Chemical index of alteration

Geochemical processes associated with weathering and soil formation are dominated by alteration of feldspars (and volcanic glass), feldspars accounting for 70% of the upper crust, if the relatively inert quartz is discounted (Taylor and McLennan, 1985). Advancing weathering leads to a shift towards an aluminum rich composition that can be approximated by the chemical index of alteration (CIA) of Nesbitt and Young (1984)... 

Figure 12 Ternary plots of molecular proportions of Al203-(Na20 + CaO )-K20 with the Chemical Index of Alteration (CIA) scale shown on the left. Also plotted are selected idealized igneous and sedimentary minerals and the range of typical natural waters. Squares are suspended sediments from major rivers throughout the world representing a variety of climatic regimes. CaO is the silicate bound concentration only (after McLennan et al., 2003).

Table 4.9 Chemical index of alteration (CIA) values for various crustal materials. Data from Nesbitt and Young (1982), Maynard el al. (1991) and Taylor and McLennan (1985).

CHondridc Uniform Reservoir — the chondritic model for the composition of the bulk earth Chemical Index of Alteration — a measure of the degree of chemical weathering... 

The disadvantage of this procedure is that the minerals maybe physically or chemically altered during burning. Eor example, the refractive index of clay minerals is changed the color, birefringence, and pleochroism of micas is altered carbonates are destroyed and the iron sulfides are oxidized to iron oxides. 

This paper reports the chemical and physical alteration of coals from two general areas in Colorado. An index map is included as Figure 1 to help in... 

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