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 orientation of the OH-vectors in hydroxyl groups of micas of different compositions, polytype modifications, and symmetry were calculated by the method of minimization of electrostatic energy. The orientations are strongly affected by the peculiarities of structures, and even slight deviations from the ideal dioctahedral periodicity can introduce a correction of as much as 10° to the value of the polar angle. Ordering of cations in octahedra lead to twisting of the OH-bond towards the octahedron with lower charge and larger dimension. The latter is less true for cation ordering in tetrahedra. Because of these factors and the dependence of the hydrogen position on the coordinates of heavy atoms, such calculations can give reliable results only if the structure has been refined with high precision. Different polytypes of micas displayed the same orientation of the OH-vectors with respect to the axes of a separate layer, if the stacking sequence did not introduce specific distortions of the 2:1 layer.

Crystal growth theory was applied to describe edge sites of phyllosilicates. Three face configurations were found to exist. One face has one tetrahedral site per tetrahedral sheet and two octahedral one-coordinated sites per crystallographic area ac sin β, where a and c are layer dimensions and β is the angle between them. The other two faces are similar except that they have one less octahedral site which is replaced by one SiIV-O-AlVI site in this same ac sin β area. A transfer of bonding energy from the remaining octahedral site to the SiIV-O-AlVI site is believed to neutralize all edge charge on faces containing these latter sites at normally encountered pHs (pH 3–9). A similar charge rearrangement along the edges results in an apparent decrease in the permanent charge of the mineral with an increase in edge area.

On the basis of such an analysis, lath-shaped illite can be described as a very fine grained dioctahedral mica in which the apparent deficient occupancy of the octahedral sheet, presence of excess water, and measurable cation-exchange capacity may in part be the result of a large ratio of edge area to total volume, with no other chemical or structural change in the mica layers. The increasing importance of edge charge relative to layer charge produces erroneous formulae for 2:1 phyllosilicates in very fine grained samples containing fewer than 2 of 3 octahedral sites occupied by cations, on the basis of a 22-charge half cell.

The crystal structure of phengite-2M1 from Rio de Oro, Spanish Sahara, was refined in space group C2/c to a residual of 3.3% with 1267 independent X-ray diffraction reflections. The composition of the mica determined by electron microprobe analysis is (K0.948Na0.051Ba0.027)(Al1.510Mg0.273Fe0.144Cr0.095Ti0.010Mn0.003)2.035(Si3.253Al0.747)O10(OH)2. The cell dimensions are a = 5.2153(5), b = 9.043(2), c = 19.974(9) Å, β = 95.789(9)°, and V = 937.2(3) Å3. The substitution of Si for Al in the tetrahedral sheet and larger divalent cations for Al in the octahedra allows the amount of distortions that are generally required to alleviate tetrahedral-octahedral sheet lateral misfit in muscovite, such as tetrahedral rotation and octahedral flattening, to be reduced. The O…H vector is nearly horizontal and points slightly into the octahedral sheet where it may be involved in hydrogen bond contacts inside M(1). In contrast to previously reported refinements of all but one other phengite, no ordering of tetrahedral cations was found in the study specimen. This disorder is considered to be due to the low-pressure, high-temperature amphibolite facies environment of crystallization.

TheAu-Cu (±Pb, Zn, Ag) prospects of Lavras do Sul, southernmost Brazil, arehosted in Neoproterozoic granitic and volcanogenic rocks. Mineralization occurs in structurally controlled N40°W to E-W quartz veins; sericite (±chlorite) and sulfides are the main secondary minerals in associated wall rocks.

In the present contribution we use petrography (optical microscopy and scanning electron microscopy (SEM)), mineralogy (X-ray diffraction (XRD) with polytypes, FWHM, decomposition of diffraction patterns), and crystal chemistry of samples from several prospects to document the spatial and temporal evolution of sericitic alteration of veins and wall rocks associated with gold.

