A new model is proposed to explain, within the framework of the theory of spiral growth of Frank, the formation on inhomogeneous mica polytypes. This model relates the interaction and cooperative growth of two components (spirals and/or crystals) to produce a new stacking sequence. Depending on the relative orientation between the two components, a mismatch of the interlayer positions occurs, which is compensated through either a growth defect or a crystallographic slip at the octahedral (O) sheet. Both these adjustments transform the Ml layer into the M2 layer. These two types of layers have the same chemical composition but differ in cation distribution in the O sheet. The coalescence and cooperative growth of crystals occurs in fluid-rich environments and is most frequent in druses and volcanic fumaroles. These environments favor the inhomogeneous polytypes, especially those with complex stacking sequenc¬es. In addition, the Ml → M2 transformation is most probable in micas with an oxybiotitic composition, where the removal of the OH dipole strengthens the interlayer bonding and the presence of high-charge cations destabilizes the O sheet. Three examples of inhomogeneous polytypes of titaniferous oxybiotite from Ruiz Peak (a volcanic environment where many inhomogeneous polytypes have been reported) are presented.

The three-dimensional crystal structures of illite-smectite (I-S) in K-bentonite samples from the Appalachian Basin are characterized by rotational disorder in the stacking sequence of 2:1 illite layers, different proportions of n60° rotations (as opposed to n120°) in the rotated layers, and layers with centrosymmetric trans-vacant (tv) octahedral sites that are randomly interstratified with noncentric cis-vacant (cv) layers. The proportion of cv interstratification in the I-S increases with tetrahedral A1 and decreases with octahedral Mg and Fe content. The I-S minerals in the northern Appalachian basin K-bentonites are characterized by high (79% average) proportions of cv (Pcv) layers. In contrast, I-S from equivalent K- bentonites from the southern Appalachian basin has low Pcv values (38% average). These values do not correlate with expandability or rotational disorder. The geographic distribution of these I-S structural parameters may have resulted from possibly short-term, hot, and advective fluid migrations that differed in Mg concentrations and/or other physical and chemical parameters.

Thermal maturity of the Lower Cretaceous Sindong and Hayang groups in the Gyeongsang Basin, Korea, was investigated using the Kübler illite “crystallinity” index (KI) which is based on the numerical expression of the 10-Å peak width after calibration to the Crystallinity Index Standard scale. The metamorphic grade of the Sindong and Hayang groups ranges from late diagenetic zone to high anchizone. Depth of burial was not a major factor controlling KI variation in the basin because Hayang mudrocks have higher thermal maturity than the underlying Sindong mudrocks. Short-lived heating by the emplacement of the Upper Cretaceous plutonic rocks is responsible for the higher thermal maturity in the Hayang mudrocks. A linear NNE-SSW trending belt is drawn by connecting the lowest KI values, and it seems to reflect a hidden fault trace beneath the Hayang Group. Emplacement of Upper Cretaceous plutonic rocks and introduction of hydrothermal fluids along the inferred fault may have been the main cause of the lowering of KI values along this belt. This inferred fault is interpreted to represent the buried eastern boundary of the Nagdong Trough, which was the main depositional site for the Sindong Group in the western part of the Gyeongsang Basin. Deposition of the Hayang Group occurred in the enlarged basin when the basin extended eastward. This study provides an example that KI values can be used in the reconstruction of an early history of basin evolution.

The clay particles in a kaolin deposit from Brazil were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), analytical transmission electron microscopy (ATEM), and electron paramagnetic resonance (EPR) to examine the relationships between morphological and chemical properties of the crystals and to relate these properties to formation conditions. The XRD patterns show the dominant presence of kaolinite with minor amounts of gibbsite, illite, quartz, goethite, hematite, and anatase. ATEM observations show two discontinuities in the deposit as indicated by changes in morphology and size of the kaolinite crystals. At the base of the deposit, hexagonal platy and lath-shaped particles (mean area of 001 face = 0.26 μm2) maintain the original fabric of the parent rock which characterizes an in situ evolution. In the middle of the deposit a bimodal population of large (mean area of 001 face > 0.05 μm2) and small (mean area of 001 face < 0.05 μm2) sub-hexagonal platy kaolinite crystals occurs. This zone defines the boundary between the saprolitic kaolinite and the pedogenic kaolinite. Near the top of the profile, laths and irregular plates of kaolinite, together with sub-hexagonal particles, define two different depositional sources in the history of formation of the deposit. Crystal thickness as derived from the width of basal reflections and the Hinckley index are compatible with the morphological results, but show only one discontinuity. At the base of the deposit, kaolinite has a low- defect density whereas in the middle and at the top of the profile, kaolinite has a high-defect density. Likewise, EPR spectroscopy shows typical spectra of low-defect kaolinite for the bottom of the deposit and typical spectra of high-defect kaolinite for the other portions of the deposit. Despite the morphological changes observed through the profile, the elemental composition of individual kaolinite crystals did not show systematic variations. These results are consistent with the deposit consisting of a transported pedogenic kaolinite over saprolite consisting of in situ kaolinized phyllite.

