Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

A synopsis of events related to the assembly of eastern Gondwana

▪ Abstract Mixed-layer clay minerals are intermediate products of reactions involving pure end-member clays They come from natural environments ranging from surface to low-grade metamorphic and hydrothermal conditions Most often mixed layering is essentially two component, but more complicated interstratifications have also been documented Variable tendency to form regular 1:1 interstratifications has been observed and explanations of this phenomenon have been proposed Mixed-layer clays are either di- or trioctahedral; di/trioctahedral interstratifications are rare Most mixed-layer clays contain smectite or vermiculte as a swelling component Exceptions are all trioctahedral: serpentine/chlorite in low-temperature environments, and mica/chlorite and talc/chlorite at high temperatures Solid state transformation and dissolution/crystallization are the two mechanisms responsible for the formation of different mixed-layer clays In general, the weathering reactions that produce mixed layering are revers

Nature of mixed-layer clays and mechanisms of their formation and alteration

This paper compares mechanisms of the reaction of smectite to illite, in light of structural models for interstratified illite/smectite (US). The crystal structure of US has been described previously by a nonpolar and polar 2:1 layer model. In a nonpolar model, individual 2:1 layers are chemically homogeneous, whereas a polar model assumes a 2: l layer can have a smectite charge on one side and an illite charge on the other side. Several kinds of data support the polar model; however, more deter- minations of the negative charge of expandable sites in US are needed to confirm such a model. Assuming a polar 2:1 layer model for I/S, we compare the mineralogical and geochemical consequences of several reaction mechanisms for smectite illitization: 1) solid-state transformation (SST), 2) dissolution and crystallization (DC) and 3) Ostwald ripening (OR). Features of an SST model are the replacement of smectite interlayers by illite intedayers, resulting in gradual changes in interlayer ordering, polytype, chemical and isotopic composition and crystal size and shape. Several SST models are possible depending on the nature of the reaction site (framework cations, polyhedra or interlayers). In contrast, DC models allow for abrupt changes in the structure, composition and texture of I/S as illitization proceeds. Several DC models are possible depending on the nature of the rate-controlling step, for example, diffusional transport or surface reactions during crystal growth. The OR model represents the coarsening of a single mineral where the smallest crystals dissolve and nucleate onto existing larger crystals, allowing for evolution in the overgrowth but not in the template crystal. An SST mechanism, involving either reacting polyhedra or reacting interlayers, seems to best model illitization in rock-dominated systems such as bentonite. A DC mechanism seems to best model illitization in fluid-dominated systems such as sandstone and hydrothermal environments. Both DC and SST mech- anisms can occur in shale. Differences in reaction mechanism may be related to permeability. An OR model poorly describes illitization because of the progressive mineralogical and chemical changes in- volved. For many geologic environments, it is important to consider alternate origins for US such as kaolinite illitization and detrital. Further work is needed to clarify the DC crystal growth process in terms of a structural model of I/S and to determine which specific SST or DC model best characterizes illitization in geologic systems.

Comparison of structural models of mixed-layer illite/smectite and reaction mechanisms of smectite illitization

Formation of clay minerals under hydrothermal influence is the result of rock alteration by circulating hot water in the Earth’s crust. A pre-existing rock-forming mineral assemblage is altered to a new set of minerals which are more stable under the hydrothermal conditions of temperature, pressure, and fluid composition. The interaction of hot water and rocks forms a spatially and temporally regular zonal pattern of new clay minerals, as the fluid with cooling temperature moves through the surrounding rock mass. This chapter discusses the formation of clay minerals in such dynamic processes of hydrothermal alteration. The approach is one of clay-mineral facies formed under conditions of massive alteration in the rocks. The chemical and mineralogical changes which occur on the scale of a rock or rock mass are considered to have been dealt with in the preceding chapter. The exact process of change via local, vein-influenced exchange processes is ignored for simplicity (see Chap. 6).