Hexagonal, coarse-grained 2M1 phengite-rich alteration (± illite) is best developed with coarse-grained primary growth (comb) quartz + pyrite ± Au veins and altered wall rock from the western portion of the granitic complex (phyllic alteration). Pure phengite was recognized by narrow XRD profiles (FWHM ⩽ 0.2°2θ CuKα) of the <5 µm particle-size fraction, non-expandable d001 X-ray reflections and interlayer charge (IC) >0.9 per O10(OH)2.

Towards the eastern zones of the granitic complex and in the volcanogenic rocks, wider XRD profiles (FWHM values ⩾ 0.2°2θ CuKα) were decomposed. They contain mixtures of coarse- to fine-grained, lath-like crystals of both 2M1 and 1M illite (non-expandable d001 X-ray reflections, IC between 0.85 and 0.89 per O10(OH)2) with expandable d001 reflections associated with lath-like, fine-grained crystals of ordered (R ⩾ 1) illite-rich I-S (80–90% of illite; IC of ∼0.8 per O10(OH)2), and minor amounts of regularly ordered (R = 1), illite-rich I-S mixed layers (75% of illite; IC of ∼0.74 per O10(OH)2). The dioctahedral clay association of illite + illite-rich I-S mixed layers (intermediate argillic alteration) is best developed in quartz + pyrite ± Au veins, breccias, and wall-rock alteration from the eastern portion of the granitic complex and in the volcanic area. Quartz from veins and breccias has fine-grained primary growth, recrystallization, and replacement textures, similar to those in epithermal deposits.

The overall distribution of the dioctahedral clays indicates that the study area represents a fracture-controlled, tilted, porphyry to epithermal deposit, with telescoping alteration features observed in the east of the mining district. Deeper levels of exposure of a large hydrothermal system are observed in the west of the mining district, as shown by higher-rank dioctahedral minerals (phengite) that crystallize at relatively high temperatures (Tphe ≈ 300°C, phyllic alteration) associated with coarse-grained, primary-growth quartz veins, similar to those observed in porphyry deposits. On the other hand, shallower levels of exposure are observed in the east of the study area, associated with abundant, lower-rank dioctahedral clay minerals (illite + illite-rich I-S mixed layers, intermediate argillic alteration) that crystallize at relatively lower temperatures (TI-S ≈ 120–200°C).

Available data show that gold is associated with phengite, but that lower-rank, overprinting alteration characterized by illite-I-S may have locally modified the original gold grades.

This study corroborates interpretations suggesting that the Makrotantalon Unit on Andros represents a tectonic slice with Pelagonian affinity in the nappe stack of the Cycladic Blueschist Unit. Previously reported Cretaceous 40Ar–39Ar dates of a garnet-glaucophane schist from the Makrotantalon Unit could not be reproduced by Rb–Sr geochronology, but this is not an indication of contamination with excess Ar. Instead, the newly dated samples record disturbance of the Rb–Sr isotope system by partial recrystallization. Subsets of these phengite populations, representing the smaller grain-size fractions, yielded low-precision dates ranging from c. 21 to c. 15 Ma that document deformation-related resetting and recrystallization of a presumably Cretaceous white mica population. Although these Miocene dates cannot be linked with blueschist-facies metamorphism, they provide time constraints on the formation of shear zones that overprinted the original thrust contact during exhumation. The geological relevance of a Cretaceous high-pressure event is confirmed by a Rb–Sr date of c. 121 Ma for an epidote-glaucophane schist collected further away from the tectonic contact. The occurrence of a second blueschist-facies event in the Eocene is verified by Rb–Sr dates of two epidote-glaucophane schists (c. 40 Ma and c. 44 Ma) that can unambiguously be assigned to the Makrotantalon Unit.

