The presence of certain minerals in the sand fraction and their alterations can help to assess the intensity of the weathering. Comparison between the values of the ratios of unstable and stable minerals has been an effective help to establish alteration indices between the different soil horizons.

Degree of evolution of the soil
The degree of evolution of a soil can be assessed by very different parameters and one of them is its mineralogy. This is done by comparing the mineralogy of the parent rock with that of the soil and the very important analysis of its variation with depth. Unstable minerals decrease as the horizons approach the surface of the land.

In general, values of the ratios between unstable and stable minerals are used, in fine sand and silt fractions (RUHE, 1956) .
Table II gives a summary of the behaviour according to the depth of a series of pairs of minerals in the coarse sand fraction of a set of soils. In the table, you can see that the vast majority of these soils show net growths of the ratios of biotite/quartz (in 80% of the soils), plagioclases/quartz (71%), orthoclase/quartz (74%) and feldspars (plagioclases + orthoclase + microcline)/quartz (73% ) depending on the depth of the sample, a distribution that reflects a decrease in the intensity of the alteration as the depth in the soil increases.

It is also possible to assess the degree of evolution of the soil on the basis of the alteration intensity that more or less unstable mineral grains have in the different soil horizons.

Alteration indices

Finally, the decrease in the percentage of unstable minerals in accordance with evolution can be used to calculate some alteration indices that serve to quantify the alteration of every soil horizon. Good results can be obtained working with the plagioclases/quartz, feldspars/ quartz and biotite/quartz ratios and calculating an alteration index on the basis of the value of this ratio in the lowest, and logically least altered, horizon and dividing this value by the value of this ratio in every horizon. It can be observed how young soils have indices with low values with distributions that are not, or hardly at all, marked with the depth, while evolved soils have high values of the mineral alteration index and moreover, its distribution with depth shows a marked decrease.

The following figure shows the variation of an index based on the quartz/feldspars ratio in the coarse sand fraction of a chronosequence of soils on alluvial terraces (River Almar, Salamanca). As the soil is older, this leads to: 1.) a more and more regular growth of this ratio with depth; 2.) higher and higher values of this ratio. These facts confirm that as the soil gets older (as the topographic elevation of every terrace increases) the evolution of the soil increases.