Formation of Clay Minerals in Hydrothermal Environments

Morphological characteristics of illitic clay minerals from a hydrothermal system

Textural and geochemical characteristics of the Ajali Sandstone, Anambra Basin, SE Nigeria: Implication for its provenance

Stacking structures in pyrophyllite, Al 2 Si 4 O 10 (OH) 2 , were investigated mainly by using high-resolution transmission electron microscopy (HRTEM). The specimens examined were large lath-shaped crystals (Berosovska, Urals, Russia) and massive aggregates of fine platy crystals (Nohwa, southwest Korea). Both specimens showed powder X-ray diffraction (XRD) patterns similar to those reported previously as the 2 M polytype. The common stacking sequence in the two specimens is not monoclinic with two-layer periodicity as previously reported, but a uniform orientation of the 2:1 layers and near complete disorder of two alternative directions of interlayer displacement, i.e., lateral displacement between the two tetrahedral sheets across an interlayer region. The directions of interlayer displacement are about ±2π/3 from that of the intralayer shift (lateral displacement between the two tetrahedral sheets within a 2:1 layer). Simulation of powder XRD patterns by this stacking model closely approximates the experimental pattern. Elongation of the lath-shaped Berosovska crystals corresponds to the direction of the intralayer shift, as seen in illite-1 M . 2:1 layers with different orientations, and interlayer displacement almost parallel to the intralayer shift, were occasionally observed as stacking faults. Such disorder occurs more frequently in the massive Nohwa specimen than in the Berosovska specimen. Sub-micrometer domains of the 2 M stacking sequence with regular alternation of the two directions of interlayer displacement were found in the Nohwa specimen.

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Stacking structures in pyrophyllite revealed by high-resolution transmission electron microscopy (HRTEM)

The weathering process of a granitic rock was mineralogically investigated with XRD, XRF, SEM and TEM at one weathered profile (Ca. 20m in height) in the western part of Cameroon where climate is the intermediate of humid and arid tropical condition. Halloysite and kaolinite are main constituent clay minerals in the granite saprolite (weathered granite). Halloysite predominantly occurs at the lower part of the investigated profile associating with small amount of kaolinite. Numerous spherical-shaped halloysite were observed in the bottom of the profile under TEM. The granite saprolite at the top part of the profile consists mainly of platy kaolinite with small amount of tubular halloysite. Detailed observation of morphological variation by TEM together with X-ray analysis revealed that the spherical-shaped halloysite crystals (0.05-0.5μm) are first unrolled and then transform to tubular halloysite (0.05-2μm), and finally unrolled again producing platy kaolinite (0.2-3μm). Close relationship between crystal size and morphology is obtained.

Morphological transformation of kaolin minerals from granite saprolite in the western part of cameroon

The K-Ar ages of sericites collected from sericite and kaolin deposits in the Chugoku district were determined and their formation stages were discussed. Sericites from sericite deposits are dated as follows; Ohgin (31.1 Ma), Iwaya (36.4 Ma), Nabeyama (45.6 Ma), Yoshida (48.5 Ma), Unnan (50.3 Ma), Igi (50.6 Ma), Hida (56.9 Ma), Ohguni (61.4 Ma) and Kumano (77.5 Ma). Sericites from Komaki, Kohtachi and Toyosaka kaolin deposits are dated at 55.6 Ma, 76.5 Ma and 79.5 Ma, respectively. The results indicate that the formation ages of sericite and kaolin deposits are comparable to or slightly younger than those of respective hosts or nearby granitic rocks whose age data are available at the present. Therefore, the mineralization stages of these clay deposits appear to be concluded to correspond with the hydrothermal stages of granitic activities in the Chugoku district during the period from Late Cretaceous to Palaeogene.

https://www.jstage.jst.go.jp/article/shigenchishitsu1951/38/209/38_209_279/_pdf

Ryuji Kitagawa

Papers

Morphological characteristics of illitic clay minerals from a hydrothermal system

Textural and geochemical characteristics of the Ajali Sandstone, Anambra Basin, SE Nigeria: Implication for its provenance

Stacking structures in pyrophyllite revealed by high-resolution transmission electron microscopy (HRTEM)

Morphological transformation of kaolin minerals from granite saprolite in the western part of cameroon

K-Ar Ages of the Sericite and Kaolin Deposits in the Chugoku District, Southwest Japan