Thermodynamic interpretation of weathering indices and its application to engineering properties of rocks

Weathering indices typified by the Miura index (W^o_m) that are the ratios of geochemically mobile to relatively immobile elements can be shown to be logarithmically related to the free energy of the weathering reaction. This can be demonstrated for silicate minerals; other extensive properties such as the moduli of elasticity and rigidity are similarly correlated. The mass strength envelope implied by the corresponding o-log W plot concurs well with plotted values for rocks, suggesting that the correlation is valid for intact rocks as well.By a combination of the Miura index and an index believed to show not only the state of weathering but also the susceptibility to further weathering (the Parker index W^o_p), it is possible to obtain estimates of the activation energy for the incongruent aqueous dissolution of minerals, which compare favourably with experimental determinations. Although the engineering behaviour of rocks is generally governed by physical parameters, and thus by short-term mechanical or physico-chemical forces, there are some instances where failure has been attributed to the chemistry of the rock itself. For these rocks a good correlation is obtained between a chemical weatherability index (log W^o_m/W^o_p) and rock durability, as measured by the Los Angeles abrasion test.