The dissolution in 1 M HC1 of Al-, Mn-, and Ni-substituted hematites and the influence of metal substitution on dissolution rate and kinetics of dissolution were investigated. The inhomogeneous dissolution of most of the hematites investigated was well described by the Avrami-Erofe'ev rate equation, kt = √[-ln(l − α)], where k is the dissolution rate in time, t, and α is the Fe dissolved. Dissolution of Al-substituted hematite occurred mostly by edge attack and hole formation normal to (001), with the rate of dissolution, k, directly related to surface area (SA). Dissolution of rhombohedral Mn- and Ni-bearing hematites occurred at domain boundaries, crystal edges, and corners with k unrelated to SA. The morphology of Mn- and Ni-substituted hematites changed during dissolution with clover-leaf-like forms developing as dissolution proceeded, whereas the original plate-like morphology of Al-bearing hematite was generally retained. Acid attack of platy and rhomboidal hematite is influenced by the direct (e.g., metaloxygen bond energy, hematite crystallinity) and indirect (e.g., crystal size and shape) affects associated with incorporation of foreign ions within hematite.

Imogolite is a tubular aluminosilicate which is common in Andosols and Spodosols. The high pH at point-of-zero charge at the outer parts of the tube and the anomalously high chloride adsorption of imogolite suggested that there may be structural charge associated with this mineral. The structural charge may arise because of changes in bond valence imposed by the incorporation of orthosilicate anions in a gibbsite-type sheet. By using a Basic Stem Model approach, it is shown that the surface charge properties of imogolite are explained if the mean Al-O bond valence of the outer -Al2OH groups is higher than the inner -Al2OHSiO3 groups. Hence, a weak positive charge is developed on the outer tube walls whereas a negative charge develops in the tubular pores. The best model fits were obtained where either one or two units of structural charge per unit cell of tube were assumed. The model may also explain why imogolite tubes are normally aggregated in large bundles in close hexagonal packing, because bound counterions may hold the tubes together. However, to arrive at good model descriptions, the deprotonation of -Al2OH groups must occur at a higher pH than that expected when assuming that all surface oxygens form two hydrogen bridges with H2O. A more precise structure of imogolite is required to test fully this hypothesis.

Dioctahedral vermiculite occurs in an isolated metagabbro klippe (Kurançalı Metagabbro) that belongs to the Central Anatolian Ophiolites from central Turkey. Both the metagabbro and the structurally underlying high-grade metamorphic rocks are intruded by granitic rocks. The Kurançalı Metagabbro is characterized by its well-developed compositional layering, and the presence of vermiculitized phlogopiterich layers. Petrographic and mineralogie studies show that the primary mineral phases in the host rock are diopside, tschermakitic hornblende, Fe-rich phlogopite, and plagioclase. Secondary minerals are hornblende, actinolitic hornblende, Fe-rich phlogopite, and vermiculite. A two-phase history of alteration involving acidic weathering and alkaline metasomatism is suggested for the dioctahedral vermiculite and secondary Fe-rich phlogopite, respectively. The alteration of phlogopite to dioctahedral vermiculite proceeded both along cleavage planes and at crystal edges. The vermiculite is colorless to pale yellow with weak pleochroism and shows optical continuity with the parent mineral. Vermiculite flakes, analyzed semi-quantitatively by scanning electron microscope-energy dispersive analysis (SEM-EDS) and electron microprobe (EMP), are characterized by partially expanded interlayers, K depletion, and Mg and/or A1 enrichment. X-ray diffraction (XRD) and differential thermal analysis-thermal gravimetric (DTA-TG) analyses indicate that phlogopite is not a pure phase, although it is the dominant one. The XRD patterns show the presence of both dioctahedral vermiculite having dehydrated interlayers and hydroxy-Al interlayers, and interstratified phlogopite-vermiculite. The transformation of phlogopite to vermiculite is thought to represent an initial stage of weathering in an acidic environment.

The alteration process of a subvolcanic rock with calcic plagioclase, pyroxene, and olivine as major components was investigated by X-ray diffraction (XRD) and analytical and transmission electron microscopy (TEM/AEM). Experimental interaction with 1 M NaOH solution led to the formation of dioctahedral beidellite to Fe-rich montmorillonite after 1 and 3 d of reaction. This range of smectite composition is similar to that from natural subvolcanic-derived soil formed from the same parent material. After 14 d of reaction, a berthierine-smectite (B-S) interstratified clay had partially replaced the smectite. Although, the presence of smectite interlayers prevented analysis of pure berthierine, berthierine-rich BS interstratifications have a composition similar to pure berthierine. After 40 d, the alteration process led to a 7-Å S interstratification whose composition falls between greenalite and lizardite. A series of amorphous materials were also found in the 14 and 40-d experiments. The most abundant of these is a Si-CaFe-rich material, whose chemical composition approaches that of the starting rock. In contrast, two other amorphous materials had a smectitic composition.