The structure of a lepidolite-2M1 from Biskupice, Czechoslovakia, has been redetermined. Violations of systematic extinctions and of monoclinic equivalences plus the results of a second harmonic generation test indicate that the true symmetry most likely is C1̄. The deviation of the data set from C2/c symmetry, however, proved to be too small to permit a statistically significant refinement in C1̄. Refinement in C2/c symmetry indicated no ordering of tetrahedral cations but ordering of octahedral cations so that M(1) = Li0.93R2+0.06Fe3+0.01 and M(2) = Al0.58Li0.35□0.07. The tetrahedra are elongated to form trigonal pyramids with a rotation angle of 6.2°. The anomalous orientation of the thermal ellipsoid for the F,OH anion plus the large equivalent isotropic B value of 2.58 for F,OH and of 1.74 for the interlayer K cation, whose position is partly restricted in C2/c symmetry, suggest a lower symmetry than C2/c.

The compositions of this sample and of a second lepidolite-2M1 from Western Australia fall outside the stability field of lepidolite-2M1 in the synthetic system. Structural control of the stacking sequence is discounted on the basis of the structural similarity of the lepidolite unit layers. Crystallization parameters are considered more important than composition or the structure of the unit layer in determining the stability and occurrence of different layer-stacking sequences in lepidolite.

Long-range ordering of tetrahedral cations in micas is favored by phengitic compositions, by the 3T stacking sequence of layers, and by tetrahedral Si:Al ratios near 1:1. Phengites of the 1M, 2M1, and 2M2 polytypes are said to show partial ordering of tetrahedral cations, although the amounts of tetrahedral substitutions are small and the accuracies of determination are not as large as desired. The 3T structures of muscovite, paragonite, lepidolite, and protolithionite show tetrahedral ordering, as do the 2M1 brittle micas margarite and an intermediate between margarite and bityite. Muscovite-3T and margarite-2M1 are also slightly phengitic relative to their ideal compositions. Examples of octahedral cation ordering in micas are more abundant and are to be expected when cations of different size and charge are present. Octahedron M(1) with its OH,F groups in the trans orientation tends to be larger than the mean of the two cis octahedra as a result of the ordering of cations and vacancies. In some samples ordering has reduced the true symmetry to a subgroup of that of the ideal space group. If ordering in subgroup symmetry results in ordered patterns of different geometries but similar energies in very small domains, the average over all unit cells may simulate long-range disorder.

The method for prediction of Gibbs free energies of formation, based on the parameter ΔGO=Mz+(clay) characterizing the oxygen affinity of the cation Mz+, on the smectites, considered as hydrated clay minerals, has been used for micas and brittle micas, and yielded underestimated values. This method of prediction can be improved by a new set of parameters ΔGO=Mz+(clay), characterizing the electronegativity of a cation in a specific site (interlayer, octahedral, tetrahedral in the 10 Å minerals), determined by minimizing the difference between experimental Gibbs free energies and calculated Gibbs free energies of formation from constituent oxides. By considering the crystal structure of 10 Å and 14 Å minerals, and assuming the same electronegativity of cations, ΔGO=Mz+(o), in the octahedral sheets, an attempt is made to determine the electronegativity of cations in the brucitic sheet, ΔGO=Mz+(b). The results indicate that this prediction method compared to other determinations, gives values within 0.25% of the experimentally-estim ated values. The relationships between ΔGO=Mz+(clay) corresponding to the electronegativity of a cation in the interlayer, octahedral, tetrahedral or brucitic sites and known ΔGO=Mz+(aq) were thus determined, allowing the determination of the electronegativity of transition metal ions and trivalent ions in each of the four sites and consequently contribute to the prediction of Gibbs free energies of formation of different micas and chlorites. Examples are given for low-Fe clinochlore whose solubility is measured experimentally and the results appear excellent when compared with experimental values.

More than 19,000 analytical data mainly from the literature were used to study statistically the distribution patterns of F and the oxides of minor and trace elements (Ti, Sn, Sc, V, Cr, Ga, Mn, Co, Ni, Zn, Sr, Ba, Rb, Cs) in trioctahedral micas of the system phlogopite-annite/siderophyllite-polylithionite (PASP), which is divided here into seven varieties, whose compositional ranges are defined by the parameter mgli (= octahedral Mg minus Li). Plots of trace-element contents vs. mgli reveal that the elements form distinct groups according to the configuration of their distribution patterns. Substitution of most of these elements was established as a function of mgli. Micas incorporate the elements in different abundances of up to four orders of magnitude between the concentration highs and lows in micas of ‘normal’ composition. Only Zn, Sr and Sc are poorly correlated to mgli. In compositional extremes, some elements (Zn, Mn, Ba, Sr, Cs, Rb) may be enriched by up to 2–3 orders of magnitude relative to their mean abundance in the respective mica variety. Mica/melt partition coefficients calculated for Variscan granites of the German Erzgebirge demonstrate that trace-element partitioning is strongly dependent on the position of the mica in the PASP system, which has to be considered in petrogenetic modelling.

This review indicates that for a number of trace elements, the concentration ranges are poorly known for some of the mica varieties, as they are for particular host rocks (i.e. igneous rocks of A-type affiliation). The study should help to develop optimal analytical strategies and to provide a tool to distinguish between micas of ‘normal’ and ‘abnormal’ trace-element composition.

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.

Li-mica (zinnwaldite and/or lepidolite)—topaz—albite granites in the Tregonning—Godolphin pluton and similar rocks in the St. Austell pluton appear to be petrogenetically unrelated to the spatially associated biotite granites. Evidence is provided by lack of development of Li-mica granites at roof zones of biotite granites and markedly different trends and composition fields in bivariate plots such as Li vs. Cs, Rb vs. Sr and Nb vs. Zr. Thus, differentiation of biotite granite magma is unlikely to have generated Li-mica granite magma, as also, on its own, is partial melting of biotite granite or biotiteabsent residual lower crust. However, partial melting of biotite-rich residual rocks involving biotite breakdown could yield a trace alkali- and F-enriched melt, although this would require marked femic mineral, K-feldspar and anorthite fractionation, and Na-enrichment. It is proposed that volatiles derwed from either a mantle source or the crust/mantle interface have aided metasomatism of either residual S-type crust that earlier provided S-type biotite granite magma, or basic (biotite-rich) granitoid, to produce a low-temperature, low-viscosity Li-mica granite melt that rose rapidly in the crust soon after the emplacement of associated biotite granites.