Hydrothermal tobelitic phyllosilicates modeled as ISII (R3) ordering with a minimum of 2–3% and a maximum of 6–8% interstratified smectite occur in veins and as replacement of fossils in hydrothermally altered black shale. These heavy metal-rich phyllosilicate veins formed during a Mesozoic-aged, regional-scale hydrothermal event that affected an area which encompasses the Mercur Au district (Wilson and Parry, 1990a, 1990b). Associated minerals include kaolinite, quartz, chlorite, Fe-oxides, I/S (R1, 45% smectite), and pyrite. N and O contents of NH4 phyllosilicates determined by microprobe analysis range from 0.19 to 1.78 and 48.6 to 52.9 elemental wt. %, respectively. Infrared absorption analysis indicates N occurs as NH4+. Very high O analyses are probably caused by contamination with kaolinite. A representative structural formula for the tobelitic material is [(NH4)0.36K0.36Na0.03]-(Al1.91Mg0.13Fe0.03)(Si3.21Al0.79)O10(OH1.88F0.12).

Correlation plots of data from microprobe analyses indicate an atypically high correlation between interlayer charge and octahedral layer charge and no correlation between (K+Na) and N. More typical correlations between N and (K+Na) and between interlayer charge and tetrahedral layer charge are obtained if 2–8% of a beidellitic smectite are factored out of the analyses. This amount of smectite is consistent with modeling of X-ray diffraction data using the computer program NEWMOD (Reynolds, 1985).

Possible sources of NH4 are from introduction by hydrothermal fluids or from thermal degradation of organic matter prevalent within the host rocks during low-grade metamorphism. The occurrence of NH4 phyllosilicate veins in unoxidized shale and the limited occurrence of NH4 phyllosilicates within the host shales suggests a hydrothermal source for the NH4.

The Hercynian metamorphic event is poorly characterized in internal zone complexes of the Betic Cordillera (Spain), as it has been, to a great extent, overprinted by the mineral assemblages formed during the Alpine event. Identification of the signals of the Hercynian episode is easier in series largely unaffected by the Alpine event, such as Intermediate units between the Maláguide and the Alpujárride Complexes, which consist of a set of thrust slices. With the aim of characterizing the Hercynian paragenesis, a detailed comparison of the mineral assemblages of Paleozoic and overlying Triassic sequences, unaffected by the pre-Alpine event, was carried out. Mineral assemblages were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, electron microprobe, and transmission-analytical electron microscopy. A rapid increase in the illite crystallinity values was observed at the Triassic—Paleozoic transition in the upper tectonic slices. In addition, the diagenetic to anchizonal dickite-, sudoite-, and pyrophyllite-bearing assemblages, characterizing the Triassic rocks, contrast with Paleozoic assemblages consisting of white K-mica ± paragonite + chlorite + mica-chlorite and chlorite-vermiculite mixed layers + garnet, suggesting that this assemblage corresponds to the Hercynian metamorphic event. This assemblage records temperatures on the order of 400ºC and an intermediate pressure regime. Paleozoic rocks contain, in addition, tobelite, which comprises some of the detrital grains and strongly masks the illite crystallinity values. Tobelite has been identified only in the upper thrust slices, suggesting that changes in the detrital input is primarily responsible for the disappearance of tobelite at the transition from the Maláguide to the Alpujárride domain. Tobelite appears finely intergrown with white K-mica and its origin is uncertain. It could have been inherited as tobelite, but a more likely hypothesis is that the intergrowths of white K-mica and tobelite were formed at low temperature from an NH4-bearing mica precursor.

In the Nuussuaq Basin, West Greenland, a thick succession of Tertiary dolerites has penetrated Upper Cretaceous mudstone. The mixed-layer minerals of mudstone core samples have been analyzed by X-ray diffraction, solid-state 29Si and 27A1 magic-angle spinning nuclear magnetic resonance, Mössbauer and infrared spectroscopies, thermal analysis, chemical analysis, stable isotopes (18O/16O), and K/Ar dating. The mixed-layer minerals include for each sample two mixed-layer phases consisting of pyrophyllite, margarite, paragonite, tobelite, illite, smectite and vermiculite layers. The main, 80 m thick intrusion resulted in the formation of pyrophyllite, margarite, paragonite and tobelite layers. However, the tobelite layers are absent in samples <21 m from this intrusion. Furthermore, chlorite was formed and kaolinite destroyed in samples adjacent to minor intrusions and at distances <60 m from the large intrusion. For the first time, the detailed, complex mixed-layer structures formed during contact metamorphism of kaolinitic, oil-forming mudstones have been investigated accurately. The formation of tobelite layers reveals that oil formation has taken place during contact metamorphism. Furthermore, K/Ar dating of mixed-layer minerals from shale indicates that the intrusives are of early Eocene age. The 80 m thick intrusive is responsible for the main mixed-layer transformations, whereas two thin (3 m and 0.5 m thick) intrusions contribute little. Thus, the detailed mixed-layer investigation has contributed significantly to the understanding of the regional geology and the contact metamorphic processes.

FTIR spectroscopy has been applied to NH4+-exchanged dioctahedral clay minerals to determine the molecular environment of NH4+ and to quantify N concentration. FTIR under vapourpressure control, coupled with heating and freezing treatments has shown that NH4+ ion symmetry varies with the nature of clay minerals. NH4+ has a perfect tetrahedral symmetry in hydrated or dehydrated smectites and belongs to the Td symmetry group. The NH4+-bending vibration is centred at 1450 and 1425 cm–1.

The Si4+-Al3+ substitution in dioctahedral clay minerals induces the loss of symmetry elements of the NH4+ tetrahedron which acquires a C2v symmetry. As a consequence, the Td –C2v transition can be used to characterize the smectite–illite transition. Quantification of NH4+ content per half unit cell is provided by nNH4 = k[NH4]/[OH] where [NH4]/[OH] is the band area ratio of the NH4+-bending vibration to the OH-stretching vibration. k = 1.1 for hydrated smectite, 0.9 for dehydrated smectite and 0.8 for illite or tobelite. The bending vibration of NH4+ is chosen for the calculation because it is not affected by superimposed contributions.

A transmission electron microscopy (TEM) investigation was undertaken in order to elucidate the nature of the structural disorder in a tobelite specimen from the sedimentary rocks of the Armorican sandstones (western France). This structural disorder may be the origin of residual electron-density maxima in difference-Fourier maps reported previously in single-crystal XRD studies of tobelite. The TEM investigation of tobelite confirmed that it is the 2M2 polytype in subfamily-B, but the ordered sequence is interrupted by numerous stacking faults parallel to (001), for which the stacking vectors belong to both subfamilies-A and -B of mica polytypes, with a prevalence of the latter. Chemical heterogeneity depending on the Si/Al ratio and Na and Mg concentration was observed at the nanoscale among different mica lamellae in a single crystal. The observed variations are consistent with a change in mica chemistry leading to interlayer vacancies which may cause shortening of the interlayer separation, as revealed by the single-crystal structure refinements.