Interlayer Sheet Research Articles

The crystal structure of charmarite – the first case of a 11 × 11 Å superstructure mesh in layered double hydroxides

Abstract Charmarite, Mn4Al2(OH)12CO3⋅3H2O, is a hydrotalcite supergroup member (or layered double hydroxide, LDH) with a previously unknown crystal structure and a Mn2+-analogue of quintinite (commonly erroneously reported as ‘2:1 hydrotalcite’). The single-crystal X-ray diffraction (XRD) data were obtained from the specimen from Mont Saint-Hilaire, Québec, Canada and are best processed in the space group P $\bar{3}$ , a = 10.9630(4), c = 15.0732(5) Å and V = 1568.89(12) Å3. The crystal structure has been solved by direct methods and refined to R1 = 0.0750 for 3801 unique reflections with Fo > 2σ(Fo). The charmarite structure has long-range periodicity in the xy plane due to $2\sqrt 3$ a’ × $2\sqrt 3$ a’ scheme (or 11 × 11 Å) determined for LDHs for the first time (where a’ is a subcell parameter ≈ 3.2 Å). This periodicity is produced by the combination of two superstructures formed by: (1) Mn2+ and Al3+ ordering in the metal-hydroxide layers [Mn4Al2(OH)12]2+ according to the $\sqrt 3$ a’ × $\sqrt 3$ a’ pattern and (2) the (CO3)2– ordering according to the 2a’ × 2a’ pattern in the [CO3(H2O)3]2– interlayer sheet in order to avoid close contacts between adjacent carbonate groups. The $2\sqrt 3$ a’ × $2\sqrt 3$ a’ superstructure is an example of the adaptability of the components of the interlayer space to the charge distribution of the metal-hydroxyl layers. The Mn2+ and Al3+ cations have a large difference in size, which apparently leads to the considerable degree of their order as di- and trivalent cations resulting in a higher degree of statistical order of the interlayer components. Both powder and single-crystal XRD data show that the samples studied belong to the hexagonal branch of two-layer polytypes (2T or 2H) with d00n ≈ 7.57 Å. The chemical composition of the samples studied is close to the ideal formula. The Raman spectrum of charmarite is reported and the band assignment is provided.

Dominant Role of Hole Transport Pathway in Achieving Record High Photoconductivity in Two-Dimensional Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) with mobile charges have attracted significant attention due to their potential applications in photoelectric devices, chemical resistance sensors, and catalysis. However, fundamental understanding of the charge transport pathway within the framework and the key properties that determine the performance of conductive MOFs in photoelectric devices remain underexplored. Herein, we report the mechanisms of photoinduced charge transport and electron dynamics in the conductive 2D M-HHTP (M=Cu, Zn or Cu/Zn mixed; HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) MOFs and their correlation with photoconductivity using the combination of time-resolved terahertz spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and density functional theory (DFT) calculations. We identify the through-space hole transport mechanism through the interlayer sheet π-π interaction, where photoinduced hole state resides in HHTP ligand and electronic state is localized at the metal center. Moreover, the photoconductivity of the Cu-HHTP MOF is found to be 65.5 S m-1 , which represents the record high photoconductivity for porous MOF materials based on catecholate ligands.

Dominant Role of Hole Transport Pathway in Achieving Record High Photoconductivity in Two‐Dimensional Metal–Organic Frameworks

AbstractMetal–organic frameworks (MOFs) with mobile charges have attracted significant attention due to their potential applications in photoelectric devices, chemical resistance sensors, and catalysis. However, fundamental understanding of the charge transport pathway within the framework and the key properties that determine the performance of conductive MOFs in photoelectric devices remain underexplored. Herein, we report the mechanisms of photoinduced charge transport and electron dynamics in the conductive 2D M−HHTP (M=Cu, Zn or Cu/Zn mixed; HHTP=2,3,6,7,10,11‐hexahydroxytriphenylene) MOFs and their correlation with photoconductivity using the combination of time‐resolved terahertz spectroscopy, optical transient absorption spectroscopy, X‐ray transient absorption spectroscopy, and density functional theory (DFT) calculations. We identify the through‐space hole transport mechanism through the interlayer sheet π–π interaction, where photoinduced hole state resides in HHTP ligand and electronic state is localized at the metal center. Moreover, the photoconductivity of the Cu−HHTP MOF is found to be 65.5 S m−1, which represents the record high photoconductivity for porous MOF materials based on catecholate ligands.