To reveal the factors that determine the different ranges of compositional variations in high- and low-temperature Al-rich K-dioctahedral micas, relationships between structural parameters and cation composition were analysed for: (1) a series of synthetic 2M1 muscovite–phengite–aluminoceladonite samples; and (2) Al-rich, K-dioctahedral 2M1 micas with previously published refined structural data. The dependences of the unit-cell parameters on cation composition and the variations in tetrahedral and octahedral lateral dimensions and sheet thicknesses, interlayer distances and tetrahedral rotation angles were analysed and compared with those found previously for the series 1M trans-vacant (tv) illite–1M aluminoceladonite. The similarities in the variations of unit-cell parameters with cation composition observed in 2M1 and 1M natural and synthetic K-dioctahedral micas imply that these variations are controlled by similar – albeit not identical – structural factors. A major structural factor is the readjustment of the differently sized tetrahedral and octahedral sheets, which is realized in a different manner in micas formed under different pressure and temperature conditions.

Triassic sequences from 'intermediate units' between the Alpujárride and the Malâguide complexes (Betic Cordilleras, Spain) of the westernmost part of the Cordilleras (Casares area) occur as four superimposed tectonic units; the uppermost unit shows lithological characteristics similar to those of the Malâguide complex, changing progressively at increasing depth, towards lithologies typical of the Alpujárride complex. The units studied, with a maximum thickness of ~400 m, record important variations in metamorphic pressures, according to the b parameter of white micas: from low-pressure metamorphism (in the unper unit) to high-pressure faciès series (in the deepest one). The mean b values range from 8.988 Å in the uppermost unit (Crestellina) to 9.042 Å in the lowermost one (Jubrique). The lowest metamorphic grade is represented by mineral assemblages consisting of phengite + intermediate Na-K white mica ± Fe-chlorite ± sudoite ± pyrophyllite, which record temperatures of ~300°C and pressures of 1.5—3 kbar. At increasing tectonic depth, intermediate Na-K mica and pyrophyllite disappear and the metamorphic assemblages consist of phengite ± paragonite ± margarite + Mg-chlorite ± sudoite, which record minimum pressures of ~7 kbar and temperatures in the order of 400—450°C. These mineral assemblages provide evidence of the passage from collisional to extensional geotectonic settings. The units showing different metamorphic patterns were juxtaposed tectonically, after the development of metamorphic mineral assemblages.

Phengite and chlorite have undergone decomposition during pyrometamorphism caused by the intrusion of a dolerite feeder pipe into Dalradian greenschists in Argyllshire, Scotland. All reaction products are extremely fine grained. Transmission electron microscopy has revealed that phengite pseudomorphs consist of biotite, spinel, mullite, sanidine and phengite, and that chlorite pseudomorphs consist of combinations of chlorite, spinel, orthopyroxene, magnetite, cordierite and biotite. Although the reactions were short-lived and did not go to completion, microprobe analysis and phase diagram analysis have revealed that there has been significant chemical interaction between the phyllosilicates and the surrounding rock. Numerous orientation relationships exist between the original minerals and their reaction products; the close-packed planes in the precursor phyllosilicates were inherited by their reaction products.

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.

Metamorphic rocks of undoubted crustal origin have been described in recent years, principally from Mediterranean collision zones that have been subjected to PT conditions along very low geothermal gradients (∼ 7°C/km) and have reached pressures up to 30 kbar. MgAl-rich metapelites develop particularly diagnostic high-pressure minerals and mineral assemblages that have been and are being studied experimentally in model systems involving the components K2O, MgO, Al2O3, TiO2, SiO2, P2O5, and H2O up to pressures of 50 kbar and temperatures of 1000°C.

