Sediments from depths to 670 m in the Barbados accretionary complex and transecting the décollement zone have been studied by transmission and analytical electron microscopy (TEM/AEM). The sediments consist of claystone and mudstone intercalated with layers of volcanic ash. Smectite comprises the bulk of the noncalcareous sediments and forms a continuous matrix enveloping sparse, irregular, large grains of illite, chlorite, kaolinite and mixed-layer illite/chlorite of detrital origin at all depths. The detrital origin of illite is implied by illite-smectite textural relations, well-ordered 2M polytypism, and a muscovite-like composition. K is the dominant interlayer cation in smectite at all depths, in contrast to the Na and Ca that are normally present in similar rocks.

Deeper samples associated with the décollement zone contain small (up to 100 Å thick) illite packets included within still-dominant subparallel layers of contiguous smectite. AEM analyses of these packets imply illite-like compositions. Selected area electron diffraction (SAED) patterns show that this illite is the 1Md polytype. Packets display step-like terminations like those seen in illite of hydrothermal origin. The data collectively demonstrate that smectite transforms progressively to illite via a dissolution-recrystallization process within a depleting matrix of smectite, and not by a mechanism of layer replacement. This illite seems to form at depths as shallow as 500 m and temperatures of 20°-30°C, which is in marked contrast to the much higher temperature conditions normally assumed for this transformation. This implies that the high water/rock ratios associated with the décollement zone are significant in promoting reaction.

Prograde evolution of illite/smectite occurring in an unusually-thick (8000 m) sequence of Mesozoic-Cenozoic sediments in the Basque-Cantabrian Basin, Spain, has been studied using XRD and TEM/AEM. The sediments, which are only slightly tilted, cover the range from smectite to illite, and the most deeply buried ones are unique in that they span the range from diagenesis through low-grade metamorphism (anchizone), with no apparent overprinting due to tectonic deformation.

Pelites are absent from the shallow section, but smectite occurs in marls as high-charge, K-dominant and low-charge, K-poor anastomosing arrays of layers. At intermediate depths, authigenic clay is identified largely as R1 I/S, coexisting with packets of R≫3 I/S (nearly pure illite), where illite-like and smectitelike layers can be identified by contrast in TEM images, which is consistent with XRD data. The authigenic clay of the deepest samples consists of illite with no or almost no expandable layers, which occurred as packets with layers largely subparallel to bedding, K-deficient composition in comparison with muscovite, 1Md-like SAED patterns, and 100 Å mean packet thickness. There is no evidence of deformation stress-induced, non-bedding-parallel clays in the deepest samples. Detrital micas with either a phengiterich or a phengite-poor composition range dominate the phyllosilicate fraction of all the samples. Detrital micas show no changes over the diagenesis/metamorphism range and appear to have behaved as if isolated from authigenic clays.

Authigenic clays occur as bedding-subparallel packets that evolved during passive burial metamorphism through dissolution/crystallization of less-evolved clays. Where illite-like and smectite-like layers can be identified, TEM images imply a discontinuous series in which packets of R1 I/S (50% I) transform to packets of nearly-pure illite, that is, an Ostwald-step-rule-like sequence. Such immature illite remains unmodified with further burial, and is apparently the potential predecessor of stress-induced, highly-evolved mica of higher-grade, tectonically-deformed pelites.

The evolution of texture, structure and chemical composition of chloritic clays in coeval pairs of metabasites and metapelites of a prograde sequence from the Bükk Mountains has been investigated using electron microscopy techniques. Samples are from the Bükkium (innermost Western Carpathians, Hungary) that underwent Alpine metamorphism, ranging from late diagenesis to epizone for pelites and from prehnite-pumpellyite to greenschist facies for the metabasites.

Although bulk-rock compositions, textures and primary minerals are different, chlorite evolved at similar rates in coeval metabasites and metasediments, but along different paths. The principal similarities in the prograde sequence are a decrease in the percentage of interstratified material in both dioctahedral and trioctahedral phyllosilicates and increase in thicknesses of chlorite and illite crystallites. The principal difference is in the type of interstratification in chlorite, with berthierine in metapelites, and smectite (saponite) in metabasites, although smectitic mixed layers also occur in the former. The evolution of trioctahedral phyllosilicates is marked by a decrease in the number of mineral species with increasing grade, chlorite, sensu stricto, being the only trioctahedral mineral at higher grades. This is consistent with the trend in reaction progress where both metastable systems (metabasites and metapelites) tend toward the same end-member, thermodynamically stable chlorite, as well as texture (crystal size), and where all intermediate states are metastable, and determined by the Ostwald step rule.

