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It is often necessary to know, in addition to the types of clay minerals, the total amount of clay or clay minerals in a reservoir rock. Traditionally, this is done by quantitatively separating a specific size fraction ( <2 or <5 pm), by gravity sedimentation or centrifugation, from the bulk rock. Figure 1 shows types of minerals that are present in different size fractions in a sandstone reservoir rock. Kaolinite is present even in the 20 pm particle size range. Traditional methods would have underestimated the amount of clay or clay minerals in this sample. In this specific case, a different approach was used to estimate the amount of clay minerals. The sample was spiked with an internal standard, and the only non clay minerals, quartz and feldspar, were quantified to within 1-2 wt% using the Rietveld method (Iyengar, 1995). Computer Simulation and Modeling The advent of fast and powerful personal computers has made characterization of minerals by XRD very efficient. Furthermore, these computers have helped rationalize the unpredictable behavior of certain minerals in nature. A very good example is the nature of illite/smectite in sediments. These occur either as discrete illite and smectite or interstratified with each other in various proportions. It is often important, as discussed later, to know the nature of their occurrence. Computer simulation and modeling have made this task easier. XRD patterns of these two minerals in various proportions (Fig. 2) have been generated as an identification aid (Reynolds, 1980; Hower, 1981). Similar synthetic patterns have been generated for other combinations of minerals including chlorite and kaolinite. Several programs (Newmod, Matchmod, Foxfire, etc.) are available for generating such synthetic patterns (Brindley and Brown, 1980; Reynolds and Walker, 1989). Several computer programs (POWD12, Rietveld, etc.) are also available for generating a powder pattern from single-crystal data.