In the present review the following synthetic phases and phase assemblages are discussed, emphasizing their water-pressure-temperature stability fields (approximated in parentheses here), their reaction relationships, and their known or potential occurrences in metamorphic rocks. Sudoite (0 to ∼ 12 kbar, 150? to 380°C) occurs in very low-grade metapelites. Mg-carpholite (∼ 7 to ∼ 45 kbar, ∼ 200 to 600°C) is found in subducted metabauxites, metapelites, and related quartz veins. Mg-chloritoid (18 to 45 kbar?; 400 to 760°C) has not been found in nature as pure or nearly pure end-member; it requires silica-deficient environments. Yoderite, known in nature only from a single talc-kyanite schist occurrence, has only a small stability field (9 to 18 kbar?, 700 to 870°C?), cannot coexist with quartz, but may be stabilized by Fe3+. Pyrope (∼ 15 to at least 50 kbar, ∼ 700°C to melting), with or without relic coesite inclusions, occurs spectacularly in quartzites. Mg-staurolite (∼ 14 to some 90 kbar?, 700 to 1000°C), recently discovered as inclusions in pyrope, requires silica-deficiency. MgMgAl-pumpellyite is a new synthetic phase in which Mg totally replaces Ca of normal pumpellyite; because of its very high-pressure, low-temperature stability (∼ 37 to at least 55 kbar, < 400 to 780°C) it may not form within our globe. Ellenbergerite, the new high-pressure mineral forming inclusions in pyrope, apparently exhibits a rather composition-dependent stability with Ti-ellenbergerite, requiring higher pressures (> 20 kbar) than P-bearing, Ti-free members; a pure hydrous Mg-phosphate with ellenbergerite structure was synthesized at 10 kbar. Phengites, the widespread MgSi-substituted muscovites, require increasingly high water pressures (up to ∼ 20 kbar) for higher degrees of substitution, but the Al-celadonite end-member is not stable under any conditions; the compositions of phengites coexisting with limiting assemblages such as phlogopite, K-feldspar, and an SiO2 phase are useful geobarometers. The common assemblage Mg-chlorite + Al2SiO5 (mainly kyanite) has an extensive stability field ranging from near zero to 31 kbar at temperatures varying from some 320 to ∼ 760°C depending on pressure. The whiteschist assemblage talc + kyanite (6 to ∼ 45 kbar, 550 to 810°C) plays an important role in collision zone metamorphism as it forms from the greenschist assemblage chlorite + quartz at low grades but is also known to break down into pyrope + coesite at the highest grade observed thus far. The assemblage talc-phengite (11 to at least 35 kbar, 300? to 820°C depending on pressure), on the other hand, is well known from subducted metapelites. At pressures of 15–20 kbar and temperatures of 400–650°C a very K,Mg-rich, siliceous fluid forms as a consequence of the mutual reaction of the minerals K-feldspar and phlogopite (biotite) which are very common in crustal rocks including granites. Such fluids are bound to cause metasomatism in neighbouring mantle rocks which, upon subsequent increase of temperature, produce post-collisional ultrapotassic, lamproitic melts.

The results of a microstructural study by backscattered scanning electron microscopy and a microchemical study using X-ray microprobe analysis of phyllosilicate intergrowths from sandstones, shales, metagreywackes, and low-grade schists are presented. The microstructural study revealed that the intergrowths thicken and become more coherent with metamorphic grade; the intergrowths change from incoherent to coherent in the anchizone. The increasing coherency is mirrored by an increase in the crystallinity indices of the illites/phengites. Chemical analysis of the individual intergrowth phases was difficult in the sediments and no systematic compositional variations were recorded. However, clear compositional trends with increasing metamorphic grade emerged in the phengites from the metagreywackes and schists, but in the chlorites only slight compositional changes were recorded.

Lithium micas of the zinnwaldite and phengite–Li-phengite series occur as characteristic minerals in Permian Li-F-(P) granites of the western Gemeric unit (Western Carpathians) accompanied by topaz, tourmaline, Nb, Ta, Ti, Sn oxides and aluminophosphates. The calculated Li2O contents of all the mica analysed, together with Rb2O and Cs2O were confirmed by LA-ICP-MS analyses for all the identified micas. Samples from three localities were investigated: two surficial (Surovec, Vrchsúl’ová); and one drill hole (Dlhá dolina). Zinnwaldite (polylithionite) occurs in the upper level of the Dlhá dolina granitic intrusion and in the nearby shallow satellite body of Surovec. The lower level porphyritic granites contain only siderophyllite. The Vrchsúl’ ová micas are closer in composition to Li-annite and siderophyllite. Dioctahedral micas are mostly phengites, although zinnwaldite-bearing granites are rich in late-crystallizing Li-phengite, which extensively replaces earlier zinnwaldite. The secondary Liphengite and phengite are interpreted as products of Alpine metamorphism during Cretaceous burial and subsequent exhumation of the Gemeric unit. Reactions are suggested explaining the formation of Li-phengite by reaction of zinnwaldite with phengite or with muscovite. All mica types were investigated by Mössbauer spectroscopy, which showed high degrees of oxidation (25–50% Fe3+ of total Fe) with the exception of zinnwaldite from Vrchsúl’ová, which may have preserved an original, reduced value of 10%. The metamorphic assemblage present permitted calculation of P-T-X conditions: T = 184°C, P = 320 MPa, with oxidation of siderophyllite to phengite + goethite and fO2 at ΔNN = 4.7, confirming the low-grade conditions of the Alpine metamorphism in agreement with previous estimates.

