SEDIMENTARY ROCKS/Clays and Their Diagenesis 65 Figure Field emission scanning electron microscopy (SEM) image of authigenic chlorite enclosed by detrital clay in mudrock size, and a reduction in abundance of crystal lattice defects This property has been widely used as an indicator of diagenetic grade, and the results may be correlated with vitrinite reflectance The Kubler crystallinity index is a measure of the width at half height of the glycol-solvated illite 001 reflection The Weaver sharpness ratio is the ratio of the illite 001 ˚ ) reflection to the height of the low-angle side of (10 A the reflection at 10.5 A˚ , also on the glycol-solvated trace The validity of using illite crystallinity or the sharpness ratio has been much debated, but the recent finding that the thickness of fundamental illite particles follows a unique evolution has permitted the refinement of illite crystallinity into a precise measurement of mean crystal thickness It should be noted that the inclusion of detrital metamorphic mica in the analysis will result in an overestimation of crystallinity Chlorite smectite It is also probable that both dissolution/precipitation and layer transformation occur in different settings In both bentonites and mudrocks, most smectite is dioctahedral, which appears to be more prone than trioctahedral smectite to illitization The latter is more likely to react to form chlorite via interstratified chlorite/smectite minerals Illitization in Bentonites Bentonites are ash falls that have undergone extensive devitrification to dioctahedral smectite (usually montmorillonite) Because they have a very simple mineralogical assemblage (most mudrocks contain not only more than one clay type, but a mixture of smectites and illite–smectites), and are often almost monomineralic, ancient bentonites have been extensively used to study the process of illitization of smectite Comparison of different bentonites, or single bentonites which have undergone variable heating during burial, shows that Si4ỵ, Ca2ỵ, and Naỵ are lost from the bed and Kỵ is gained as the smectite is illitized It should be noted that the supply of Kỵ is the rate-limiting step in the illitization of most bentonites because they are Kỵ deficient Thus, the most potassic (illitized) portions of many bentonite beds are frequently the margins Where the enclosing sediment is limestone, illitization will be restricted to any Kỵ present within the bentonite bed Illite Crystallinity and Illite Sharpness Ratio With increasing temperature, illite in mudrocks undergoes an increase in crystallinity, as measured by the 001 reflection sharpness on XRD traces This is due to the loss of smectite layers, increased particle Si4ỵ, Al3ỵ, Fe2ỵ, and Mg2ỵ, released from the dissolution of smectite and kaolinite, may react to form chlorite This is usually detected as a down-hole increase in chlorite, although it can be argued that mineralogical changes may also result from a shift in provenance or climate To obtain unequivocal evidence of authigenesis, it is usually necessary to use an imaging technique, such as SEM or TEM analysis, to demonstrate the face-to-edge arrangement of euhedral platelets The chlorite shown in Figure was investigated because XRD patterns for a Tertiary mudrock sequence showed unusually high chlorite concentrations at around 2.6 km burial depth A sample from the same depth, analysed ˚ by TEM, confirmed the presence of 14 A˚ and A lattice fringes, and X-ray analyses confirmed that it was an iron-rich chlorite Authigenic chlorite in mudrocks can also form by the replacement of biotite; commonly, replacement is partial, resulting in chlorite–biotite ‘stacks’ Kaolinite Authigenic kaolinite in mudrocks is much more abundant than was previously thought before backscattered electron imaging made mudrock petrography a real possibility Previously, a high kaolinite content in an argillaceous rock was assumed to indicate that the clay was formed through tropical weathering, and that it was consequently a climatic indicator Kaolinite in mudrocks typically replaces muscovite and phengite mica, and cements microfossil cavities Replacement of detrital mica by kaolinite is a hydrolysis reaction that releases Kỵ It characteristically occurs during early diagenesis, whilst the