New insights into the nature of glauconite

AbstractGlauconite must be assessed as mica-rich mica-smectite R3 interstratified mineral, with the pure end-member mica also having intrinsic K-deficient chemical characteristics (K+ ~ 0.8 apfu). This assertion is in accordance with our X-ray diffraction (XRD) and high-resolution tranmission electron microscopy (HRTEM) studies and chemical analyses by electron probe microanalysis (EPMA) of mature glauconites in Cenozoic Antarctic sediments that indicate that: (1) It consists of a glauconite-smectite (R3 ordered) mixed-layer silicate, composed mainly of mica-type layers (>90%), but displaying slightly different proportions of Fe(III)-smectite layers (<10%). (2) More mature glaucony grains are characterized by major K+ and VIFe2+ (mica layers) and minor VIFe3+ (smectite layers) content in the interstratified glauconite-smectite. (3) Potassium is stabilized at the interlayer site by the octahedrally coordinated Fe2+. (4) Microtexture of the glauconite crystals are comparable with those of other micas and illite minerals, with straight, defect-free lattice fringes of ~10 Å spacings glauconite packets characteristic of mica with minor interstratified poorly crystalline smectite layers. In addition, our new findings give insights into the glauconitization process and at the same time investigate the potassium-deficient character of the dioctahedral mica “glauconite.” These findings show that glauconite crystallizes by a layer-growth mechanism at the expense of a poorly crystalline smectite precursor and that smectiteto-glauconite transformations are accompanied by a gradually higher octahedral charge deficiency (Fe2+/Fe3+) stabilized by K+ uptake into the interlayer sheet.

Effect of Brass Interlayer Sheet on Microstructure and Joint Performance of Ultrasonic Spot-Welded Copper-Steel Joints

Solid-state ultrasonic spot welding (USW) inevitably offers a potential solution for joining dissimilar metal combination like copper (Cu) and steel (SS). In this study, the USW has been performed on Cu (UNS C10100) and SS (AISI 304) with brass interlayer by varying various welding parameters, aiming to identify the interfacial reaction, changes in microstructure and weld strength. The highest tensile shear and T-peel failure loads of 1277 and 174 N are achieved at the optimum conditions like 68 µm of vibration amplitude, 0.42 MPa of weld pressure and 1 s of weld time. The fractured surface analysis of brass interlayer and AISI 304 stainless steel samples reveals the features like swirls, voids and intermetallic compounds (IMCs). These IMCs are composed of CuZn and FeZn composite-like structures with ~1.0 μm thickness. This confirms that the weld quality is specifically sensitive to the levels of input parameter combinations as well as the type of material present on the sonotrode side.

Powder XRD and TEM study on crystal structure and interstratification of Zn-chlorite (baileychlore)

Baileychlore is Zn-end member trioctahedral chlorite, named by Audrey C. Rule and Frank Radke in 1988 for the honor of Professor Sturges W. Bailey of the University of Wisconsin – Madison, USA. Baileychlore occurs as dark green chlorite on calcite veins from garnet-vesuvianite skarn clasts at Red Dome ore deposit, Chillagoe, Queensland, Australia. The baileychlore has been studied by using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray energy-dispersive analytical electron microscopy analyses to determine crystal structure and interstratified layers. Baileychlore with stacking disorder displays streaking reflections of 0k0 (≠6n) hhl (h ≠ 3n). Unit-cell parameters for baileychlore (type I polytype) with a space group of C$ \bar 1$ are: a = 5.351(3), b = 9.266(5), c = 14.418(8) Å, α = 89.741(3)°, β = 96.741(4)°, and γ = 90.122(2)°. The strong lines of the measured XRD pattern [d(Å)(I)(hkl)] are: 14.331(7.151)(90.5)(002); 4.574(23.2)(1$ \bar 1$0, 11$ \bar 1$); 3.572(38.5)(004); 2.653(31.4)($ \bar 1$31, 200, 13$ \bar 1$); 2.406(49.4)(202, $ \bar 1$33, 13$ \bar 3$); 1.543(27.6)($ \bar 3$31, 060, 33$ \bar 1$), respectively. Reitveld refinement provides a composition (Zn2.49Al0.09Fe2+ 0.09□0.33)0.61− for the octahedral sheet and (Si3.53Al0.47)0.47− for the tetrahedral sheets within the 2:1 layer with (Al1.08Fe1.08Mg0.84)1.08+ for the interlayer sheet. The refinement results indicate that baileychlore is an intergrowth of type I and II polytypes. High-resolution TEM images show stacking disorder of baileychlore with small amount of isolated smectite layers.