Diagenetic clay minerals usually occur as heterogeneous assemblages of submicroscopic layers consisting of different structure types such as illite, smectite and chlorite, with variable composition within a given structure type, and with highly variable concentrations of imperfections. The dimensions of mixed-layering, the semi-coherent to coherent nature of the structures across the layering, and compositional heterogeneity occur at a scale well below that of an individual thermodynamic phase. These relations imply that most clays are not distinct minerals or phases, and that assemblages of clays in shales and mudstones are incompatible with the phase rule. Such relations are better evaluated in terms of the formation of metastable materials with each small unit having unique chemical properties, rather than as a small number of stable homogeneous phases. Consequently, treatment of most clay minerals in terms of equilibrium stability with either a thermodynamic or experimental approach is subject to error.

Chemical reactions involving most clay minerals are best understood with kinetic models. These involve a great variety of parameters such as time, fluid/rock ratio, deformation history, nature of starting materials and transformation mechanisms, as well as the variables, such as temperature, pressure and composition, that are commonly used to define equilibrium. Solubility experiments on the stabilities of clay minerals are unlikely to attain equilibrium at low temperatures. Moreover, the activity of soluble species may be controlled by surface equilibria, or by absorbed or exchangeable cations. Interpretations of available experiments on the solubility of illite vs. other mineral assemblages are in violation of Schreinemakers’ rules and indicate lack of equilibrium.

Predictable sequences of clay minerals as a function of temperature are best understood through the Ostwald step rule, in which clay mineral assemblages undergo reactions in response to kinetic factors that represent reaction progress rather than an approach to equilibrium. Currently used clay mineral thermometers (illite crystallinity, smectite/illite reaction, chlorite composition) are not based on equilibrium reactions. Such systems are not accurate thermometers and therefore have questionable utility.

The purpose of this report is to describe the appropriate use of indices relating to crystallinity, such as the ‘crystallinity index’, the ‘Hinckley index’, the ‘Kübler index’, and the ‘Árkai index’. A ‘crystalline’ solid is defined as a solid consisting of atoms, ions or molecules packed together in a periodic arrangement. A ‘crystallinity index’ is purported to be a measure of crystallinity, although there is uncertainty about what this means (see below). This report discusses briefly the nature of order, disorder and crystallinity in phyllosilicates and discusses why the use of a ‘crystallinity index’ should be avoided. If possible, it is suggested that indices be referred to using the name of the author who originally described the parameter, e.g. ‘Hinckley index’ or ‘Kübler index’, or in honor of a researcher who investigated the importance of the parameter extensively, e.g. ‘Árkai index’.

Pelitic and basic rocks occurring within prograde sequences in Portugal, Spain and Hungary have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The minerals formed in typical prograde reactions define the general sequences, but smectite, chlorite-smectite (corrensite) and/or berthierine were found to have replaced chlorite, whereas kaolinite and mixed-layer illite-smectite replaced illite-muscovite. Alteration occurred under conditions normally associated with diagenesis, subsequent to regional metamorphism, and we therefore refer to such processes with the term “retrograde diagenesis”. In the cases studied and in other cited examples, reactions occurred on a regional basis via pervasive fluids under open-system conditions inferred to be related to tectonic stress. The observed alterations could generally not be inferred from XRD data, although the presence of pure smectite in sediments other than bentonite is suggestive of retrograde relations, especially where other minerals are consistent with a higher grade of diagenesis. Retrograde diagenesis is readily observed through imaging of textures by TEM, however. Textural features show that retrograde reactions are more common than generally assumed, and that care should be used in interpreting geological events where appropriate textural relations are not seen.

Well-cuttings and core samples from four Gulf Coast areas have been examined by high-resolution scanning-transmission electron microscopy and other techniques to detail the chemical, mineralogical and textural changes during diagenesis. Wells in each of the four areas exhibit a distinct smectite-to-illite (S-I) transition, but the estimated depths for the transition vary significantly: 5600 to 8800 ft for the onset of the transition, and 7500 to 13 600 ft for the completion (70–80% illite). Calculated temperatures also vary significantly: 58° to 92°C for the onset, and 88° to 142°C for the completion. No pattern is evident for characteristic temperatures of the S-I transition. These data suggest that it is inappropriate to use the S-I transition for determining absolute diagenetic temperatures in geological settings similar to Gulf Coast sequences. TEM data imply the presence of two distinctly different kinds of domains, interpreted as smectite (with some mixed-layer component?) and illite. Initially, smectite domains dominate and contain subparallel layers of variable thickness. As the transition proceeds, abundant dislocations provide avenues for diffusion of both necessary reactants (K and Al) and products (Na, Si, Fe, Mg); illite packets grow within a shrinking matrix of smectite. Variable temperatures for the transition may be due to several factors, including chemical heterogeneity of the original smectite, variation in water/rock ratio, porosity differences, diverse chemical character of available fluids, and the particular physical environment for diagenesis. It is suggested that actual depths and temperatures for the transition are primarily a function of kinetic factors associated with a reaction for which at least one of the principal phases is metastable; the transition therefore is highly dependent on local geological factors.

