MINERALOGICAL MAGAZINE, JUNE 1986, VOL. 50, PP. 257-66 Muscovite solid solutions in the system K20-MgO-FeO-A1203-SiO2-H20: an experimental study at 2 kbar PH2o and comparison with natural Li-free white micas GILLES MoNIER Laboratoire de P&rologie, Universit6 d'Or16ans, 45046 Orlrans Cedex, France AND JI~AN-LouIs ROBERT Centre de Recherche sur la Synthrse et Chimie des MinSraux, G.I.S.C.N.R.S.-B.R.G.M., 1A rue de la Frrollerie, 45071 Odrans Cedex 2, France ABSTRACT. This paper presents the results of an experi- K EY W OR O S : muscovite, phengite, solid solution, crystal- mental study of muscovite solid solutions in the system chemistry, experimental mineralogy, granites, hydro- K20-M~+O-A1203 SiO2-H20 (HF), with M2+= thermal alteration. Mg 2+ or Fe 2§ in the temperature range 300 700~ under 2 kbar P.~o, Muscovite solid solutions can be described, in this system, as the result of two substitutions. NATURAL lithium-free white micas are generally One is the phengitic substitution (x), which preserves the described as solid solutions between the muscovite pure dioctahedral character of the mica; the second is the end member K(A12R)(Si3AI)Olo(OH)2, where [] biotitic substitution (y), which leads to trioctahedral stands for an octahedral vacant site, and the micas and does not change the composition of the celadonite end member K(AIM z+ [3)Si4010(OH)2, tetrahedral layer Si3AI. The general formula of muscovite with M 2 + = Mg 2., Fe E+, thus, they are considered in this system is K(A12_~_2y/aM2+yOl_y/a)(Sia+~All_x) to belong to the so-called phengitic series. The OIo(OH,F)2. Both substitutions x and y are more substitutional mechanism which describes this extensive at lower temperatures. The extent of solid solu- tion decreases drastically with increasing temperature. series can be written For T > 600 ~ the phengitic substitution (x) becomes AlV',AtTM ~ (M: +)v',Siw. (1) negligible, but some biotitic substitution (y) persists. This unsymmetrical decrease of the solid solution of muscovite If x is the amount of substitution, according to with increasing temperature is similar to that previously equation (1), the muscovite end member has x = 0 observed in phlogopite, the micas with a tetrahedral layer and the Al-celadonite end member has x = 1. The composition of SiaAl being the most stable. The behaviour purely dioctahedral character is maintained. This of muscovite solid solutions in the ferrous system is series has been the subject of several experimental qualitatively identical to that observed in the magnesian investigations (Crowley and Roy, 1964; Velde, one, but the extent of solid solution is smaller than with Mg 2§ Fluorine neither changes the size nor the shape of 1965, 1980; Green, 1981) and of many studies of the solid solution fields but increases their stability by micas in metamorphic rocks. These works show an about 50 ~ increase of the extent of the phengitic substitution A comparison of these experimental results with data [mechanism (1)] with increasing water pressure. on natural muscovites is presented. Most natural primary The possibility of solid solution between diocta- (magmatic) granitic muscovites lie very close to the hedral and trioctahedral micas has been rarely muscovite end member, in agreement with their high- considered; this kind of solid solution involves a temperature origin. Low-temperature muscovites (300 second substitutional mechanism: 400 ~ typically muscovites from hydrothermally altered granitic rocks, can have high x and y values. The rate of 2/3A1vt, 1/3 []w ~ (M 2 +)vk (2) the biotitic substitution y can reach 0.6, which corresponds to an octahedral occupancy of 2.2 atoms per formula unit If y is the amount of substitution, according to (based on 11 oxygens), consistent with the experimental equation (2), the muscovite end member has y = 0 data. and the biotite end member (phlogopite with (~ Copyright the Mineralogical Society 258 G. MONIER AND J.-L. ROBERT M 2+= Mg and annite with M 2+= Fe2+), has starting proportion used was Fe 3 +/Fe ~ = 3/2; this y = 3, with the formula KM2+(Si3AI)OIo(OH)2. technique leads to the rapid attainment of a In this series, the number of octahedral vacant reproducible equilibrium in which ferrous iron is sites is variable. Recently, Massonne (1981) has widely prevalent (Julliot et al., 1985). In addition, taken this possibility into account in his experi- the system may be considered to be roughly mental work on phengites between 5 and 30 kbar. It buffered by the couple Ni-NiO, set by the alloy is noteworthy that most natural muscovite solid constituting the pressure-vessel. solutions are combinations of substitutions (1) and After the experiments the products were identi- (2) and can be described by the two parameters fied and characterized under the polarizing