The thermodynamic and geochemical significance of the Stiff and Davis stability index

The Stiff and Davis stability index (S&DSI) has never had a clear thermodynamic or geochemical definition. In the present work, an analysis of its formulation allowed it to be defined as the saturation index (SI) of a calcium carbonate polymorph, investing it with clear thermodynamic significance. This analysis also allowed the K coefficient, an essential component of the S&DSI, to be defined; until now, no clear definition has been available. Finally, the experiment performed by Stiff and Davis that led to their announcement of the S&DSI was reproduced by geochemical modelling. The results obtained indicate that calcite was probably the calcium carbonate polymorph used in the construction of this index.

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► A thermodynamic definition is found for the stability index of Stiff and Davis S&DSI index.
► S&DSI index is equal to the saturation index, SI of a polymorph of calcium carbonate.
► We found a thermodynamic meaning for the coefficient K of Stiff and Davis S&DSI index.
► Using geochemical modeling, we have found geochemical significance to the S&DSI index.
► The S&DSI index is equal to the saturation index of calcite mineral.