The diagenetic evolution of kaolin and illitic minerals in sandstones is described here. The structural characterization of these minerals, the possible reaction pathways leading to their crystallization, and the origin of the fluids involved are discussed specifically.

While early precipitation of kaolinite is in general related to flushing by meteoric waters, subsequent diagenetic kaolinite-to-dickite transformation probably results from invasion by acidic fluids of organic origin. Dickite is the stable polytype in most sandstone formations and the kaolinite-to-dickite conversion is kinetically controlled.

The conventional model of kaolin illitization, assuming a thermodynamic control in a closed system, is discussed and compared to an alternative model in which illitization of kaolin is not coupled to dissolution of K-feldspar (Berger et al. , 1997). In the latter model, illite crystallization at the expense of kaolin implies that an energy barrier is overcome either by an increased K+/H+ activity ratio in solution or by a considerable temperature increase.

Authigenic kaolin and illitic minerals during burial diagenesis of sandstones: a review

Replacement of kaolinite by dickite has been observed to occur with increasing depth of burial in sandstones from three different basins on the Norwegian continental shelf. In the Garn Formation (Middle Jurassic) of Haltenbanken, samples from 1.4-2-7 km below the sea floor (110°C) contain kaolinite, whereas deeper than 3.2 km (130°C) mainly dickite is present. In the Statfjord Formation (Late Triassic-Early Jurassic) from Gullfaks and Gullfaks Sor Fields, transformation of kaolinite to dickite occurs at ~3.1 km below the sea floor (120°C) From the Sto and Nordmela Formations (Lower to Middle Jurassic) to the Troms Area, kaolin polytypes have been identified in only two shallow and two deep samples, but the results are consistent with the transformation depth determined in two other areas studied. These occurrences are significant because they allow the temperature of the kaolinite/dickite transformation to be established with greater confidence than had been possible previously. Also the observation of this transformation in all three areas so far examined indicates that it may be a general and predictable feature of kaolinbearing sandstones worldwide and therefore a potentially reliable paleogeothermometer. In most cases, the kaolinite occurs as relatively large vermicular crystals, whereas dickite forms more euhedral, blockier crystals. This morphological difference, together with the nature of the structural difference in octahedral occupancy between the kaolinite and dickite, suggests that the transformation occurs by dissolution and reprecipitation, rather then in the solid state.

Depth-dependent transformation of kaolinite to dickite in sandstones of the Norwegian continental shelf

Publisher Summary This chapter discusses mineralogy and petrology of burial diagenesis and incipent metamorphism in clastic rocks The phase-modification processes referred to as “late-diagenetic” or “epigenetic” (or catagenetic) are metamorphic in nature Hence, the distinction between burial diagenesis and lowest-grade regional metamorphism is largely artificial and arbitrary The modification of various kinds of sedimentary rocks upon burial has largely been studied by investigators from different sub-disciplines within the earth sciences: (1) changes in mineralogy of clay-rich sedimentary rocks by sedimentary petrographers and clay mineralogists; (2) authigenic mineral assemblages in tuffs and volcanic sediments by metamorphic petrologists; and (3) changes in the constitution of coaly and other organic matter by coal petrologists, and organic geochemists The growing awareness of the overlap in space of the physical conditions and of the chemical influences controlling these phenomena warrants renewed efforts to correlate the changes in various types of sedimentary rocks in order to arrive, ultimately, at an integrated picture of the phase changes in different types of sedimentary rocks upon burial and incipient metamorphism This chapter intends to contribute to such an integrated picture

Chapter 5 Mineralogy and Petrology of Burial Diagenesis (Burial Metamorphism) and Incipient Metamorphism in Clastic Rocks

The SEM, XRD, FTIR and DTA analyses of different size-fractions of clay material from sandstone reservoirs which have experienced a large range of burial conditions have been used to examine the different steps of the depth-related kaolinite-dickite reaction. Dickite progressively replaced kaolinite within a range of burial depths estimated between about 2500 m and 5000 m. The kaolinite-to-dickite reaction proceeds by gradual structural changes concomitant to crystal coarsening and change from booklet to blocky morphology. The crystallization of dickite proceeds by two distinct paths: (1) Accretion of new material from either dissolution of smaller unstable kaolinite crystals and/or detrital minerals (chiefly feldspars), on early-formed coarser metastable kaolinite crystals which exert extended morphological control on the growing crystals. (2) Neoformation of ordered dickite which will continue to grow by a dissolution-crystallization process. The kaolinite-to- dickite reaction is kinetically controlled and anomalies in the kaolinite/dickite ratio observed in certain sandstone reservoirs may be used to assess the timing of invasion by hydrocarbons.

Kaolinite-to-dickite reaction in sandstone reservoirs

Near-atomic resolution TEM imaging has been successfully applied to determine the stacking defect structures in kaolin minerals, especially kaolinite. The specimen studied is from the middle stage of the depth-related kaolinite-to-dickite transformation in a sandstone reservoir. Several high-quality images were recorded on films in which the tetrahedral and octahedral positions in the kaolinite unit layers are clearly resolved, despite the obstacle of electron radiation damage. Electron diffraction and high-resolution imaging of dickite showed that few stacking defects exist in this polytype. On the other hand, kaolinite crystals contain a high density of stacking defects. These defects or stacking disorders are formed by a mixture of two kinds of lateral interlayer shifts, t 1 (approximately − a /3) and t 2 (− a /3 + b /3), between adjacent layers. Disorder due to the coexistence of B and C layers (dickite-like stacking sequence) was never observed. These results provide not only an unambiguous settlement for the long -standing controversy of the defect structures in kaolinite, but also a new clue to understanding the kaolinite-to-dickite transformation mechanism.

Determination of defect structures in kaolin minerals by high-resolution transmission electron microscopy (HRTEM)

SUMMARY
The paper presents the dynamics of the structural change of kaolinite and its polymorphous (polytype) modifications at different stages of sedimentary rock formation.

Sedimentogenesis is characterized by a gradual disordering of structure of triclinic kaolinite, the degree of disorder produced being proportional to the intensity and duration of the influence of the hydrodynamic medium.

Epigenesis (deep) is connected with gradual transformation of kaolinite into dickite through the stage of mixed-layer growths inheriting the structure of twin rotations of disordered kaolinite from the sedimentogenesis stage.

In metagenesis filling of fissures with dickite or nacrite takes place. The former is peculiar to the general thermodynamic medium of deep epigenesis/metagen-esis, whereas nacrite is an indicator of intense stress and seems to be due to transformation of dickite.

S. A. Losievskaya

Papers

Genetic Interpretation of the Polymorphism of the Kaolinite Group in Sedimentary Rocks