micro- (x, y). So, the general formula of muscovites scope, by scanning electron microscopy and by is 2 + Vl " lV K(Alz-x-2rl3Mx+rDl r/3) (S13+xAll-x) O10 X-ray diffraction. The cell dimensions of micas are (OH) 2. As will be shown later, high values of y are of interest, and specially the cell parameter b = observed in low-pressure white micas; therefore, 6" d060, classically used to characterize micas. The this study deals with the extent of muscovite solid lattice spacing d060.33i- (given as d600 in the tables) solutions in the system K20-M 2 +O-AI203-SiO 2- was systematically measured, with pure silicon H20, considering both substitutions (1) and (2), added as an internal standard, the radiations used under 2 kbar PH2o, as a function of temperature being Cu-Ke (2 = 1.5418 A) for magnesian micas between 300 and 700 ~ Two series of experiments and Co-Ke (2 = 1.7902 A) for ferrous micas. The were performed, one with M 2+ = Mg 2+ and one micas obtained as a single phase have also been with M 2+ = Fe z+. In addition, fluorine was added studied by infra-red spectrometry; the results of this to the system for some runs. Several other im- study wilt be given in a forthcoming paper. portant substitutions are possible in muscovites, The results are represented (fig. 1) in a triangle especially those involving Fe 3+, Ti *+, and M 2 + A1 Si (atomic proportions; A1 = AlW+ AlV[ interlayer cations; they are not considered here. This triangle is a section nearly parallel to the base M 2 + -A1-Si of a K-M 2 § -A1-Si tetrahedron repre- Experimental method senting the experimental system. All the micas The extent of the solid solution domain stable at each temperature has been determined experi- Cor .o~Mu mentally by using classical methods of hydro- .:~ "~,!~ thermal synthesis. The starting products were gels of appropriate composition, i.e. having variable AIS values ofx and y, prepared according to the method .z/i"L described by Hamilton and Henderson (1968). 100 mg of each gel were introduced into a gold tube ,,r "625V-7/? b.375 together with 15 microlitres of distilled water, before arc welding. The use of cold-seal pressure ,.o{/z '~'~ h.sPh vessels permitted the simultaneous control of x=0 y.3x y.x =0 temperature and pressure. The temperature was tst( measured with a thermocouple located inside the wall of the pressure vessel; the accuracy of measure- 1.80/ ment is estimated to be • 5 ~ The pressure was measured with a Bourdon-type manometer, with an estimated accuracy of + 50 bars. The minimum 2.40?/ run duration was chosen as a function of tempera- ture: 8 days above 600~ 12 days at 600~ 2.7/ 3 weeks at 500 ~ 1 month at 400 ~ and 3 months 3.oo/~Phl,Ann at 300~ Previous work on similar systems (Robert, 1976, 1981) has shown that these durations FIG. 1. Positions of mica end members, principal joins are satisfactory to obtain reproducible equilibria. and starting experimental compositions in the diagram For the synthesis of micas in a fluorine-bearing M z+ AI Si (atomic proportions). (o) magnesian com- system, KF was added to a starting gel depleted in positions; (o) ferrous compositions. The sum x + y = M z + is indicated close to the experimental points. Mu = potassium so as to preserve the bulk potassium muscovite end member, Ph = phengite, Cel = celadonite stoichiometry of the starting product. end member, Phl,Ann = phlogopite and annite end In the iron-bearing system, the starting product members, East = eastonite, Sid = siderophyllite, Cor = was a mixture of two gels, one containing iron as corundum, Als = aluminosilicate, San = sanidine, Qz = Fe ~ and the other containing iron as Fe 3 § the right quartz. MUSCOVITE SOLID SOLUTIONS 259 studied here belong to this plane, since all of them All the micas situated between the first two joins contain one K atom per formula unit. In this plane are solid solutions between dioctahedral and tri- three joins are important: the join muscovite- octahedral micas. The micas which belong to the celadonite (x), i.e. the phengitic series--on this join, join muscovite-phlogopite (or annite) have a con- the micas are purely dioctahedral; the join phlogo- stant composition in their tetrahedral layer: Si~AI. pite-eastonite (Robert, 1976, 1981) in the mag- nesian system or annite-siderophyllite (Rutherford, Experimental results in the magnesian system t973) in the ferrous system--this join is parallel to the previous one and corresponds to purely tri- The different starting compositions are given in octahedral micas; and the join muscovite-phlogo- Tables I-IV in terms of x, y, and F content; the run pite (or annite), (y), along which all the micas are temperature is also indicated in these tables, to- solid solutions between dioctahedral and triocta- gether with the nature of the final phase assem- hedral micas.