Fig. 2.5 A typical XRD pattern with mineral identification for two different soils (modified from Sreedeep 2006)

           It is understood that the area under the peak of diffraction pattern gives the quantity of each phase present in the specimen. However, quantitative determination of mineral composition in soils based on simple comparison of diffraction peak heights or area under peak is complex and uncertain because of different factors such as mineral crystallinity, hydration, surface texture of the specimen, sample preparation, non-homogeneity of soil samples, particle orientation etc. The method of quantification will be more precise for those soils with less number of minerals. Al-Rawas et al. (2001) have discussed about constant mineral standard method and constant clay method for quantification of clay minerals. In the first method, increasing quantity of clay are added to the fixed mass of known standard and the difference in X-ray diffraction intensity when the specimen changes from 100 percent standard to 100 percent clay is noted. The peak area ratio for each component is then plotted against percentage of clay, based on which regression equation is determined. This regression equation is further used for mineral quantification. In the second method, known weight of pure standard mineral is added to clay containing the same components, and the change in the reflection peak-area intensity of each component is measured to estimate the weight proportion of that component.
           The fundamental discussion on the theory of XRD is quite extensive and cannot be dealt in this course. Interested readers can go through literature available on XRD in detail (Whittig and Allardice 1986; Moore and Reynolds 1997; Chapuis and Pouliot1996; Manhaes et al. 2002).

2.1.5 Applications of soil mineral analysis in geoenvironmental engineering
As explained above, the soil-water and soil-water-contaminant interaction and hence reactivity is greatly influenced by the mineralogy.
           Chapuis and Pouliot (1996) have demonstrated the use of XRD for determining bentonite content in soil-bentonite liners employed in waste containment. Predicting global hydraulic performance of liner is very difficult with small scale permeability test conducted in the field. There are no methods available for the prediction of global permeability from small scale permeability test. For this purpose, the XRD quantified bentonite content is used for understanding the global hydraulic performance of liners. The soil used in this study was subjected to heating at 550 °C in order to reduce its tendency for hydration, there by eliminating the possibility of variation in diffraction intensity due to difference in hydration. An internal standard was used for controlling X-ray absorption and has been added to all specimens in equal quantity. In this study, authors also indicate the usefulness of using XRD for knowing the quality and constancy of bentonite supplied for the project.
           When there are problems associated with expansive soils, the best method for identifying the problem is by conducting XRD and checking for expansive clay minerals. Bain and Griffen (2002) highlights that acidification of soil can be understood by understanding the transformation of minerals. This is mainly due to the fact that micas get transformed to vermiculite by weathering process under acidic condition. Velde and Peck (2002) have shown that crops can affect the clay mineralogy of the soils on which they are grown over periods of time. The influence of fertilizer addition on cropping can be studied by analyzing transformation of soil mineral in the field where the cropping has been done. By analyzing mineralogy, the land use practices can be assessed.