Joining of hybrid AA6063-6SiC p -3Gr p composite and AISI 1030 steel by friction welding

Abstract Joining of metals and aluminium hybrid metal matrix composites has significant applications in aviation, ship building and automotive industries. In the present work, investigation is carried out on Friction Welding of AISI 1030 steel and hybrid AA6063-6SiC p -3Gr p composite, that are difficult to weld by fusion welding technique. Silicon carbide and graphite particle reinforced AA6063 matrix hybrid composite was developed successfully using stir casting method and the joining feasibility of AISI1030 steel with AA6063-6SiC p -3Gr p hybrid composite was tried out by friction stud welding technique. During friction stage of welding process, the particulates (SiC & Graphite) used for reinforcement, tend to increase the viscosity and lead to improper mixing of matrix and reinforcement. This eventually results in lower strength in dissimilar joints. To overcome this difficulty AA1100 interlayer is used while joining hybrid composite to AISI 1030 steel. Experimentation was carried out using Taguchi based design of experiments (DOE) technique. Multiple regression methods were applied to understand the relationship between process parameters of the friction stud welding process. Micro structural examination reveals three separate zones namely fully plasticized zone, partially deformed zone and unaffected base material zone. Ultra fine dynamically recrystallized grains of about 341 nm were observed at the fully plasticized zone. EDX analysis confirms the presence of intermetallic compound Fe 2 Al 5 at the joint interface. According to the experimental analysis using DOE, rotational speed and interlayer sheet thickness contribute about 39% and 36% respectively in determining the impact strength of the welded joints. It is found that joining with 0.5 mm interlayer sheet provides efficient joints. Developed regression model could be used to predict the axial shortening distance and impact strength of the welded joint with reasonable accuracy.

High-angle annular dark field scanning transmission electron microscopic (HAADF-STEM) study of Fe-rich 7 Å–14 Å interstratified minerals from a hydrothermal deposit

AbstractThe distribution of octahedral cations in the two component layers of a 7 Å–14 Å interstratified mineral with a bulk chemical composition (Fe4.122+Mg0.07Mn0.01Al1.69□0.11)(Si2.56Al1.44) O10(OH)8was investigated using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) in combination with the image simulations. In the 14 Å component layers, comparison between the observed and simulated images revealed that the M4 sites of the interlayer sheets were occupied preferentially by Al together with a small amount of Fe; the other M1, M2 and M3 sites were occupied by dominant Fe and residual Al in equal proportions. Two types of octahedral sheets with disordered and ordered cation distributions were recognized in the 7 Å component layers. The two types of sheets were similar to the octahedral sheet of the 2:1 layer and the interlayer sheet in the 14 Å layer above, respectively. Irregular vertical stacking and lateral contact of the different component layers in structure and chemistry characterized the interstratification, which may be caused by rapid precipitation and accretion of the component layers in hydrothermal environments.

Crystal Structure Refinement of Synthetic Pure Gyrolite

Pure calcium silicate hydrate – gyrolite was prepared under the saturated steam pressure at 473 K temperature in rotating autoclave. The crystal structure of synthetic gyrolite was investigated by X-ray diffraction and refined using Le Bail, Rietveld and crystal structure modelling methods. Background, peak shape parameters and verification of the space group were performed by the Le Bail full pattern decomposition. Peculiarities of interlayer sheet X of gyrolite unit cell were highlighted by Rietveld refinement. Possible atomic arrangement in interlayer sheet X was solved by global optimization method. Most likelihood crystal structure model of gyrolite was calculated by final Rietveld refinement. It was crystallographically showed, that cell parameters are: a = 0.9713(2) nm, b = 0.9715(2) nm, c = 2.2442(3) nm and alfa = 95.48(2) o, beta = 91.45(2) °, gamma = l20.05(3) °. DOI: http://dx.doi.org/10.5755/j01.ms.21.1.5460

Characterization of Fe-smectites and their alteration potential in relation to engineered barriers for HLW repositories: The Nui Nua clay, Thanh Hoa Province, Vietnam

