscovite in this system is K(Al2−x−2y∕3M2+x+y□1−y∕3)(Si3+xAl1−x)O10(OH,F)2. Both substitutions x and y are more extensive at lower temperatures. The extent of solid solution decreases drastically with increasing temperature.For T > 600°C, the phengitic substitution (x) becomes negligible, but some biotitic substitution (y) persists. This unsymmetrical decrease of the solid solution of muscovite with increasing temperature is similar to that previously observed in phlogopite, the micas with a tetrahedral layer composition of Si3Al being the most stable. The behaviour of muscovite solid solutions in the ferrous system is qualitatively identical to that observed in the magnesian one, but the extent of solid solution is smaller than with Mg2+. Fluorine neither changes the size nor the shape of the solid solution fields but increases their stability by about 50°C.A comparison of these experimental results with data on natural muscovites is presented. Most natural primary (magmatic) granitic muscovites lie very close to the muscovite end member, in agreement with their high-temperature origin. Low-temperature muscovites (300–400°C), typically muscovites from hydrothermally altered granitic rocks, can have high x and y values. The rate of the biotitic substitution y can reach 0.6, which corresponds to an octahedral occupancy of 2.2 atoms per formula unit (based on 11 oxygens), consistent with the experimental data.

ABSTRACT. This paper presents the results of an experimental study of the miscibility gap between trioctahedral and dioctahedral micas in the system K20 Li20-MgOFeO AI203-SiO2-H20-HF at 600~ under 2 kbar Pu2o. 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 do6 o. 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.

Dans les deux types de leucogranites constituant le massif de Millevaches (leucogranites occidentaux et leucogranites orientaux), la composition des phyllites dioctaédriques et des feldspaths ainsi que l'état structural de ces derniers conduisent à caractériser les différentes étapes de cristallisation : magmatique (I), tardi à postmagmatique (II) et hydrothermale (III). Les minéraux primaires ou de cristallisation magmatique (muscovites riches en Ti et Na et feldspaths monocliniques et relativement riches en sodium — Or 86) ont une composition constante à l'échelle de l'affleurement. A l'échelle du massif, leur changement de composition reflète l'évolution magmatique du granite. Entre les minéraux primaires, les éléments tels que Fe, Mg, Ti, Rb, Ba... montrent des coefficients de partage bien définis. Les phénomènes de seconde génération (II) — cristallisation de muscovites pauvres en Ti et Na, cristallisation de feldspaths potassiques (Or 91 à 98) tricliniques, l'inversion partielle mono à triclinique des feldspaths I et le remplacement partiel de l'albite par le microcline — nécessitent tous la présence d'eau et ont lieu immédiatement après la cristallisation magmatique sous l'action des fluides résiduels (transformations deutériques). Les transformations III (illite-smectite) sont liées à des altérations hydrothermales très postérieures à la consolidation du granite, le long de la bordure orientale faillée du massif. La répartition des transformations deutériques, dans les deux leucogranites du Sud-Millevaches amènent à conclure que les leucogranites orientaux diffèrent des leucogranites occidentaux par une histoire cristallogénétique complexe liée à la présence d'eau dans les stades tardi à post-magmatiques, tandis que l'histoire des leucogranites occidentaux est pratiquement réduite à la cristallisation magmatique. Ces différences sont reliées à la teneur originelle en eau de chaque magma et à leurs conditions de refroidissement, elles-mêmes en relation avec le niveau de mise en place.

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