This paper reports new geochronological data from the island of Andros, one of the less-studied islands of the Cycladic blueschist belt in the central Aegean Sea. On Andros, two tectonic units can be distinguished, the Makrotantalon unit and the Lower unit, which are separated by a low-angle normal fault, related to large-scale regional extension. Mineral assemblages indicate greenschist-facies P–T conditions for the last metamorphic overprint of both units. In contrast to the structurally lower unit, unambiguous indications for an earlier high-pressure stage were not recognized in rocks collected above the tectonic contact. Owing to a polyphase metamorphic evolution and incomplete resetting of the Rb–Sr isotope system during overprinting, phengite geochronology indicates a wide range in dates between c. 104 and 21 Ma for the Makrotantalon unit, as observed in rocks of similar structural position elsewhere in the Cyclades. The new Rb–Sr data support the interpretation, but are not conclusive evidence, that tectonic slices within the hanging wall were affected by two periods of Cretaceous metamorphism (c. 100–90 Ma and c. 80–70 Ma) and a Miocene event (c. 21 Ma). Tectonic juxtaposition was accomplished around c. 21 Ma. The Lower unit is correlative with the Cycladic high-pressure occurrences. Rb–Sr phengite dating yielded the same range in ages as determined elsewhere in the region for white mica of high-pressure rocks (c. 50–40 Ma) and their overprinted, greenschist-facies derivatives (c. 23–21 Ma). An age gradient towards the tectonic contact with the overlying Makrotantalon unit is not developed. The new results fit well into the previously established chronological framework for the larger study area. Indications for regional differences in the timing of the HP stage and/or the greenschist-facies overprint have not yet been found.

Single grains of detrital white mica from two different synorogenic sediments in the Southern Urals were analysed using the in situ ultraviolet laser ablation Ar–Ar dating technique to discriminate between age signatures associated with a high-pressure signal (phengites) from those related to muscovite only. Two disparately aged sandstone formations of Neoproterozoic (Upper Vendian) and Upper Devonian (Famennian) age were formed by the erosion of high-relief source areas with contemporaneously exhumed high-pressure rocks. A bimodal distribution of ages and chemical compositions can be detected in the two detrital populations. There is no age overlap between the two populations, reflecting completely different source areas containing high-pressure rocks of different ages. Within the Upper Vendian sandstones, detrital white mica from a 571–609 Ma age group is phengitic in composition (Si 3.3–3.41 per formula unit), while an older 645–732 Ma age group is comprised of muscovite composition grains only. The first group is compatible with the time of late exhumation and emplacement of a source area containing high-pressure rocks, the Neoproterozoic Beloretzk terrane. The older age range is compatible with a long history of cooling and the allochthonous nature of this terrane. Detrital white mica from the Famennian sandstones (Zilair Formation) comprises one age group (342–421 Ma) containing phengite (Si 3.21–3.39 per formula unit) and muscovite, and a second group (446–496 Ma) containing muscovite only. While the derivation of the second group cannot be correlated with any as yet known regional data, the first age group indicates the earliest arrival of high-pressure rocks at the surface along the suture zone after Late Devonian arc–continent collision.