The stability of smectite-rich clay minerals is of interest because they could be candidates for engineered barriers for high-level radioactive waste repositories. This research characterized the chemical and mineralogical properties of the Nui Nua clay which forms from the weathering of the Nui Nua serpentinized ultramafic–mafic massif (Thanh Hoa Province, Viet Nam) using several methods (including Fourier transform infrared spectrometry and transmission electron microscopy). The Nui Nua clay, taken from Co Dinh and My Cai valleys, is composed of Fe-smectites as the main phase with minor phases of normal smectite, quartz, talc, chlorite, kaolinite, amphibole, antigorite, feldspars and magnetite. The Fe-smectites were characterized as mixed-layer minerals (including Fe-montmorillonitic as an end-member) composed of illitic or dioctahedral vermiculitic layers and Fe-rich smectitic layers. The proportion of the smectitic layer is approximately 80%; the interlayer sheet is dominated by Ca and Mg, while the octahedral sheet is dominated by Fe3+ (not Al). The stability of the Nui Nua smectites was also investigated by a simulation in saturation of 1M NaCl and deionized water under kinetic impaction. The average tetrahedral-Si content of the smectites increased or decreased depending on the “dynamic solution” or hydraulic regime. By chemical identification, the alteration of Fe-smectites is mainly increased by the smectitization process. This research suggests that the Nui Nua clay is a potential candidate for an engineered barrier because during the alteration process, neo-formation of a montmorillonitic layer occurs.

Formation of hydroxy-interlayer vermiculite (HIV) in rhizosphere soils under German camomile cultivation and manure application

AbstractThis study examines the effect of German camomile (Matricaria camomilla L.) cultivation on the weathering of minerals present in the clay fraction of five different soil series modified or not unmodified (control) with cattle manure. A factorial experiment was performed in a randomized complete block design (RCBD) with three replications. At harvest time the rhizosphere soil was separated from bulk soil and the mineralogy was examined by X-ray diffraction (XRD). Trioctahedral chlorite transformed to hydroxy-interlayer vermiculite (HIV), while kaolinite disappeared. The presence of HIV was identified by an increase in the intensity ratio of the 1.0 and 1.4 nm peaks after Na-citrate pretreatment and K-saturation. It is suggested that Al3+ is released during dissolution of kaolinite and subsequently enters into the interlayer sheet of chlorite, and consequently the brucite layer disappears. This was verified by a significant Mg2+ increase in the soil solution in three out of five experiments at harvest time compared to pre-planting. The pots amended with manure showed the same changes as the pots without manure. The results of this study may explain the presence of unexpected clay minerals in regions with unsuitable conditions for their formation.

Flexible and Transparent Dielectric Film with a High Dielectric Constant Using Chemical Vapor Deposition-Grown Graphene Interlayer

We have devised a dielectric film with a chemical vapor deposited graphene interlayer and studied the effect of the graphene interlayer on the dielectric performance. The highly transparent and flexible film was a polymer/graphene/polymer 'sandwich-structure' fabricated by a one-step transfer method that had a dielectric constant of 51, with a dielectric loss of 0.05 at 1 kHz. The graphene interlayer in the film forms a space charge layer, i.e., an accumulation of polarized charge carriers near the graphene, resulting in an induced space charge polarization and enhanced dielectric constant. The characteristic of the space charge layer for the graphene dielectric film, the sheet resistance of the graphene interlayer, was adjusted through thermal annealing that caused partial oxidation. The dielectric film with higher sheet resistance due to the oxidized graphene interlayer had a significantly lower dielectric constant compared to that with the graphene with lower interlayer sheet resistance. Oxidizing the graphene interlayer yields a smaller and thinner space charge density in the dielectric film, ultimately leading to decreased capacitance. Considering the simplicity of the fabrication process and high dielectric performance, as well as the high transparency and flexibility, this film is promising for applications in plastic electronics.

NUMERICAL SIMULATION ON JOINING OF CERAMICS WITH METAL BY FRICTION WELDING TECHNIQUE

The joining of ceramic and metals can be done by different techniques such as ultrasonic joining, brazing, transient liquid phase diffusion bonding, and friction welding. Friction Welding is a solid state joining process that generates heat through mechanical friction between a moving workpiece and a stationary component. In this article, numerical simulation on thermal analysis of friction welded ceramic/metal joint has been carried out by using Finite Element Analysis (FEA) software. The finite element analysis helps in better understanding of the friction welding process of joining ceramics with metals and it is important to calculate temperature and stress fields during the welding process. Based on the obtained temperature distribution the graphs were plotted between the lengths of the joint corresponding to the temperatures. To increase the wettability, aluminium sheet was used as an interlayer. Hence, numerical simulation of friction welding process is done by varying the interlayer sheet thickness. Transient thermal analysis had been carried out for each cases and temperature distribution was studied. From the simulation studies, it is found that the increase in interlayer thickness reduces the heat affected zone and eventually improves the joint efficiency of alumina/aluminum alloy joints.

