64 SEDIMENTARY ROCKS/Clays and Their Diagenesis result of acid dissolution, especially by organic acids However, feldspar dissolution can occur at low, neutral, or high pH Indeed, the rate of feldspar dissolution is kinetically controlled Hence feldspar (and mica) dissolution increases with increasing temperature, and therefore depth Moreover, organic acids have low buffering capacities and therefore not influence pH greatly It should be noted that the reaction results in the release of silica and, through smectite dehydration, the release of water The source of aluminium is probably mostly feldspar, but the means of transporting sufficient dissolved aluminium to the reaction site has not been entirely resolved, as the aluminium solubility varies enormously with pH, but in most geological situations is rather low Carboxylic acids are claimed to have the capacity to complex with aluminium, thereby increasing the amount that can be held in solution However, such acids are unlikely to have much effect on aluminium solubility in complex (i.e., natural) systems It should also be noted that the reaction yields a potential source of quartz cement It is largely agreed that the reaction is kinetically controlled, although, if the proposed kinetic equations are applied to the older sedimentary basins, the amount of illitization is vastly overestimated; this may be because the total heating to which they have been exposed has been overestimated How close K feldspar needs to be to the site of illitization probably depends on the fluid flow, the degree of sandstone/mudrock interbedding, and the overpressure In the Mahakam Delta Basin in Indonesia, it has been shown that the K feldspar alteration in both sandstone and mudrock is restricted to the upper km of sediment, whereas illitization occurs at greater depths, thus necessitating an open system for Kỵ transfer at depth In contrast, in the Gulf Coast and the Tertiary mudrock/sandstones of the North Sea, diagenesis may be a nearly closed system The illitization of smectite commences at approximately 70 C, and peaks at approximately 120– 130 C However, in sedimentary basins with high geothermal gradients, this will occur at shallower depths than in those with lower geothermal gradients Time, overpressure, pore fluid composition, and hydrothermal activity are also important factors in clay burial diagenesis In general, there is sufficient K feldspar and mica for this not to be an inhibiting factor; clay diagenesis in clay-rich basins is most likely to be inhibited by overpressure which restricts fluid movement If a source of Kỵ is lacking, illite will not be formed, except where intense organic diagenesis releases NH4, which is able to form ammonium illite The illitization of smectite in mudrocks proceeds via random, mixed-layer, smectite-rich, illite–smectite to ordered, illite-rich, illite–smectite Ordering commences at about 35% expandable layers The maximum illite content is typically 80% This sequence has been recognized in a wide variety of settings The analysis of the expandability and thermal histories of basins ranging in age from Precambrian to Quaternary has indicated that the composition of illite–smectite in mudrocks is primarily controlled by the maximum palaeotemperature Hence, illite–smectite may be used as a geothermometer for mudrocks, although, as pore fluid overpressure and a lack of Kỵ may occasionally be more than minor controls on the illitization process, this should always be performed with caution The interpretation of mixed-layer illite–smectite in terms of fundamental particles and interparticle diffraction, in the early 1980s, triggered much research into the true nature of illite–smectite The concept of interparticle diffraction implied that, during illitization, mixedlayer crystals were not two chemically distinct clay ˚ thick (Figure 2) minerals, but single illite layers 10 A When these fundamental particles were analysed by XRD, diffraction between particles created the illusion of smectite interlayers With increasing diagenetic maturity, these particles grow in three dimensions and the apparent smectite layers decrease This interpretation is not intended to imply that smectite does not exist! The mechanism implies the dissolution of smectite and the precipitation of illite, which initially exists as fundamental particles However, it is still argued from transmission electron microscopy (TEM) data that layer-by-layer replacement of smectite by illite occurs High-resolution investigations of the illitization of smectite have shown coherently scattering domains of interstratified illite-expandable layers (Figure 3), expandable layers within coherently scattering domains of illite, and domains of illite within Figure Lattice fringe transmission electron microscopy (TEM) image of interstratified illite expandable clay (the expand able layers have been fixed to prevent collapse in the electron beam)