An XRD-TEM study was carried out on low-grade metapelites (Transantarctic Mountains) to determine the cause of apparent inconsistences in grade as determined by IC and independent geologic relations. The illite crystallinity (IC) data indicate that the three units investigated (BT, RBT, MS) were affected by very low- (IC = 0.24°Δ2θ in BT) to low-grade metamorphism (0.19°Δ2θ in MS). In all three samples, mica crystals are of a size typical of the epizone, but the mean size increases from BT to RBT and MS, due to the increasing strain features from BT to MS. These results indicate that strain-induced reduction in crystal size was retained in BT samples (with anomalously high IC values). Microtextures in RBT and MS (with smaller IC) samples suggest a recovery of sub-grain boundaries. A decrease in crystal size may occur with increasing grade where strain rates are high relative to the rate of recrystallization.

The purpose of this report is to describe the appropriate use of indices relating to crystallinity, such as the ‘crystallinity index’, the ‘Hinckley index’, the ‘Kü bler index’, and the ‘Árkai index’. A ‘crystalline’ solid is defined as a solid consisting of atoms, ions, or molecules packed together in a periodic arrangement. A ‘crystallinity index’ is purported to be a measure of crystallinity, although there is uncertainty about what this means (see below). This report discusses briefly the nature of order, disorder and crystallinity in phyllosilicates and discusses why the use of a ‘crystallinity index’ should be avoided. If possible, it is suggested that indices be referred to using the name of the author who originally described the parameter, as in ‘Hinckley index’ or ‘Kübler index’, or in honour of a researcher who investigated the importance of the parameter extensively, as in ‘Árkai index’.

An unusually complete sequence of pelitic rocks ranging from diagenetic to greenschist-facies metamorphic grades occurs in southern Sierra Espun˜a, Spain. Prograde and retrograde reactions have been studied by X-ray diffraction and electron microscopy (SEM, TEM and AEM). The prograde reaction series, with reactions facilitated by tectonic stress, includes: (1) R4 interstratified illite-smectite in the diagenetic Malaguide Complex that preserves the variable orientation of original smectite packets, and has 1Md polytypism; (2) chemically heterogeneous illite and Na-K dioctahedral white micas that progressively evolve toward chemical and textural equilibrium in the anchizonal Intermediate Units; and (3) thick, defect-free packets of phengite, paragonite and clinochlore which have a typical metamorphic texture, in the Alpujarride Complex. Two superimposed retrograde episodes produced: (a) sudoite at near-peak metamorphic conditions and (b) dioctahedral smectite during low-temperature retrograde diagenesis.

Microanalytical data on Plio-Pleistocene sediments from a Salton Sea Geothermal Field well define the evolution of textures and mineral assemblages from sediment to metamorphic rock in an open hydrothermal system.

At shallow depths detrital grains occur within a fine-grained matrix of I-S and illite. As depth and temperature increase, grains lose their detrital characteristics; they subsequently form a continuous intergrown array of irregular domains, with phyllosilicates occupying the progressively decreasing pore space.

Illite and chlorite are authigenic phyllosilicates down to 2500 m, where biotite occurs. Authigenic phyllosilicates occur as subhedral crystals (200 –600 Å thick). Alkali feldspar has end-member compositions, implying exchange of alkalis with convected fluids. Changes in texture imply dissolution of detrital phases and neocrystallization of authigenic phyllosilicates directly from pore fluids. Changes with depth are not time-dependent for phyllosilicates: they formed simultaneously at all depths as a result of a single hydrothermal event in an open system.