Temperature and stress fields in electron beam welded Ti-15-3 alloy to 304 stainless steel joint with copper interlayer sheet

Electron beam welding of Ti-15-3 alloy to 304 stainless steel (STS) using a copper filler metal was carried out. The temperature fields and stress distributions in the Ti/Fe and Ti/Cu/Fe joint during the welding process were numerically simulated and experimentally measured. The results show that the rotated parabola body heat source is fit for the simulation of the electron beam welding. The temperature distribution is asymmetric along the weld center and the temperature in the titanium alloy plate is higher than that in the 304 STS plate. The thermal stress also appears to be in asymmetric distribution. The residual tensile stress mainly exists in the weld at the 304 STS side. The copper filler metal decreases the peak temperature and temperature grade in the joint as well as the residual stress. The longitudinal and lateral residual tensile strengths reduce by 66 MPa and 31 MPa, respectively. From the temperature and residual stress, it is concluded that copper is a good filler metal candidate for the electron beam welding of Ti-15-3 titanium alloy to 304 stainless steel.

Two-stage interlamellar silylation of synthetic gyrolite with n-butyldimethylchlorosilane

The direct interlamellar silylation of synthetic gyrolite with n-butyldimethylchlorosilane was carried out. The silylation reaction was performed in two stages at temperatures of 50 °C and 60 °C, respectively. In the first stage of the reaction process, silylation was confined predominantly to the interlayer sheet, resulting in the formation of 3:2 organo-phyllosilicates. In the second stage of the reaction, the silylation depleted the brucite-like calcium oxide (hydroxide) sheets within the 3:2 organo-phyllosilicates, leading to the formation of organic polysiloxane sheets characterized by a modification of the original silicate structure. The synthesized gyrolite and silylated derivates formed in the two-stage reaction process were characterized using XRD, 29Si MAS NMR, FTIR, SEM, and elemental analysis techniques.

Effect of Short Annealing Times on the Magnetoelectronic Properties of Co/Pd-Based Pseudo-Spin-Valves

We investigated the effects of short annealing times on the magnetoelectronic properties of pseudo-spin-valves (PSV) with perpendicular magnetic anisotropy based on Co/Pd multilayers using a contact hot plate. In order to study the time scale at which the degradation of film properties occurs for possible application in perpendicular MgO-based magnetic tunnel junctions (MTJ), the results were compared against our previous study of Co/Pd PSV based on vacuum annealing. With contact annealing for up to 90 s, no significant changes to the current-in-plane giant magnetoresistance (CIP-GMR), interlayer coupling, sheet resistance and layer coercivities were observed for up to 200 degrees C. At 350 degrees C, a 39 to 46% decrease in CIP-GMR was observed for annealing times of 30 to 90 s, respectively, slightly lower than that observed for vacuum annealing at 230 degrees C for 1 h. Similar results were also obtained for interlayer coupling, sheet resistance and layer coercivities, indicating that short annealing times allow for reduced interlayer diffusion at higher temperatures. However, it is clear that significant degradation of GMR performance occurs at 350 degrees C and above even for annealing times as short as 30 s, indicating the potential difficulty of realizing Co/Pd-based perpendicular MgO-MTJ.

Electron beam welding of Ti-15-3 titanium alloy to 304 stainless steel with copper interlayer sheet

Electron beam welding of Ti-15-3 titanium alloy to 304 stainless steel with a copper sheet as interlayer was carried out. Microstructures of the joint were studied by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). In addition, the mechanical properties of the joint were evaluated by tensile test and the microhardness was measured. These two alloys were successfully welded by adding copper transition layer into the weld. Solid solution with a certain thickness was located at the interfaces between weld and base metal in both sides. Regions inside the weld and near the stainless steel were characterized by solid solution of copper with TiFe 2 intermetallics dispersedly distributed in it. While weld near titanium alloy contained Ti-Cu and Ti-Fe-Cu intermetallics layer, in which the hardness of weld came to the highest value. Brittle fracture occurred in the intermetallics layer when the joint was stretched.

