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

The Tethyan Himalayan detrital record shows that India–Asia terminal collision occurred by 54 Ma in the Western Himalaya

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.

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

SUMMARY

This paper presents the main trends in epigenetic research in the U.S.S.R. in the last years:



1
Study of epigenetic zonality in thick series of sedimentary rocks of various ages and tectonic regions, (zones of unaltered clay matrix, of altered clay matrix, of quartzitic structures and hydromica-chloritic cement, and of prick-like structures and phyllite-like schists).

2
Principles of stadial analysis and mineral indicators of different epigenetic stages (clay minerals, organic matter, zeolites, feldspars, sulfides).

3
Facies of regional epigenesis connecting the main petrographic families of unaltered terrigenic rocks (quartz sandstones with kaolinitic matrix and the same composition of associated clays, acidic arkoses with hydromicaceous clays, lithoclastic sands with several matrix components and associated clays, and volcanogenic greywackes) with “related” facies of regional metamorphism.

4
Some practical aspects of epigenesis are: for petroleum geology: maximum depth of burial according to porosity, interruption of quartz, feldspar, sphene, epidote, carbonate, biotite, and clay mineral epigenesis by oil migration; and for ore geology: (a) initial epigenesis: ore impregnation (e.g., volkonskoite in wood remains) due to intrastratal solution of heavy minerals and leaching of rock fragments; (b) deep epigenesis: e.g., galena impregnations due to release of trace elements during dissolution of rock forming minerals e.g., feldspars; and (c) metagenesis: “Alpine” veins across and along bedding, filled, e.g., with copper ores.

Main aspects of the epigenesis problem

SUMMARY

The structure of multilayer polytype modifications of layer silicates may be regarded as the result of a combination of regularly oriented single layers Accordingly, the optical constants of a single layer (of a monolayer polytype modification) may be used for the calculation of optical constants of any other multilayer polytype modification of a given type of layers



The formulas connecting the constants of a polytype modification of layer silicates have enabled the establishment of the main parameters (Ng, Nm, Np, 2V) of the muscovites 1M and 2M2 according to the known optical constants of the muscovite 2M1, and the computation of the new constants of kaolinite and nacrite based on the known parameters of dickite The optical constants established theoretically were confirmed by direct measurements for muscovite 1M, muscovite 2M2 (gambelite) and kaolinite The optically positive character of kaolinite with +2V = 25° has been established



The assumption of the optical mixibility of finely dispersed polyminerallic media, in which the dimensions of monotype crystals are smaller than the visible light waves, made it possible to determine the quantitative relations of clay minerals in the mixtures based on optical constants of oriented aggregates

V. D. Shutov

Papers

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

Main aspects of the epigenesis problem

New aspects of the optical study of layer silicates