This report embodies recommendations on zeolite nomenclature approved by the International Mineralogical Association Commission on New Minerals and Mineral Names. In a working definition of a zeolite mineral used for this review, interrupted tetrahedral framework structures are accepted where other zeolitic properties prevail, and complete substitution by elements other than Si and Al is allowed. Separate species are recognized in topologically distinctive compositional series in which different extra-framework cations are the most abundant in atomic proportions. To name these, the appropriate chemical symbol is attached by a hyphen to the series name as a suffix except for the names harmotome, pollucite and wairakite in the phillipsite and analcime series. Differences in spacegroup symmetry and in order—disorder relationships in zeolites having the same topologically distinctive framework do not in general provide adequate grounds for recognition of separate species. Zeolite species are not to be distinguished solely on Si : Al ratio except for heulandite (Si : Al < 4.0) and clinoptilolite (Si : Al ⩾ 4.0). Dehydration, partial hydration, and over-hydration are not sufficient grounds for the recognition of separate species of zeolites. Use of the term ‘ideal formula’ should be avoided in referring to a simplified or averaged formula of a zeolite.

Newly recognized species in compositional series are as follows: brewsterite-Sr, -Ba; chabazite-Ca, - Na, -K; clinoptilolite-K, -Na, -Ca; dachiardite-Ca, -Na; erionite-Na, -K, -Ca; faujasite-Na, -Ca, -Mg; ferrierite-Mg, -K, -Na; gmelinite-Na, -Ca, -K; heulandite-Ca, -Na, -K, -Sr; levyne-Ca, -Na; paulingite-K, -Ca; phillipsite-Na, -Ca, -K; stilbite-Ca, -Na.

Key references, type locality, origin of name, chemical data, IZA structure-type symbols, space-group symmetry, unit-cell dimensions, and comments on structure are listed for 13 compositional series, 82 accepted zeolite mineral species, and three of doubtful status. Herschelite, leonhardite, svetlozarite, and wellsite are discredited as mineral species names. Obsolete and discredited names are listed.

Brinrobertsite, an ordered, mixed-layered, dioctahedral pyrophyllite-smectite (P/S), occurs in a metabentonite in the Ordovician Nant Ffrancon Formation near Bangor, N Wales. It comprises ~30% of the metabentonite, in association with quartz (~50%) and chlorite (clinochlore; 20%) which replaced glass shards and fine-grained glass matrix. Transmission electron microscopy (TEM) images show sequences of dominant ~24 Å 001 lattice fringes inferred to correspond to 2:1 layers with alternate pyrophyllite-like (low-charge) and smectite-like (higher-charge) interlayers (i.e. R1 ordering). The hk0 diffraction patterns are mostly hexanets with some spotty circles, implying that layers are largely coherently related, but with some turbostratic stacking. Collective data show that d100 = 5.2, b = 9.1, and d001 = 24–25 Å, assuming monoclinic or pseudomonoclinic symmetry. The composition, as determined by energy dispersive spectral analysis, is (Na0.22K0.07Ca0.06)(Al3.81Mg0.08Fe0.08)(Si7.84 Al0.16)O20(OH)4·3.54H2O, as consistent with the sum of the compositions of pyrophyllite-like and smectite-like units. Water content was determined by DTA/TGA analysis. The powder diffraction patterns have a principal peak with d001 = 24.4 Å. Patterns of air-dried and glycol-saturated brinrobertsite, including Na- and Ca-saturated and untreated samples, were modelled satisfactorily as R1-ordered P/S by the program NEWMOD-for-Windows. The unique composition of brinrobertsite relative to R1 IS, which is ubiquitous in metabentonites, was caused by leaching of alkalis and alkaline-earth elements by hydrothermal fluids associated perhaps with a nearby intrusion, as demonstrated by bulk-chemical analyses of the metabentonite. The crystal structure is modelled as having Al/Si distributions symmetrical by reflection across interlayers. This causes all 2:1 layers to be equivalent in having one tetrahedral sheet with little or no Al, and the other with significant Al substitution, giving rise to alternate high- and low-charge interlayers. Geological evidence suggests that brinrobertsite is a back-reacted product of hydrothermal alteration in the sequence: glass → pyrophyllite → brinrobertsite.

Manganoan berthierine, sometimes only a few layers thick, is preserved interlayered with manganoan chlorite (chamosite) in small recrystallized patches in cherty metapelagites matamorphosed in the prehnite–pumpellyite facies at Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand. The Mn occupies 11–12% of the octahedral sites, making this the most Mn–rich berthierine reported so far. The ratio (Fe+Mn)(Fe+Mn+Mg), 0.67–0.70, is at the lower end of those of previously reported occurrences, making these amongst the most Mg-rich berthierines known. Manganoan berthierine and interlayered manganoan chlorite also make up ~11–13% of the cryptocrystalline groundmass in which the berthierine is believed to have crystallized under early diagenetic conditions near a deep-ocean–sediment interface. The occurrence of berthierine in the recrystallized patches lends support to the possibility of a stability field at the temperature of prehnite-pumpellyite facies recrystallization in association with chlorite of similar but not identical composition.