XRD and HRTEM analyses of stacking structures in sudoite, di-trioctahedral chlorite

The stacking structures of sudoite, di-trioctahedral chlorite from two different localities are investigated by powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD analyses using a Gandolfi camera revealed that the structures of both specimens are similar, corresponding to a one-layer II bb -4 (1 A ) polytype. HRTEM observations indicate that the stacking sequence is characterized by a largely uniform intralayer shift of a /3 in the −X 1 direction (X i represent the directions along the pseudohexagonal axes) and by an interlayer displacement of similar magnitude in either the −X 2 or −X 3 direction. Stacking disorder is primarily caused by the mixing of interlayer displacements in the two directions. This disorder is more common in the lath-shaped crystalline specimen from Berezovsk, Russia than in the fine platy crystalline specimen from Ottre, Belgium. DIFFaX simulations of the powder XRD patterns for this stacking model reproduced the observed features well for both specimens. The stacking configuration of these sudoite specimens is considered to be controlled by the corrugation of basal oxygen planes in the 2:1 layer and the corresponding deformation in the brucite-like interlayer sheet.

Cation-site partitioning in Ti-rich micas from Black Hill (Australia): A multi-technical approach

AbstractThe crystal chemistry of Ti-rich trioctahedral micas of plutonic origin, cropping out at Black Hill (South Australia) has been investigated by combining electron microprobe analysis, single crystal X-ray diffraction, Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Chemical analyses have shown the samples taken to be quite homogeneous and Ti-rich (TiO2 ≈ 7 wt.%). Mössbauer investigation yielded Fe2+/Fe3+ ≈ 30. X-ray photoelectron spectroscopy analysis seems to suggest the occurrence of three Ti species: octahedral Tr4+(60%), octahedral Ti3+(26%), and tetrahedral Ti4+(14%). The analyzed sample belongs to the 1M polytype and the relevant crystal data from structure analysis are: a = 5.347(1) Å, b = 9.261(2) Å, c = 10.195(2) Å, β = 100.29°(1). Anisotropic structure refinement was performed in space group C2/m, and converged at R = 2.62, Rw = 2.80. Structural details (the c cell parameter, the off-center shift of the M2 cation towards 04, the bond-length distortions of the cis-M2 octahedron, the interlayer sheet thickness, the projection of K−O4 distance along c*, the difference <K−O>outer-<K−O>inner) support the occurrence of the Ti-oxy substitution (VIR2+ + 2(OH)− ⇌ Ti4+ + 202− + H2) in the sample. Analysis of structural distortions as a function of the Ti content revealed that the positions of the oxygens 03 and 04 are displaced in opposite senses along [100]. This produces an enlargement of the M1 site with respect to the M2 site and a shortening of the interlayer distance. This trend seems to be in common with other Ti-rich 1M micas of plutonic origin.

Characterization of coexisting NH4- and K-micas in very low-grade metapelites

Organic-rich Carboniferous shales with associated coal seams, from the Bacia Carbonífera do Douro-Beira (north Portugal), have been studied by TEM as well as by a variety of other methods. NH 4 and K micas and berthierine form small subparallel packets of a few layers separated by lowangle boundaries. One- and two-layer ordered polytypes, with some spot enlargement typical of minor disorder, occur in the NH 4 micas (i.e., metamorphism tobelites). They exhibit all the characteristics commonly described for sub-greenschist-facies, including a lack of textural and chemical equilibrium. The compositions of the 2:1 layers vary considerably for both K- and NH 4 -micas, and except for Ti, exhibit similar compositional ranges. The most significant compositional variations are explained by phengitic substitution (Si from 3 to 3.25, Fe + Mg from 0.1 to 0.3), but no evidence of an illitic substitution has been found. NH 4 in tobelites was determined by analysis of NH 3 using Nessler’s reagent, basal spacing, and 1-(K + Na). The resulting values indicate NH 4 contents ranging from 30 to 59% of the interlayer site occupancy. At very low temperatures of metamorphism (e.g., North Sea), the intergrowths of NH 4 and K in micas is on the nanometer-scale. With an increase in temperature (e.g., Pennsylvania), NH 4 - and K-micas are present as different minerals. The Douro- Beira samples represent a still higher-temperature case and, therefore, NH 4 and K can be present in the same interlayer sheet but with one cation being dominant. NH 4 - and K-dominated micas have segregated into well-separated packets with scarce intergrowths and almost no mixed-layers. Hence they show a solvus relationship.